geography case study japan earthquake 2011

Case Study: How does Japan live with earthquakes?

Japan lies within one of the most tectonically active zones in the world. It experiences over 400 earthquakes every day. The majority of these are not felt by humans and are only detected by instruments. Japan has been hit by a number of high-intensity earthquakes in the past. Since 2000 there are have been 16000 fatalities as the result of tectonic activity.

Japan is located on the Pacific Ring of Fire, where the North American, Pacific, Eurasian and Philippine plates come together. Northern Japan is on top of the western tip of the North American plate. Southern Japan sits mostly above the Eurasian plate. This leads to the formation of volcanoes such as Mount Unzen and Mount Fuji. Movements along these plate boundaries also present the risk of tsunamis to the island nation. The Pacific Coastal zone, on the east coast of Japan, is particularly vulnerable as it is very densely populated.

The 2011 Japan Earthquake: Tōhoku

Japan experienced one of its largest seismic events on March 11 2011. A magnitude 9.0 earthquake occurred 70km off the coast of the northern island of Honshu where the Pacific and North American plate meet. It is the largest recorded earthquake to hit Japan and is in the top five in the world since records began in 1900. The earthquake lasted for six minutes.

A map to show the location of the 2011 Japan Earthquake

The earthquake had a significant impact on the area. The force of the megathrust earthquake caused the island of Honshu to move east 2.4m. Parts of the Japanese coastline dr[[ed by 60cm. The seabed close to the focus of the earthquake rose by 7m and moved westwards between 40-50m. In addition to this, the earthquake shifted the Earth 10-15cm on its axis.

The earthquake triggered a tsunami which reached heights of 40m when it reached the coast. The tsunami wave reached 10km inland in some places.

What were the social impacts of the Japanese earthquake in 2011?

The tsunami in 2011 claimed the lives of 15,853 people and injured 6023. The majority of the victims were over the age of 60 (66%). 90% of the deaths was caused by drowning. The remaining 10% died as the result of being crushed in buildings or being burnt. 3282 people were reported missing, presumed dead.

Disposing of dead bodies proved to be very challenging because of the destruction to crematoriums, morgues and the power infrastructure. As the result of this many bodies were buried in mass graves to reduce the risk of disease spreading.

Many people were displaced as the result of the tsunami. According to Save the Children 100,000 children were separated from their families. The main reason for this was that children were at school when the earthquake struck. In one elementary school, 74 of 108 students and 10 out of 13 staff lost their lives.

More than 333000 people had to live in temporary accommodation. National Police Agency of Japan figures shows almost 300,000 buildings were destroyed and a further one million damaged, either by the quake, tsunami or resulting fires. Almost 4,000 roads, 78 bridges and 29 railways were also affected. Reconstruction is still taking place today. Some communities have had to be relocated from their original settlements.

What were the economic impacts of the Japanese earthquake in 2011?

The estimated cost of the earthquake, including reconstruction, is £181 billion. Japanese authorities estimate 25 million tonnes of debris were generated in the three worst-affected prefectures (counties). This is significantly more than the amount of debris created during the 2010 Haiti earthquake. 47,700 buildings were destroyed and 143,300 were damaged. 230,000 vehicles were destroyed or damaged. Four ports were destroyed and a further 11 were affected in the northeast of Japan.

There was a significant impact on power supplies in Japan. 4.4 million households and businesses lost electricity. 11 nuclear reactors were shut down when the earthquake occurred. The Fukushima Daiichi nuclear power plant was decommissioned because all six of its reactors were severely damaged. Seawater disabled the plant’s cooling systems which caused the reactor cores to meltdown, leading to the release of radioactivity. Radioactive material continues to be released by the plant and vegetation and soil within the 30km evacuation zone is contaminated. Power cuts continued for several weeks after the earthquake and tsunami. Often, these lasted between 3-4 hours at a time. The earthquake also had a negative impact on the oil industry as two refineries were set on fire during the earthquake.

Transport was also negatively affected by the earthquake. Twenty-three train stations were swept away and others experienced damage. Many road bridges were damaged or destroyed.

Agriculture was affected as salt water contaminated soil and made it impossible to grow crops.

The stock market crashed and had a negative impact on companies such as Sony and Toyota as the cost of the earthquake was realised.  Production was reduced due to power cuts and assembly of goods, such as cars overseas, were affected by the disruption in the supply of parts from Japan.

What were the political impacts of the Japanese earthquake in 2011?

Government debt was increased when it injects billions of yen into the economy. This was at a time when the government were attempting to reduce the national debt.

Several years before the disaster warnings had been made about the poor defences that existed at nuclear power plants in the event of a tsunami. A number of executives at the Fukushima power plant resigned in the aftermath of the disaster. A movement against nuclear power, which Japan heavily relies on, developed following the tsunami.

The disaster at Fukushima added political weight in European countries were anti-nuclear bodies used the event to reinforce their arguments against nuclear power.

Privacy Overview

Pin It on Pinterest

You must be logged in to post a comment.

Case Study - North East Japan Earthquake and Tsunami (2011)

The earthquake off the coast of Honshu, Japan's largest island, on 11th March 2011, was the most powerful earthquake ever to hit Japan. It recorded 9.0 on the Richter Scale, also making it the fifth most powerful earthquake of all time anywhere in the world. 

The quake struck on the 11th March 2011.

The earthquake measured 9 on the Richter scale.

The epicentre was 130 km from the coast of Sendai in North East Japan.

Japan lies in many plate boundaries.

The earthquake was caused by a destructive plate boundary: the Pacific plate was sining under the Eurasian plate.

The plate boundary is known as the Pacific Ring of Fire, an extremely active tectonic area.

The ocean trench and the subduction zone were in the Pacific Ocean.

As the Pacific plate slipped under it released an enormous amount of tension at the focus. This has been building up over centuries and caused massive primary seismic waves. 

The more dangerous secondary seismic waves made their way towards the easter side of Japan.

The shock waves also caused a tsunami.

There were 508 aftershocks of magnitude 5, 6 and 7.

Environmental

The earthquake shifted the earth's axis by 25 cm.

The earth has changed shape as a result of the earthquake.

Japan's north-east coast moved 3 metres out to sea.

Parts of the coast dropped over 1 metre, causing tsunami defence walls to become smaller.

The earthquake ripped down the infrastructure in the area affected.

The Fukushima nuclear power plant was affected by both the tsunami and the earthquake. Meltdown occurred.

Liquefaction appeared in the cracks in the ground. Liquefaction is when water is pushed up to the surface and through the cracks, like a sponge releasing water.

10 billion tonnes of water spread out across Japan after the tsunami.

At Sendai the water travelled 10 km inland.

The stress released in the 2011 earthquake may trigger a bigger earthquake further south in Japan, in places such as Tokyo.

15,000 people died and 5,000 people were missing, presumed dead.

The Fukushima nuclear plant was affected and sent out radiation. Some people were evacuated from the nearby area.

The airport at Sendai was destroyed.

The temporary shelters in Sendai were full.

Huge areas were without power.

Services such as schools and hospitals were shut or destroyed for a long period of time.

Mountain residents worried about the corpses being swept up the rivers in the water.

Hawaii had to evacuate people living near the sea. Luckily no one died.

In Minamisanriku 50% of the population died and 95% of the buildings were destroyed.

People had to rebuild their lives; many people had lost everything.

Human Response

The immediate response to the earthquake was automatic warnings to the Japanese people, on mobile phones and on television.

There was a one-minute warnings for earthquake and a 20-minute warnings for tsunami.

The Hawaii Pacific Earthquake facility also sent warnings to countries surrounding the Pacific, and as a result only one person died in a country other than Japan (on the California coast).

More data was collected from this earthquake than from any other disaster to date.

The Japanese military were immediately on the scene, clearing debris in the towns that had been demolished.

Aid was sent by different countries to help ease the impact of the disaster. Rescue groups were also sent.

Every school and business had performed earthquake drills and these saved many lives.

The Japanese people remained calm and waited for help; there was not a mass panic.

Click here to return to the Home page.

• Earth Science

japan earthquake 2011 case study sheet

Related documents.

Add this document to collection(s)

You can add this document to your study collection(s)

Add this document to saved

You can add this document to your saved list

Suggest us how to improve StudyLib

(For complaints, use another form )

Input it if you want to receive answer

• International
• Education Jobs
• Schools directory
• Resources Education Jobs Schools directory News Search

Japan earthquake, 2011 case study

Subject: Geography

Age range: 11-14

Resource type: Worksheet/Activity

Last updated

20 September 2022

• Share through email
• Share through twitter
• Share through linkedin
• Share through facebook
• Share through pinterest

This is a case study on the cause, effect and response to the Japan earthquake and tsunami in 2011. Based on pedagological research, the layout has been designed to appeal to readers, whilst its structure has been chunked for ease of reading.

Tes paid licence How can I reuse this?

Your rating is required to reflect your happiness.

It's good to leave some feedback.

Something went wrong, please try again later.

This resource hasn't been reviewed yet

To ensure quality for our reviews, only customers who have purchased this resource can review it

Report this resource to let us know if it violates our terms and conditions. Our customer service team will review your report and will be in touch.

Not quite what you were looking for? Search by keyword to find the right resource:

The GeoMessenger

A blog to pool knowledge on, and interest in, our planet.

Thank you for visiting my blog

Case study: japan earthquake and tsunami (11th march 2011).

• Twice as big as a magnitude 8.9
• 10 times bigger than a magnitude 8.0
• 1,000 times bigger than a magnitude 6.0
• 1,000,000 times bigger than a magnitude 3.0

• There have been over 11,000 Japan earthquake victims so far (as of 30/03/11), with over 17,000 still missing - many of which will also be dead. The predicted death toll is 18,000 however this is likely to increase. 

• There are currently 244,000 refugees who are seeking shelter in the Japanese earthquake relief camps. With many children still unable to find their parents it is worried that the majority may be 'Japan earthquake orphans'. The question therefore lies in what to do with them, foreigners looking to adopt have been turned down as Japan wants to keep them in their home country and culture. (Contrasting the Haiti orphan crisis) 

• The tsunami caused a near catastrophic meltdown of a nuclear power plant which exploded multiple times and released radioactive material into the air causing dangerous levels in foodstuffs grown within tens of kilometres.

• The water supply in many areas is unsafe to drink due to radioactivity and some of the plant workers risking their lives to cool the plant's core were exposed to massive overdoses and are now in hospital. Japan earthquake relief organisations have been handing out water bottles to survivors.

• Boats were dragged inland and left deserted, houses were ripped from their foundations and scattered among the remains of others, some even stacked on top of each other.

• Devastation.

We don't know the number of victims, but I pray that every single person can be saved.

73 comments:

Nice work Morgan! Mr A

yeah, nice work. thnks for all the information

it was okay , i need more information such as primary and secondary effects.

good work....but you could also state some of the effects of the nuclear plany disaster too

helloooooo people

very helpful and nice information...! i appreciate the case study most...!! gud work..keep it up..!

Thank you everyone for the comments :) Glad it has been interesting and useful!

Very Useful thanks, can use this in my uni work :)

THNK U SO MUCH 4 D INFORMATION!

Thanks for your comments and suggestions and I am glad to reply.

That's okay, I did work hard to make sure there was a lot of facts and information in there, not just pretty pictures :P If any one has suggestions for future posts then feel free to say!

good job....thank you

thanx soooooooo much!!!I got a lot of info. Awesome job........especially the video.Good u got it on this web page.Thanx again.

nice work....thanks a lot.......it helped me a lot in making my sistr's school project...:)

good work itwas helpful

Thanks for all the comments :) appreciate the thanks!

thanks! was really informative..helped a lot in makin the project...sad for the japanese! :(

hey did the earthquake and tsunami occur in the whole japan at once?

It didn't all occur at exactly the same time: the seismic waves that we define as an earthquake take time to travel through the ground, just as water waves (like the wall of the tsunami) take time to travel through the water. So although it was all one main earthquake (excluding the many aftershocks) it was not felt by the whole impacted area at one point in time. It could take 30seconds or more to reach other areas. In fact, this time-lapse is used as an early warning system in Los Angeles where, if an earthquake occurs in one area of the fault, warnings will be sent out to the other areas giving them a short warning (10-40 seconds ..ish) to move lifts to floors to stop people getting trapped, start emergency shut-downs of power stations, cancel plane landings, evacuate buildings and so on. Hope that helped :)

what thsnks????????? i feel like crying for the japs

its very informative......thanx

thanks helped me with geography! really good, feel so sorry for the poor people!:(

very nyc information but.. i need a quote for .. dis tragedy.. can u help me.

There is the one at the bottom by Japan's Emperor Akihito: "We don't know the number of victims, but I pray that every single person can be saved." or "Very few Japanese players in the infrastructure sector are willing to invest time and resources internationally, in the wake of current scenario due to the tsunami and earthquake which struck Japan earlier this year." SOURCE: ECONOMIC TIMES 2011-06-19 "Stable electric supply is indispensable for Japan’s reconstruction from the disaster and its economic recovery." (Referring to the 30+ shut down nuclear plants out of the country's 54) SOURCE: ARKANSAS ONLINE 2011-06-19 "The earthquake, tsunami and the nuclear incident have been the biggest crisis Japan has encountered in the 65 years since the end of World War II. We're under scrutiny on whether we, the Japanese people, can overcome this crisis." Prime Minister Naoto Kan "The supermarkets and convenience stores in and around Tokyo are still bare. Every time a delivery of food arrives, it's gone within an hour. ... I've been to two supermarkets already and there is nothing to be bought." James Stewart, Tokyo resident, e-mail to CNN Hope that helps :)

this is gr8.. it did my enviornmental education project easily!! thanxx..

thanks a lot for the info! it ws too useful! :)

Thnx alot!! The information was extremely helpful to me in my geography project!!!!!! Thn once again!! :)

thanx a lot

amazing thing...got m full marks in my disaster management project

fantabulous & most helpful case study

wow im using this for my EM project thanks for the info

I REALLY APPRECIATE THE CASE STUDY .IT STATES THE INCIDENT VERY CLEARLY.THE INFO IS QUITE WELL STATED.

i am using this casestudy for my E.V.S project .thanks

i express my heart filled condolence for the victims

Thanks a lot for posting. very informative and useful for my case study work.

yeah...i would like to give some suggestions....firstly...the words should be written in the past tense...& information about the place where tsunami approached should be more .. u know......u should elaborate it!

It has been written in the past tense, for example "The Japan earthquake was absolutely devastating" and "people had became desensitised by so many false alarms and assumed tsunami walls could handle it". Some sections, for instance where the death count is being discussed, are in present tense as I wrote this in March, just after the earthquake. This meant that bodies were still being found and the death toll was on the increase. The earthquake hit the east coast of Japan, covering pretty much the whole length of the country but focusing most heavily on areas like Kesennuma City.

well done mate, Thank You Brazil, RJ

I find this very interesting, for the Subduction Zone to be able to make such and unbelievable/ devastating earthquake and tsunami. The thing is will it ever happen again in other boundaries along the Pacific Coast, or even the Atlantic Coast??

Events like this could happen in other subduction zones, have a look online to find a map that shows the subduction areas. The thing is, for an event like this, pressure has to build and build and then be released in one massive event. That means that there cannot be 'fault creep' which is where the two plates gradually move past one another. They must be locked together, unable to move, with pressure building and building until they overcome the resistance. Only in a subduction zone that does not show signs of fault creep could this kind of event occur.

im sexy and i know it

many thanks. very useful. using it to educate others

Alright overview... I have to do one of these for my assignment and this isn't as detailed as I would have liked.. you could have included some info on further prevention and the exact location. But other than that, good job and good format!

This map shows you the exact location: http://news.bbcimg.co.uk/media/images/51638000/gif/_51638295_japan_quake_sendai_464.gif Unfortunately you cannot prevent an earthquake like this occurring, nor the tsunami. You can prevent the loss of life and damage by having higher defences but this was a very rare event and so was not expected at anywhere near this magnitude. Early-warning systems were in place however, as said above, many people didn't listen to them.

i am so thankfull a great job

I'm doing an assignment on this, to do my bibliography i need to know when this was published, could you please tell me?

Sorry for the slow reply. This may not be useful for you any more but for others with the same question: 28/Mar/11

hey could u tell me more about the epicentre and the tectonic plates causing the earthquake and some major headlines if u know which were formed in some major newspapers.and even some write out. i tried it myself but i got really few i thought that u could help me

Thankzz pepulzzzzzzzzz....

qwfqwfffffffffffff

Great information!!! I can use it for my project in the earthquake in Japan Thanks :)

Great information !! :)

Thanks, It helped me much in my geo project

nyc information but u could have provide some more

Thank youh soo much (; helped a lot with my hw

so jusus isssssssss very coming soooooooooonnnnnn........ r u ready

hello it's amy and sophie

PLEASE HAVE OUR APPROVAL JESUS FROM AMY AND SOPHIE XOXOXOXOX

Well done! This is an excellence information for me! Good job! Keep on going yeah? Hehe! :)

Wow your case study is so detailed I like it alot!!!

This comment has been removed by a blog administrator.

nice information ......this case study helped me a lot in my assignment....!!!!! thankyou.

You should have done the effects (Primary + Secondary) and the Responses (Short + Long Term) but other than that it was good!! :)

i don't like it baaaaaaaad

NICE WORK !!!!

This site helps me a lot in presentation. Ipray for the people who are effected with landslides, earthquake etc.

Very helpful as I had to do a write up on this earthquake thanks a million x

• 0 Shopping Cart

Japan Earthquake 2011

Identify the plate margin responsible for the 2011 japanese earthquake., identify the two tectonic plates responsible for the earthquake in japan in 2011., what was the magnitude of the earthquake in japan in 2011, where was the focus of the earthquake, when did the earthquake occur, identify the type of impact: some 15,894 people died, and 6,152 people were injured. 130,927 people were displaced, and 2,562 remain missing., identify the type of impact: over 800 earthquakes of magnitude 4.5 or more were recorded following the main earthquake., identify the type of impact: over 4.4 million households were left without electricity in north-east japan., identify the type of impact: the seabed near the epicentre shifted by 24 m and the seabed off the coast of the miyagi province has moved by 3 m., identify the type of impact: seven reactors at the fukushima nuclear power station experienced a meltdown. levels of radiation were over eight times the normal levels., which of the following are reasons for people living in active zones in japan.

Please select 3 correct answers

Share your Results:

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)
• Click to share on Pinterest (Opens in new window)
• Click to email a link to a friend (Opens in new window)
• Click to share on WhatsApp (Opens in new window)
• Click to print (Opens in new window)

Leave a Reply

Leave a reply cancel reply.

Your email address will not be published. Required fields are marked *

Save my name, email, and website in this browser for the next time I comment.

I agree to the terms and conditions laid out in the Privacy Policy

This site uses Akismet to reduce spam. Learn how your comment data is processed .

Please Support Internet Geography

If you've found the resources on this site useful please consider making a secure donation via PayPal to support the development of the site. The site is self-funded and your support is really appreciated.

Search Internet Geography

Latest Blog Entries

Pin It on Pinterest

Geoelectric studies in earthquake hazard assessment: the case of the Kozlodui nuclear power plant, Bulgaria

• Original Paper
• Open access
• Published: 02 September 2024

Cite this article

You have full access to this open access article

• S. Kovacikova   ORCID: orcid.org/0000-0003-2600-175X 1 ,
• G. Boyadzhiev 2 &
• I. Logvinov 3  

The study presents the results of geoelectric research for seismic risk assessment on the example of the Kozlodui nuclear power plant in Bulgaria. The image of the geoelectric structure in the study area was obtained using one-dimensional inverse electrical resistivity modeling of the full five-component magnetotelluric data and quasi-three-dimensional inverse conductivity modeling of the geomagnetic responses recorded during the summer 2021 field campaign. According to the presented results, the geoelectrically anomalous structure is divided into two levels. The near-surface anomalous structure in the immediate reach of human geotechnical activity corresponds to the electrically conductive sedimentary fill. The mid-crustal layer is coincident with the low seismic velocity zone at the brittle and ductile crust interface, revealed in previous studies. The presented results imply that the geological environment is not affected by large faults capable of transmitting seismic energy from tectonically active areas, however, in further studies, attention should be paid to the strike-slip fault systems adjacent to the study area.

Avoid common mistakes on your manuscript.

1 Introduction

Due to the contrasting electrical properties of the geological environment, geoelectric methods can be used to address a variety of engineering geological tasks related to the natural hazard assessment in karst (Satitpittakul et al. 2013 ) or landslide studies (Lapenna et al. 2003 ), in quarry operation (Magnusson et al. 2010 ), in hydrogeology (Parks et al. 2011 ) or in the construction of critical infrastructure facilities (Di et al. 2020 ).

The construction and operation of civil nuclear installations are governed by strict safety regulations issued by the International Atomic Energy Agency (IAEA). Their neglect or underestimation can lead to tragic consequences. Designing and installing nuclear facilities in tectonically active areas always pose a danger (Nadirov and Rzayev 2017 ; Ahmed et al. 2018 ), but unexpected intraplate seismicity can also be documented in stable ancient terranes (e.g. Chattopadhyay et al. 2020 ) and even apparently historically inactive faults can be potentially risky (Faure Walker 2021 ). Although these strategic facilities are currently equipped with seismic early warning systems (Wieland et al. 2000 ) and spatial displacements are monitored by geodetic networks implementing Global Navigation Satellite (GPS) System data (e.g. Savchyn and Vaskovets 2018 or Manevich et al. 2021 ), integration of other geological and geophysical data is desirable to ensure maximum safety. Seismic events can be accompanied or preceded by a range of phenomena such as isotopes emissions (Sano et al. 2016 ; Zafrir et al. 2020 ) or meteorological phenomena (Morozova 2012 ; Guangmeng and Jie 2013 ). A correlation has been observed between earthquakes and tides (Scholz et al. 2019 ). Prior to an earthquake, electromagnetic (EM) emissions may be recorded around the future epicenter as a result of tectonic forces (Mavrodiev et al. 2015 ; Petraki et al. 2015 ). When assessing seismic risk, however, recording of the natural EM field variations can be used not only for above-mentioned immediate monitoring of earthquake precursors. Due to the enhanced electrical conductivity of mineralized fluids migrating in faults and fracture systems, magnetotelluric (MT) and magnetovariational (MV exploiting only the magnetic EM field components) methods with a depth range covering levels from the earth’s surface through the crust to the mantle are established procedures, for example in geothermal exploration (e.g. Gasperikova et al. 2015 ) or studies of magmatic systems (Wynn et al. 2016 ). Likewise, the contrasting electrical properties of fluids can also be used in seismic risk studies to identify fluid pathways in faults and delineate potential hazard zones. Numerous studies address the topic of the association of low resistivity zones and seismicity with active strike-slip zones (Bourlange et al. 2012 ; Hoskin et al. 2015 ; Adam et al. 2016 ), and different scenarios are presented depending on the zone geometry, mechanical conditions of rocks, degree of deformation, porosity and hydrogeology, with both highly permeable and mechanically locked segments (e.g. Unsworth and Bedrosian 2004 ; Kaya et al. 2009 ; Ritter et al. 2014 ). Water and fluids of both surface meteoric and deep origin, penetrating fault systems, play substantial role in these systems. Shear deformation promotes the formation of interconnected networks for fluid migration, and high-pressure fluids promote fault creep. Creeping segments tend to be subject to frequent microseismicity, while rare strong earthquakes may occur at the transition between creeping and locked zones. Earthquake foci typically trace the boundary between high- and low-resistivity features, corresponding to the stress accumulation and brittle deformation zones (e.g. Convertito et al. 2020 ). Within the conductors themselves the stress is redistributed to meet the equivalent rheology and the fluid hydrodynamics. The measured MT data can thus help identify potentially risky areas and delineate zones of increased seismicity, which is crucial when designing large-scale engineering facilities.

When studying seismic hazard, it is important not only to map surface weakened geological structures with which human geotechnical activities directly interact, but also to track the deep course of faults and trace the deep origin of phenomena observed on the earth’s surface (Suzuki et al. 2000 ). The use of the MT method in solving strategic projects, such as site selection for nuclear power plants, was proposed by Adam and Vero ( 1990 ). Thus, the MT method can expand knowledge about the tectonic structure in the vicinity of objects of interest and provide additional information that can be used in evaluating the measures necessary to increase the safety of strategic facilities. As an example of such a procedure, in this paper we present the results of a case study of the deep geoelectric structure in the area of the Kozlodui nuclear power plant in Bulgaria, initiated by the National Institute of Geophysics, Geodesy and Geography of the Bulgarian Academy of Sciences (NIGGG-BAS) and the National Science Fund to update the National Emergency Prevention Action Plan.

2 Geologic setting and geophysical data

The position of the Balkan region is controlled by the dynamics of the Mediterranean seismic belt, and although the Moesian Plate, as a promontory of the East European Platform, nestled between the Southern and Serbian Carpathians and the Northern Balkans, seems to be a relatively rigid block, it nevertheless participates in the relative movements of the Eurasian, African and Arabian tectonic plates (Stanciu and Ioane 2017 ). As a result, the Moesia and its Danube part is cut by the faults of the Carpathian-Balkan arch trend and by transverse faults into a system of basement blocks and has a complex deformation behavior with neotectonic activity along a number of fault structures.

Kozlodui nuclear power plant (KNPP) is located in the southwestern, seismically least active part of the Moesian Plate (Fig.  1 a). However, the relative proximity of continuously tectonically active fault systems may carry the risk of noticeable earth movements. About 300 km northeast (see inset in Fig.  1 a) is located persistently highly geodynamically active Vrancea area with four-to-five medium depth events with magnitudes M ≥ 6.5 per century and the largest recorded shock of 7.9 (e.g. Petrescu et al. 2021 ). From the west, the Moesian Plate is bounded by a continuously tectonically active fault system (M reaching 4), including the Timok and Cerna faults (TF-CF, inset in Fig.  1 a), linking the Carpathians with the Balkanides (Bala et al. 2015 ; Vangelov et al. 2016 ; Mladenovic et al. 2019 ; Krstekanic et al. 2021 ; Oros et al. 2021 ).

Geological setting— a The major tectonic zones of Bulgaria with the position of the Moesian Plate and Vrancea seismicity zone in the incut (from Cavazza et al. 2004 ): TF-CF—Timok-Cerna fault zone, KNPP—Kozlodui nuclear power plant, PAG—Panagjurishte geomagnetic observatory, green rectangle—study area; b Simplified tectonic map of the northwestern Bulgaria (modified after Cavazza et al. 2004 ; Kounov et al. 2017 ) with red crosses of experimental site network—MV (small) and full MT (big); faults (cross-hatched belts): Blk-Sub-Balkan, NFB-Northern Forebalkan, Vlm-Vinishte-Lom (Gostilski), Tsb—Tsibritsa, Ogs—Ogosta, Isk—Iskar, SMs – South Moesian, Dnb—Danube, Mtr—Motru, Jiu—Jiu fault (Dachev and Kornea 1980 ; Georgiev and Shanov 1991 ), thick dashed line—southern border of the Moesian Plate, magenta line—electrified railway (Bulgarian State Railways: https://www.bdz.bg ), M-BS—Makresh-Black Sea seismic profile (Dachev 1988 )

Several structural complexes can be identified within the Moesian Plate. Precambrian metamorphic rocks and Upper Paleozoic (Carboniferous to Permian) formations are covered by Triassic to Cenozoic sediments. On the Bulgarian territory, two large tectonic structures are distinguished on the Moesian Plate, the Lom depression, where the KNPP is located, and the North-Bulgarian uplift. East and north-east of the Lom depression, mainly on the Romanian territory, the Alexandria depression is delimited (Chemberski and Botoucharov 2013 ). Based on geophysical data (Dachev and Kornea 1980 ; Dachev et al. 1994 ), the total thickness of sediments of the Lom and Alexandria depressions reaches about 9 km (Fig.  2 a). The thickness of the Cenozoic sediments of the Lom depression reaches 1000 m (Fig.  2 b) (Zagorchev 2009 ). The Lom depression basement is formed by lowest tectonic blocks nested among the Danube fault in the north, Northern Forebalkan in the south and Vinischte-Lom and Iskar faults in the west and east respectively, separated from each other by the Ogosta and Tsibritsa faults (Fig.  1 b). The Vinishte-Lom fault is a strike-slip feature of the larger Oltenia tectonic zone cutting across a series of tectonic structures in the central part of the Balkan peninsula (Bala et al. 2015 ).

a Depth contours of the consolidated basement in km (dashed lines); b Contours (in m) of the top of the Upper Cretaceous complex (Zagorchev 2009 ); c Schematic section of sedimentary rock resistivity in Northern Bulgaria (solid line) and the Balkan region (dotted line) (Dobrev et al. 1975 ); d Schematic S sed map of sedimentary rocks (dashed contours in Siemens) of the KNPP area (red star in all subfigures) according to Abramova et al. ( 1994 ) (private comm.); Green line—Lom depression boundary

2.1 Geoelectric characteristics

According to the results of laboratory and geoelectric field experiments (Hermance 1995 ; Haak and Hutton 1986 ; Nover 2005 and others), the electrical resistivity (ρ) of crystalline rocks of the continental crust significantly exceeds 1000 Ω∙m. Below is information on the lithological composition of sedimentary rocks and their resistivity according to Dobrev et al. ( 1975 ).

The geological section of Northern Bulgaria is characterized by a wide distribution of two types of red-bed strata: Permian–Triassic (compacted clay rocks, conglomerates, breccia conglomerates, sandstones – with ρ varying from 16 to 45 Ω∙m) and Triassic-Jurassic. Middle Triassic oil and gas bearing limestones and dolomites up to 650 m thick are characterized by ρ varying between 100 and 400 Ω m. A thick Malm-Valanginian (late Jurassic-early Kretaceous) complex with ρ varying from 130 to 3600 Ω m appears in the Moesian section. Cretaceous and Pliocene carbonate facies of the Lom depression are characterized by ρ of 40–250 Ω∙m. Regional features of the ρ distribution of sedimentary rocks according to logging data are shown in (Fig.  2 c). Similar values are also given in other publications (Nikolova 1980 ; Dachev 1988 ; Chemberski and Botoucharov 2013 ).

The most characteristic geoelectric parameter of the sedimentary cover is the integrated longitudinal conductivity (conductance) S sed  = D/ρ, where D is the layer thickness. Based on geological-geophysical and well logging data, L.M. Abramova (2013 personal communication), the initiator of previous deep EM studies in Bulgaria (Abramova et al. 1994 ), has compiled a schematic S sed map of the Balkanides and the Moesian Plate in Bulgaria. This map has been updated with new information on the thickness, composition, and geoelectric parameters of the Cenozoic sediments of the Lom depression, obtained as a result of the interpretation of MTS (MT sounding) curves (Logvinov et al. 2021 ). Using a similar method, a schematic S sed map was constructed for the Romanian territory (Demetrescu 2013 ). According to rough estimates (based on data on the thickness of sediments and their ρ), S sed of surface sediments overlying the crystalline basement rocks on the territory of the Balkanides does not exceed 50 Siemens. Figure  2 d shows the S sed map for the south of the Moesian Plate and the adjacent part of the Balkans.

2.2 Seismic results and seismicity

The study area is intersected by the quasi-latitudinal regional seismic Makresh-Black Sea profile (Figs. 1 , 3 ). From west to east along the profile, the thickness of sediments of all ages decreases. P-wave Seismic velocities for terrigenous and terrigenous-carbonate sedimentary formations of the Moesian Plate along the M-BS profile (Fig.  3 a) vary from 2 to 4.5 km/s (Dachev 1988 ). Lower velocities are typical for Cenozoic sediments.

a Structure of the earth's crust along the Makresh-Black Sea (M-BS) seismic profile (Dachev 1988 ; Dachev et al. 1994 ). 1—sedimentary layer and seismic boundaries in it, 2—the Moesian Plate basement (numbers—seismic velocities, km/s), 3—Moho boundary, 4—supposed crustal zones of reduced seismic velocity. b , c Seismicity of the KNPP region for the period of years 1973–2020 (see sources in the text); fault zones Tsb, Dnb, SMs, Isk, Mtr, Jiu, NFB—see Fig.  1 b: b earthquake hypocenters by depth (in km); c) earthquakes by magnitude (thick circles with a numeral—strongest events with M > 3); crosses—unknown magnitude. Differences in the distribution of earthquake foci in subfigures ( b ) and ( c ) are given by the absence of depths/magnitudes for some events in the catalogues mainly before 2007

According to the seismic logging results, the seismic velocity of Paleogene-Neogene terrigenous deposits does not exceed 3.2 km/s (Volvovsky and Starostenko 1996 ). Both in the upper and lower consolidated earth's crust, low-velocity layers are distinguished along the profile (Fig.  3 a). The depth to the upper boundary of these layers is about 15 km and 27–30 km, their thickness is about 5 km and the seismic wave velocity decrease with respect to the surrounding environment is 0.5–0.7 km/s (Dachev et al. 1994 ).

Earthquakes are one of the most disastrous natural phenomena, the impact of which must be taken into account in the operation of nuclear facilities. Over the past 50 years, more than 15,000 earthquakes have been registered in Bulgaria, including the area belonging to the Moesian Plate, and some 750 events have been documented in the vicinity of the KNPP ( http://crustal.usgs.gov/geophysics/htm ; http://www.isc.ac.uk/iscbulletin/search/catalogue ; http://www.emsc-csem.org/Earthquake ; http://service.iris.edu/irisws/fedcatalog/1/ ; https:// earthquake.usgs.gov/earthquakes/search/; https://doi.org/10.7914/SN/BS ). The strongest events in the area around the KNPP occurred in 1987 in the northwestern tip of the Lom depression at the Tsibritsa, Motru and Danube faults tectonic knot at a depth of 10 km with a magnitude 3.3; another with a magnitude 3.7, and apparently related to the contact of the Danube fault with the branch of the Jiu fault, occurred in 1994 at the depth of 10 km, and another, with a magnitude of 4.4, took place in 2014 northeast of the study area in Romania at a depth of 12.1 km, near the junction of the Iskar, Danube and Jiu faults. The distribution of the events closest to the KNPP by depths and magnitudes is shown in Fig.  3 b and c respectively. For some events (specifically before 2007), depths or magnitudes are not specified in the catalogs cited above (hence the differences in the Figs.  3 b and c). It can be seen that a significant number of events occur south of the KNPP within a radius of 50 km (mainly already in the Pre-Balkans). The earthquakes seem to be linked to the intercrossing of the Ogosta, Tsibritsa and Iskar faults with the Northern Forebalkan fault (Fig.  3 b, c). The last represents an element of the Balkan fold-thrust belt, a complex system thrust onto Moesia from the south and dissected by transverse and oblique faults along which lateral displacements occur. Along the Ogosta fault with its hanging NW flank, there is a step-like dip towards the west. According to Georgiev and Shanov ( 1991 ), the block between Tsibritsa and Ogosta faults is still subsiding and the seismic activity of the Ogosta, Tsibritsa and Iskar faults is likely associated with the relative subsidence of the blocks between them. In recent years, several earthquakes with a magnitude exceeding 2 have been observed to the north and west of the KNNP. Tsibritsa fault with its hanging western flank is considered a satellite of the Motru fault, stretching north of the Danube in Romanian territory, which in turn is genetically connected to the Timok–Cerna fault system linking the Carpathians with the Balkanides (see Geological setting). Motru fault is also one of sources of seismic activity in the study area. It is deep-rooted, in the northwest, on Romanian territory, it is noticeably seismically active, and both left-lateral translation and descending movements occur along it. Some events seem to be related to the contact of the Danube fault with the Jiu fault–another active fault running on the Romanian territory from the Southern Carpathians in the NW–SE direction.

3 MT experimental data and inversion results

Geoelectric measurements were performed in the summer of 2021 using two GEOMAG-2 fluxgate magnetometers owned by the Institute of Geophysics of the National Academy of Sciences of Ukraine and the Institute of Mathematics and Informatics of the Bulgarian Academy of Sciences, ensuring registration of variations of MT field components with high sensitivity threshold (Dobrodnyak et al. 2014 ). The studies belong to the category of regional experiments, the purpose of which is to identify possible conductivity anomalies in the KNPP region. MT field observations were carried out at 21 points (Fig.  1 b). The distance between observation points was 10–15 km. The density and selection of locations for the installation of observation points were limited by local infrastructure and agricultural conditions.

EM field records in the study area were affected by significant disturbances associated with the proximity of electrified railroads, pipelines, power lines and other installations. Typically, interferences from these sources can have a significant impact at distances of up to 15–20 km. Figure  1 b shows the position of the KNPP and the nearest electrified railway, the presence of which automatically limited the area of the experiment. Interference on the magnetic components of the MT field decreases in proportion to the cube of the distance from the interference source. Taking into account the above, it was decided to register the magnetic components at the closest possible distance from the KNPP.

A detailed description of the processing of the recorded data and the distortion and dimensionality analysis were presented in Logvinov et al. ( 2021 ). Data processing was performed using Ladanivsky ( 2003 ) and Varentsov ( 2007 ) codes. The first phase of the geoelectric study was completed by estimating the parameters of impedance (Z) and the vertical magnetic transfer functions (VMTF) within the single-site processing scheme. Conditioned registration of the EM field electrical components was performed at four sites (Btn, Frn, Brv, Brn, see Fig.  1 b) and as a result, Z estimates (and derived apparent resistivity and impedance phase) were obtained for periods from 20 to 6400–8100 s. Meanwhile, the VMTF parameters were estimated at all observation points in the form of real (C u ) and imaginary (C v ) induction vectors (Schmucker 1970 ), presented on maps in the form of induction arrows, for the periods from 10–20 to 4900–10800 s.

3.1 1D inversion of MTS data

The nearby electrified railway and the measuring sites layout limited the data interpretation. Therefore, the first step was to estimate the geoelectric section parameters at the sites Btn, Frn, Brv, Brn by one-dimensional (1D) inversion of the interpreted MTS curves over the entire recorded period range. The results of 1D interpretation using two different inversion codes were also presented in Logvinov et al. ( 2021 ). The D + algorithm (Parker and Whaler 1981 ) approximates the geoelectric section through a finite number of layers of zero thickness and finite conductance isolated by a non-conductive medium, while the OCCAM 1D inversion (Constable et al. 1987 ) results in a section with smoothly varying conductivity. The minimum and maximum MTS curves obtained using the Eggers ( 1982 ) method were taken as experimental in the period range from a few seconds to 10 4  s. Before applying the inversion procedure, the MTS curves had to be normalized to eliminate galvanic effects on the MT field. Galvanic distortions arise as a result of the interaction of near-surface geoelectric heterogeneities and lead to a static shift of MTS amplitude curves (Berdichevsky and Dmitriev 2008 ). The normalization consisted in restoring the position of low-frequency asymptotes reflecting the electrical conductivity of the lower levels of the tectonosphere. It is assumed that at depths exceeding 400 km, horizontal changes in electrical conductivity are small, and the MTS curves obtained in different regions should converge at periods exceeding 3 h. In practice, the normalization of MTS curves usually consists in shifting the low-frequency branches of the MTS amplitude curves (ρ curves) along the vertical axis so that they match the ρ curve corresponding to the regional geoelectric structure of the study region (if the MTS phase curves agree with the reference curve). For the study area, data from the Panagjurishte geomagnetic observatory (PAG, 24.177°E, 42.515°N, Fig.  1 a) for the years 1988–2015 were used as a reference curve (Srebrov et al. 2013 ; Ladanivskyy et al. 2019 ). For 1D inversion, the recorded MTS curves were integrated with the reference curve at periods 2⋅10 4 –2⋅10 7  s (Fig.  4 ).

Minimum and maximum experimental (circles) and model MTS curves at the sites Frn, Brv, Btn, Brn for two azimuths integrated at the period of 2·10 4  s with the reference data from the PAG observatory using D+ , OCCAM (from Logvinov et al. 2021 ) and 1D anisotropic inversion codes (Pek and Santos 2006 )

The 1D interpretation of the MT data already presented in Logvinov et al. ( 2021 ) was newly supplemented by a 1D anisotropic inversion using all components of the impedance tensor (Pek and Santos 2006 ). It should be noted here that the technique does not mean searching for real physical anisotropy in the earth and is used purely to apply the equivalent of a 1D anisotropic layered medium to the MTS curves in two directions at each site. To accommodate all impedance tensor components, the anisotropic inversion error floor in the anisotropic inversion was preset to 5%. Anomalous layers (conductors) with ρ much smaller than those lying above and below are identified on the inverse 1D models (Fig.  5 a). The differences in the distribution of geoelectric parameters calculated by OCCAM and anisotropic inversions are mainly due to the fact that in the OCCAM method, the experimental MTS curves were corrected to take into account galvanic distortion. Low resistivities of sedimentary rocks according to both inversion methods appear at depths of less than 1 km. According to the results of the anisotropic inversion, a low resistivity feature (ρ of about 10 ohmm) is identified at the Brv site at depths of about 4 km.

a Geoelectric resistivity sections according to 1D models calculated using Occam inversion procedure (from Logvinov et al. 2021 ) and 1D anisotropic inversion by Pek and Santos ( 2006 ). 1—supposed zones of reduced seismic velocity along the M-BS seismic profile (Figs. 1 , 3 ). b Earth’s crust structure along the corresponding segment of the M-BS seismic profile (see Fig.  3 a), stars—seismic events within a distance of 10 km from the sites Frn, Btn, Brv and Brn ( a ) and from the M-BS seismic profile ( b )

The resistivity of the rocks underlying the sediments exceeds 100 ohmm. In both inverse models, at Frn, Brv and Brn, a conductor with a resistivity of 10 ohmm is distinguished at depths of 20 (+/− 5) km. The most distorted records of MT field variations were obtained at Btn, which was caused by the proximity of the high-voltage power line and affected the interpretation parameters and the inversion results. Comparison of the obtained geoelectric 1D models with the seismic section along the M-BS profile (Fig.  5 b) shows the coincidence of conductors with low-velocity layers in the depth interval 15–20 km.

3.2 Quasi-3D inversion of MV data

The next step in the interpretation of the 2021 geoelectric survey data was the modeling of the conductance S distribution of the sedimentary cover and the earth's crust using a quasi-3D inverse technique based on the Price thin-sheet approach and data fitting using Tikhonov parametric functional minimization with conjugate gradient optimization and the maximum smoothness stabilizing (Kováčiková et al. 2005 ). The purpose of the quasi-3D inversion application was: (1) to determine the spatial position of anomalous features in the studied area and explain the behavior of MV parameters; (2) to compare the obtained results with other geological and geophysical data.

The thin-sheet method involves only the magnetic MT field components. VMTF data from 21 stations over the entire period range of 50–2500 s were used in the inversion. The study area (90 km × 90 km) was divided into tiles with a side of 6 km × 6 km. The cell size was chosen with respect to the applied periods and the distance between observation points. The vertical conductivity distribution in the quasi-3D model was represented by a 1D layered section (Fig.  6 a–c) selected taking into account previous geophysical and geological data, geoelectrical characteristics of the sedimentary cover (see the previous divisions) and an earlier MT survey in the Bulgarian territory by Srebrov et al. ( 2013 ). Analysis of equivalent current systems at different depths commonly used in thin-sheet modeling (Banks 1979 ) did not provide the expected depth estimate of the upper level of the crustal anomaly source due to shielding by conductive surface sediments filling the Lom depression. The smooth pattern of the current function distribution becomes unstable and breaks down at a depth of 4 km as an effect of the continuation of the field below the upper boundary of the source, in this case represented by the conductive sediments of the Lom depression (see supplementary material). Therefore, the depth of the upper boundary of the crustal anomaly was taken from the 1D inversion results, which assumed the most conductive crustal objects in the depth interval of about 15–20 km (Fig.  5 ). The initial thin-sheet model for the iterative inversion procedure was represented by a homogeneous sheet with a uniform normal conductance distribution, located at a fixed depth.

Results of the quasi-3D inversion—distribution of the conductance S (Siemens) in the thin sheet with corresponding input 1D sections: a thin sheet at the surface and recorded real and imaginary induction arrows for the period of 50 s; b thin sheet at the depth of 15 km and real and imaginary experimental induction arrows for the period of 2500 s; c two sheet model with the surface sheet (subfigure a ) and a crustal sheet at 15 km and real experimental and model induction arrows for the period of 2500 s; d experimental and model imaginary induction arrows for the same model as in the subfigure c . Faults (cross-hatched belts and other details as in Fig.  1 b; L—Lom depression, M—Moesian Plate, B—Balkans (Fore-Balkan)

Generally, the validity of the thin sheet approach is limited from below at short periods by near-surface disturbances and from above at long periods by source effects. Although given the geoelectric conditions in the Lom depression, the penetration depth at the shortest periods 50 and 100 s should allow reaching 20 and 30 km respectively, a series of inversions of geomagnetic responses at different depths at these periods showed that the best fit of the model geomagnetic responses and the experimental ones was achieved when the conductive thin sheet was placed on the surface, i.e. the resulting conductivity models reflect mainly the distribution of subsurface conductive sediments. The surface sheet substituted a sedimentary layer with an average depth of 4 km and a conductivity of 0.025 S/m (Fig.  6 a). Starting with the period of 900 s, the geoelectric image of crustal depths predominates in the conductivity models. This is accompanied by the reversal of the imaginary induction arrows pointing at short periods (50, 100 s) in the direction corresponding (or close) to the real arrows to the opposite orientation (Fig.  6 a, b). To depict the distribution of conductivity in the earth's crust, in inversions at periods of 900, 1600 and 2500 s, a thin sheet was placed at a depth of 15 km. However, the resulting conductivity model seemed to be influenced by the sub-surface sediments (Fig.  6 b). Therefore, to separate the effect of conductive sediments and the crustal anomaly source, a two-sheet model was chosen in the inversion of the VMTF’s at the periods 900, 1600 and 2500 s. The first layer with a thickness of 4 km corresponding to the average thickness of sediments of the Lom depression (Fig.  2 c) was substituted by a surface thin sheet with a fixed conductance derived from the single-sheet inversion at the period of 50 s (Fig.  6 a). The second sheet was immersed at a depth of 15 km (Fig.  6 c, d).

Modelling experiments to select the normal conductance at the thin sheet edges showed that the best data fit was obtained with a value of 100 S for the surface sheet simulating the sedimentary cover. The most satisfactory normal conductance for the crustal sheet was 1000 S (Fig.  6 a–c). In the inversion, the data weight multiplying the parametric functional (squared during the procedure) was uniform – 0.01, selected taking into account amplitudes of the recorded magnetic transfer functions (maximum 0.3). Starting with the normal conductance in the thin sheet (or two sheets), the inversion procedure converged typically after 20–35 iterations and finished reaching the data weight value between two iterations. Specifically, the presented surface model at a period of 50 s (Fig.  6 a) converged after 32 iterations, the one-sheet crustal model at 2500 s (Fig.  6 b) stopped after 24 iterations, the two-sheet model (Fig.  6 c) converged after 29 iterations. Data fit for the final two-sheet model is shown in Fig.  6 c, d.

4 Discussion

On Fig.  7 , the correlation of conductivity both in near-surface sediments and at mid-crustal depths (sub-figures a and b respectively) with seismicity is imaged. As was mentioned before in the Introduction, earthquake foci at both subfigures appear outside or at the margins of the conductors (both horizontally and vertically).

Comparison of S near the surface ( a ) and at a depth of 15 km ( b ) (from Fig.  6 a and c respectively) and seismic events above (dots) and below (crosses with focal depths) the mid-crustal conductive layer. Faults (cross-hatched belts) and other details as in Fig.  1 b and Fig.  6 ; L—Lom depression, M—Moesian Plate, B—Balkans (Fore-Balkan)

According to the quasi-3D inversion results, near-surface anomalous conductivity distribution in the study area appears to be controlled by electrically conductive sediments of the Lom depression (Fig.  6 a). One anomaly close to the junction of the Iskar and Northern Forebalkan fault appears in the area of distribution of the Pleistocene loam complexes (Angelova 2001 ). Two anomalies west and east of the river Ogosta seem to correspond to areas of Neogene (Pliocene) clays distribution (Angelova 2008 ). The anomalous conductivity area at the intersection of the Danube Tsibritsa and Motru faults (and in the confluence of the Danube and Tsibritsa rivers) may be related to the intrusion of highly mineralized water from a deeper aquifer (Toteva and Shanov 2021 ).

At mid-crustal depths (Figs. 6 c, 7 ), the basement of the most subsiding block is non-conductive, separated by the Ogs, Tsb and NFB faults from the more conductive surroundings. An anomalous electrical conductivity structure appears at the intersections of the Ogosta fault with the Danube and South Moesian faults. Moderate seismic activity and recent vertical movements have been documented on the Ogosta fault (Georgiev and Shanov 1991 ; Angelova 2008 ), however, the hypocenters are concentrated at its intersection with the Northern Forebalkan fault and mostly south of the latter, while the central part of the conductive feature itself remains unaffected by seismic events (Fig.  7 ). Most of the hypocenters are located above the depth of the mentioned conductor (black dots in Fig.  7 ). The entire western and southwestern margin of the study area is also significantly electrically conductive. This electrically anomalous area is located west of the Tsibritsa fault and southwest of the Northern Forebalkan fault, which delimits the area that already belongs to the Balkanides (Fore Balkans) from the north. The highly conductive area may be associated with the effect of the significantly strike–slip TF-CF zone west of the study area (see Fig.  1 a). It may also represent the deep source of the near-surface anomaly at the intersection of the Danube and Tsibrica faults in the above-mentioned area of occurrence of the mineralized water spring.

Previous 1D inverse models at 4 points indicated the existence of the low resistivity objects in the depth interval of 15–25 km (Fig.  5 a). This corresponds to the crustal conductive feature identified by quasi-3D modeling in the area around the Btn site; Frn and Brv are located at the edge of this conductor, while Brn is located outside the conductive area. Also, the results of the anisotropic 1D inversion do not indicate the existence of a significant decrease in electrical resistivity at crustal depths. The results of both methods also point to the existence of near-surface low-resistivity/conductive layers around the Btn and Brv points.

An anomalous conductivity mid-crustal layer with an upper boundary at approximately 15 km resulting from the thin-sheet inversion also correlates with the seismic low-velocity layer revealed by Dachev et al. ( 1994 ) (Figs. 3 a, 5 ). Earthquake foci within a 10 km radius around the Brn, Frn, Brv, Btn sites (Fig.  7 ) were also superimposed on their 1D resistivity depth distribution (Fig.  5 a). Similarly, events occurring within 10 km to both sides of the M-BS seismic profile were shown (Figs. 5 b, 7 ). From the presented sample, it can be seen that most of the events took place at shallow depths above the low-velocity layers (and none at their depths). Mid-crustal reflective low-seismic-velocity layers with an upper boundary at a depth of about 15 km were described by Gutenberg ( 1954 ) and further reported in various studies and various regions (e.g. Zorin et al. 2002 ; Zhan et al. 2020 ). In seismically active regions, low-velocity zones associated with the presence of partial melt, residual magma, heat escaping from the mantle, or frictional heating at fault zones exposed to shear between contact blocks act as waveguides channeling seismic waves during earthquakes (e.g. Zhao et al. 2000 ; Qin et al. 2018 ; Nagar et al. 2021 ). However, low velocity layers are also widespread in stable cold crust regions, where they cannot be explained by increased heat and the presence of melt. Low velocity layers can often correlate in space with electrically conductive layers (Eaton 1980 ; Vanyan et al. 2001 ) and their mutual mechanism can be interpreted as a consequence of rheological stratification and processes at the brittle/ductile crust transition, influencing the increase in porosity, geometry of pore spaces, the amount of pore fluids, their salinity and consequently controlling both elasticity and electrical conductivity of rocks (Gough 1986 ; Marquis and Hyndman 1992 ; Unsworth and Rondenay 2013 ), although graphitization along fission planes due to ductile shear is also mentioned as an alternative mechanism of increased conductivity (Simpson 1999 ; Glover and Adam 2008 ). The fluid origin is most likely associated with dehydration (Jones 1992 ), while the most reasonable explanation for the increase in porosity itself is, according to Pavlenkova ( 2004 ), the dilatancy phenomenon, again associated with the influx of hydrous fluids. The low velocity/high conductivity layers are thought to act as detachment zones, separating weak and brittle parts of the crust on which most faults, except deep fault zones cutting the whole crust, flatten. Episodic seismic events occur above such interfaces or at their periphery.

The presented results support the outputs of other available studies (Antonov 2000 ; Groudev and Petrova 2017 ) concerning the geodetic monitoring, stress tests and natural hazard assessment for the KNPP operation, which state the stability and safety of the geological environment in the study area. According to regional GPS studies by Kotzev et al. ( 2001 ) overall kinematic pattern shows that the only tectonically active structures in northern Bulgaria lie east of the domain hosting the Lom basin. The western and southern boundaries of the domain are characterized by N-S to NE-SW extension. In the northwest, a system of NE-trending faults (the Vinishte-Lom fault in the study area, Fig.  1 b) shows left-lateral movement. The eastern and southeastern boundaries of the domain (along the Yantra river east of the study area, Fig.  3 a) is not distinct, however, with moderate right-lateral strike-slip and NE-SW compression. As already mentioned in Sect.  2.2 Seismic results and seismicity, the northern boundary is formed by the Danube dip-slip fault with a recently uplifted (in response to extension) northern block. Geodetic survey by Valev et al. ( 2016 ) focused on the area around the KNPP registered only weak and slow deformation of a variable character in the KNPP area. Also, results of DInSAR studies (Differential Synthetic Aperture Radar Interferometry) by Drakatou et al. ( 2015 ) reported the stability of the region with a negligible rate of deformation ranging between − 1 and + 1.5 mm/year. Seismic surveys by the Common Depth Point (CDP) and refraction methods for the exploration of coal-bearing horizons (Yaneva and Shanov 2015 ) prove the uniformity of the tectonic regime from the end of the Dacian (Pliocene) period to the present. The oil and gas prospecting seismic studies (Toteva and Shanov 2021 ) have noted the deep Tsibritsa fault topographically predetermining the eponymous Danube tributary, however, they do not address the question of whether the fault is active.

Although the presented MT survey results are consistent with the mentioned above studies that suggest no special measures for the KNPP safety, further research should be directed towards the creation of a complete 3-D image of the study area using 3D-inversion procedures, which would depict the vertical geoelectrical structure in more detail. These would require a set of broadband fully 5-component MT measurements with reference measurements and inter-station processing, with the presented results serving as a-priori input information. The dataset should be also supplemented with MT and GDS results from the adjacent Romanian part of the Moesian platform in the north (Stanica and Stanica 2011 ) and with completely missing data from the Serbian territory, from the arched belt of faults, namely the TF-CF strike-slip system, bending The Moesian plate from the west.

5 Conclusion

Although the main focus of engineering geology is in the area within the reach of human activity and its interaction with earth processes, it should not be limited to the earth's surface, since deep tectonic processes can significantly affect any engineering and geotechnical work. The results of a case study of a geological structure in the area of the Kozlodui nuclear power plant in Bulgaria showed how the analysis of geoelectric features can complement the complex of geological and geophysical information for seismic hazard assessment.

1D inverse resistivity modeling based on MT data recorded during the summer 2021 field experiment indicated the existence of mid-crustal low-resistivity features coincident with a seismic low-velocity layer revealed by a previous regional seismic survey. The subsequent quasi-3D inversion provided insight into the sub-surface sedimentary structure as well as the electrical conductivity distribution in the mid-crust. An electrically anomalous feature with an upper boundry at a depth of about 15 km appears at the intersection of the Ogosta with South Moesian and Danube faults. The conductive western and southwestern margin of the investigated area is probably related to the strike-slip fault systems bounding the Moesian Plate from the west. The mid-crustal high electrical conductivity and low seismic velocity layer is assumed to correspond to the transition zone between the brittle and ductile crust. Seismic events may occur at its outer boundary, however, no large fault structures with the potential to transfer seismic energy from tectonically active areas were revealed in the study area. The presented results support the conclusions of previous seismic hazard studies and confirm that the Kozlodui nuclear power plant is located in an area with a stable geological environment, however, in further research, the results of studies covering the fault system linking the Carpathians with the Balkanides west of the studied area should be included.

Adam A, Vero J (1990) Application of the telluric and magnetotelluric methods in selection of sites for nuclear plants. Proc Indian Acad Sci (earth Planet Sci) 99(4):657–667

Article   Google Scholar  

Adam A, Szarka L, Novak A, Wesztergom V (2016) Key results on deep electrical conductivity anomalies in the Pannonian Basin (PB) and their geodynamic aspects. Acta Geod Geophys 52(2):205–228. https://doi.org/10.1007/s40328-016-0192-2

Abramova AM, Varentsov IM, Velev A, Gavrilov R, Golubev NG, Zhdanov MS, Martanus ER, Sokolova EYu, Schneier VS (1994) Investigation of deep geoelectric structure of Bulgaria. Phys Earth 11:59–69 (in Russian)

Google Scholar  

Ahmed N, Ghazi S, Sami J (2018) Seismicity assessment of Fukushima region, fault kinematics and calculation of PGA value for Idosawa fault in Hamadori area, Japan. Nat Hazards 92:1065–1079. https://doi.org/10.1007/s11069-018-3240-0

Angelova D (2001) Quaternary geology, geomorphology and tectonics in the Iskar River valley system, the Danubian Plain (Bulgaria). Bull Geol Soc Greece 34(1):55–60. https://doi.org/10.12681/bgsg.16943

Angelova D (2008) Integral Environmental Assessment of Ogosta River Basin, (Northwestern Bulgaria). BALWOIS 2008–196:1–15

Antonov D (2000) “Kozloduy” NPP geological environment as a barrier against radionuclide migration. Transactions 32/14 International Youth Nuclear Congress 2000: Yuth, Future, Nuclear; Envirohmeint & Safety/87, SK01K0039

Bala A, Raileanu V, Dinu C, Diaconescu M (2015) Crustal seismicity and active fault systems in Romania. Rom Rep Phys 67(3):1176–1191

Banks RJ (1979) The use of the equivalent current systems in the interpretation of the geomagnetic deep sounding data. Geophys JR Astr Soc 87:139–157

Berdichevsky MN, Dmitriev VI (2008) Models and methods in magnetotellurics. Springer-Verlag, Berlin Heidelberg. https://doi.org/10.1007/978-3-540-77814-1

Book   Google Scholar  

Bourlange S, Mekkawi M, Conin M, Schnegg P-A (2012) Magnetotelluric study of the Remiremont-Epinal-Rambervillers zone of migrating seismicity, Vosges (France). Bulletin De La Société Géologique De France 183(5):461–470

Cavazza W, Roure F, Spackman W, Stampfli G, Ziegler P (eds) (2004) The mediterranean region from crust to mantle. The Transmed Atlas. Geological and Geophysical Framework of the Mediterranean and the Surrounding Areas. In: A publication of the Mediterranean Consortium for the 32nd International Geological Congress, Florence, Italy

Chemberski HI, Botoucharov ND (2013) Triassic Lithostratigraphic Correlation in the Moesian Platform (Bulgaria–Romania). Stratigr Geol Correl 21(6):609–627. https://doi.org/10.1134/S0869593813060087

Chattopadhyay A, Bhattacharjee D, Srivastava S (2020) Neotectonic fault movement and intraplate seismicity in the central Indian shield: a review and reappraisal. J Mineral Petrol Sci J-STAGE Adv Publ 115(2):136–149. https://doi.org/10.2465/jmps.190824b

Constable SC, Parker RL, Constable CG (1987) Occam’s inversion: a practical algorithm for the inversion of electromagnetic data. Geophysics 52:289–300

Convertito V, De Matteis R, Improta L, Pino NA (2020) Fluid-triggered aftershocks in an anisotropic hydraulic conductivity geological complex: the case of the 2016 Amatrice Sequence, Italy. Front Earth Sci 8:541323. https://doi.org/10.3389/feart.2020.541323

Dachev H (1988) Structure of the Earth Crust in Bulgaria. Technique 334 (in Bulgarian with a Summary in English)

Dachev H, Bokov P, Radulesku F, Demetresku K, Lazaresku V, Polonik G (1994) Moesian Plate. In: Chekunov AV (ed) Lithosphere of Central and Eastern Europe: Young Platform and Alpine Fold Belt. Naukova Dumka, Kiev, pp 197–198 (in Russian)

Dachev H, Kornea I (1980) Moesian Platform. In: Sologub VB, Guterch A, Prosen D (eds) The structure of the crust of Central and Eastern Europe according to geophysical studies. Naukova Dumka, Kiev, pp 59–68 (in Russian)

Demetrescu C (Project Director) (2013) Scientific Report. The project “The geomagnetic field under the heliospheric forcing. Determination of the internal structure of the Earth and evaluation of the geophysical hazard produced by solar eruptive phenomena”.Program IDEI. Contract 93/5.10.2011, Stage I-III. Institute of Geodynamics Romanian Academy. http://www.geodin.ro/IDEI2011/engl/index.html

Di Q, Fu Ch, An Zh, Wang R, Wang G, Wang M, Qi Sh, Liang P (2020) An application of CSAMT for detecting weak geological structures near the deeply buried long tunnel of the Shijiazhuang-Taiyuan passenger railway line in the Taihang Mountains. Eng Geol 268:105517. https://doi.org/10.1016/j.enggeo.2020.105517

Dobrev TB, Ivanova VP, Pishalov SS (1975) Regional characteristics of physical properties of main rock complexes in Bulgaria. Geophysical collection Institute of geophysics of Acad Sci Ukraine 52 (in Russian)

Dobrodnyak L, Logvinov I, Nakalov E, Rakhlin L, Timoshin S (2014) Application of magneto-telluric stations (Geomag-02) in geoelectric studies on the territory of Bulgaria. Seminar proceedings 3, 2013 INRNE-BAS, Sofia, Bulgaria, pp 148–151

Drakatou ML, Bignami Ch, Stramondo S, Parcharidis I (2015) Ground deformation observed at Kozloduy (Bulgaria) and Akkuyu (Turkey) NPPs by means of multitemporal SAP interferometry. Theofrastos Digital Library - Department of Geology. A.P.Th. http://geolib.geo.auth.gr

Eaton GP (1980) Geophysical and geological characteristics of the crust of the Basin and Range Province. In: Studies in geophysics, continental tectonics, edited by thenational research council, division on engineering and physical sciences, commission on physical sciences, mathematics, and applications, geophysics research board, assembly of mathematical and physical sciences, geophysics study committee, Washington DC, pp 107–108. http://www.nap.edu/catalog/203.htm

Eggers DE (1982) An eigenstate formulation the magnetotelluric impedance tensor. Geophysics 47:1204–1214

Faure Walker J (2021) Fukushima: why we need to look back thousands of years to get better at predicting earthquakes. https://theconversation.com/fukushima-why-we-need-to-look-back-thousands-of-years-to-get-better-at-predicting earthquakes:156882

Gasperikova E, Rosenkjaer GK, Arnason K, Newman GA, Lindsey NJ (2015) Resistivity characterization of the Krafla and Hengill geothermal fields through 3D MT inverse modeling. Geothermics 57:246–257. https://doi.org/10.1016/j.geothermics.2015.06.015

Georgiev T, Shanov S (1991) Contemporary Geodynamics of the Western Part of the Moesian Platform (Lom Depression). Bulg Geophys J XVII 3:3–9 ( (in Bulgarian) )

Glover PWJ, Adam A (2008) Correlation between crustal high conductivity zones and seismic activity and the role of carbon during shear deformation. J Geophys Res 113:B12210. https://doi.org/10.1029/2008JB005804

Gough DI (1986) Seismic reflectors, conductivity, water and stress in the continental crust. Lett Nature 323:143–144

Groudev P, Petrova P (2017) Overview of the available information concerning seismic hazard for the Kozloduy NPP site. Prog Nucl Energy 97:162–169. https://doi.org/10.1016/j.pnucene.2017.01.007

Guangmeng G, Jie Y (2013) Three attempts of earthquake prediction with satellite cloud images. Nat Hazards Earth Syst Sci 13:91–95. https://doi.org/10.5194/nhess-13-91-2013

Gutenberg B (1954) Low-velocity layers in the Earth’s mantle. Bull GSA 65:337–348

Haak V, Hutton R (1986) Electrical resistivity in continental lower crust. Geol Soc Lond Spec Publ 24:35–49. https://doi.org/10.1144/GSL.SP.1986.024.01.05

Hoskin T, Regenauer-lieb K, Jones A (2015) Deep conductivity anomaly of the Darling Fault Zone - implications for fluid transport in the Perth Basin. ASEG Ext Abstr 1:1–4. https://doi.org/10.1071/ASEG2015ab047

Hermance JF (1995) Electrical conductivity models of the crust and mantle. In: Ahrens TJ (ed) A handbook of physical constants: global earth physics. AGU Ref Shelf I, AGU, Washington DC, pp 190–205

Jones A (1992) Electrical conductivity of the continental lower crust. In: Kay RW (ed) Fountain DM, Arcu1us RJ. Continental Lower Crust, Elsevier, pp 81–143

Kaya T, Tank BS, Tuncer MK, Rokityansky II, Tolak E, Savchenko T (2009) Asperity along the North anatolian fault imaged by magnetotellurics at Duzce, Turkey. Earth Planets Space 61:871–884

Kotzev V, Nakov R, Burchfel BC, King R, Reilinger R (2001) GPS study of active tectonics in Bulgaria: results from 1996 to 1998. J Geodyn 31:189–200

Kounov A, Gerdjikov I, Vangelov D, Balkanska E, Lazarova L, Georgiev S, Blunt E, Stockli D (2017) First thermochronological constraints on the Cenozoic extension along the Balkan fold-thrust belt (Central Stara Planina Mountains, Bulgaria). Int J Earth Sci 107:1515–1538. https://doi.org/10.1007/s00531-017-1555-9

Article   CAS   Google Scholar  

Kováčiková S, Červ V, Praus O (2005) Modelling of the conductance distribution at the eastern margin of the european Hercynides. Stud Geophys Geod 49:403–421

Krstekanic N, Willingshofer E, Broerse T, Matenco L, Toljic M, Stojadinovic U (2021) Analogue modelling of strain partitioning along a curved strike-slip fault system during backarc-convex orocline formation: Implications for the Cerna-Timok fault system of the Carpatho-Balkanides. Journ Struct Geol 149:104386. https://doi.org/10.1016/j.jsg.2021.104386

Ladanivsky BT (2003) Algorithm for processing MTS data. Fifth geophysical readings of VV Fedynsky, February 27 - March 01, 2003. Abstracts of reports, pp 134–135 (in Russian)

Ladanivsky B, Logvinov I, Tarasov V (2019) Earth mantle conductivity beneath the Ukrainian territory. Stud Geophys Geod 63:290–303. https://doi.org/10.1007/s11200-018-0347-4

Lapenna V, Lorenzo P, Perrone A, Piscitelli S (2003) High-resolution geoelectrical tomographies in the study of Giarrossa landslide (southern Italy). Bull Eng Geol Env 62:259–268. https://doi.org/10.1007/s10064-002-0184-z

Logvinov I, Boyadzhiev G, Srebrov B, Rakhlin L, Logvinova G, Timoshin S (2021) Geoelectric studies of the Kozloduy nuclear power plant region, Bulgaria. Geophys Journ 6(43):3–22. https://doi.org/10.24028/gzhv43i6.251549

Magnusson MK, Fernlund JMR, Dahlin T (2010) Geoelectrical imaging in the interpretation of geological conditions affecting quarry operations. Bull Eng Geol Environ 69:465–486. https://doi.org/10.1007/s10064-010-0286-y

Manevich AI, Kaftan VI, Losev IV, Shevchuk RV (2021) Improvement of the deformation GNSS monitoring network of the Nizhne-Kansk Massif underground research laboratory site. Seismic Instrum 57(5):587–599. https://doi.org/10.3103/S0747921050042

Marquis G, Hyndman RD (1992) Geophysical support for aqueous fluids in the deep crust: seismic and electrical relationships. Geophys J Int 110:91–105

Mavrodiev SC, Pekevski L, Kikuashvili G, Botev E, Getsov P, Mardirossian G, Sotirov G, Teodossiev D (2015) On the imminent regional seismic activity forecasting using INTERMAGNET and sun-moon tide code data. Open J Earthq Res 4:102–113. https://doi.org/10.4236/ojer.2015.43010

Mladenovic A, Antic M, Trivic B, Cvetkovic V (2019) Investigating distant effects of the Moesian promontory: brittle tectonics along the western boundary of the Getic unit (East Serbia). Swiss J Geosci 112:143–161. https://doi.org/10.1007/s00015-018-0324-5

Morozova LI (2012) Clouds are the Forerunners of Earthquakes. Sci First Hand 2(32):81–91

Nadirov R, Rzayev O (2017) The metsamor nuclear power plant in the active tectonic zone of Armenia is a potential Caucasian Fukushima. J Geosci Environ Protect 5:46–55. https://doi.org/10.4236/gep.2017.54005

Nagar M, Pavankumar G, Mahesh P, Rakesh N, Chouhan AK, Nagarjuna D, Chopra S, Ravi KM (2021) Magnetotelluric evidence for trapped fluids beneath the seismogenic zone of the Mw60 Anjar earthquake, Kachchh intraplate region, Northwest India. Tectonophysics 814:228969. https://doi.org/10.1016/j.tecto.2021.228969

Nikolova JB (1980) The experience of studying laboratory and geophysical methods of physical properties of volcanic rocks in wells (North Bulgaria). In: Proceedings of the XI congress of the Carpathian-Balkan geological association, Geophysics, Naukova Dumka, Kiev, pp 132–140 (in Russian)

Nover G (2005) Electrical properties of crustal and mantle rocks-A review of laboratory measurements and their explanation. Surv Geophys 26:593–651. https://doi.org/10.1007/s10712-005-1759-6

Oros E, Placinta AO, Moldovan IA (2021) The analysis of earthquakes sequence generated in the Southern Carpathians, Orsova june-july 2020 (Romania): seismotectonic implications. Rom Rep Phys 73:706

Parker RL, Whaler KA (1981) Numerical method for establishing solution to the inverse problem of electromagnetic induction. J Geophys Res 86(B10):9574–9584

Parks EM, McBride JH, Nelson ST, Tingey DG, Mayo AL, Guthrie WS, Hoopes JC (2011) Comparing electromagnetic and seismic geophysical methods: estimating the depth to water in geologically simple and complex arid environments. Eng Geol 117(1–2):62–77

Pavlenkova NI (2004) Low velocity and low electrical resistivity layers in the middle crust. Ann Geophys 47(1):157–169. https://doi.org/10.4401/ag-3268

Pek J, Santos E (2006) Magnetotelluric inversion for anisotropic conductivities in layered media. Phys Earth Planet Int 158(2–4):139–158. https://doi.org/10.1016/j.pepi.2006.03.023

Petrescu L, Borleanu F, Radulian M, Ismail-Zadeh A, Matenco L (2021) Tectonic regimes and stress patterns in the Vrancea Seismic Zone: Insights into intermediate-depth earthquake nests in locked collisional settings. Tectonophysics 799:228688. https://doi.org/10.1016/j.tecto.2020.228688

Petraki E, Nikolopoulos D, Nomicos C, Stonham J, Cantzos D, Yannakopoulos P, Kottou S (2015) Electromagnetic pre-earthquake precursors: mechanisms, data and models-a review. J Earth Sci Clim Change 6(1):1000250. https://doi.org/10.4172/2157-7617.1000250

Qin W, Zhang S, Li M, Wu T, Chu Z (2018) Distribution of Intra-crustal low velocity zones beneath Yunnan from seismic ambient noise tomography. Journ Earth Sci 29(6):1409–1418. https://doi.org/10.1007/s12583-017-0815-8

Ritter O, Hoffmann-Rothe A, Bedrosian PA, Weckmann U, Haak V (2015) Electrical conductivity images of active and fossil fault zones. In: Bruhn, DF Burlini L (eds) High strain zones: structure and physical properties, vol 245, Special Publication, Geol. Soc. London, pp 165–186

Savchyn I, Vaskovets S (2018) Local geodynamics of the territory of Dniestr pumped storage power plant. Acta Geodyn Geomater 15(1):189. https://doi.org/10.13168/AGG.2018.0002

Sano Y, Takahata N, Kagoshima T, Shibata T, Onoue T, Zhao D (2016) Groundwater helium anomaly reflects strain change during the 2016 Kumamoto earthquake in Southwest Japan. Sci Rep 6:37939. https://doi.org/10.1038/srep37939

Satitpittakul A, Vachiratienchai C, Siripunvaraporn W (2013) Factors influencing cavity detection in Karst terrain on two-dimensional (2-D) direct current (DC) resistivity survey: a case study from the western part of Thailand. Eng Geol 152:162–171

Schmucker U (1970) Anomalies of geomagnetic variations in the southwestern United States. Bull Scripps Inst Oceanogr Univ Calif 13:13–32

Scholz CH, Tan YJ, Albino F (2019) The mechanism of tidal triggering of earthquakes at mid-ocean ridges. Nat Communications 10:2526. https://doi.org/10.1038/s41467-019-10605-2

Simpson F (1999) Stress and seismicity in the lower continental crust: a challenge to simple ductility and implications for electrical conductivity mechanisms. Surv Geophys 20:201–227

Srebrov B, Ladanivskyy B, Logvinov I (2013) Application of space generated geomagnetic variations for obtaining geoelectrical characteristics at Panagyurishte geomagnetic observatory region. Comptes Rendus De L’acade’mie Bulgare Des Sciences 66(6):857–864

Stanciu I-M, Ioane D (2017) Regional seismicity in the Moesian Platform and the Intramoesian Fault. Geo-Eco-Marina 23:263–271

Stanica D, Stanica DA (2011) Earthquakes precursors. In: D’Amico S (ed) Earthquake research and analysis. IntechOpen, pp 79–100. https://doi.org/10.5772/28262

Suzuki K, Toda S, Kusunoki K, Fujimitsu Y, Mogi T, Jomori A (2000) Case study of electrical and electromagnetic methods applied to mapping active faults beneath the thick quarternary. Eng Geol 56:29–45

Toteva A, Shanov S (2021) Chemical composition of groundwater in the zone of slow water exchange of the Upper Pontian aquifer, Northwestern Bulgaria. Eng Geol Hydrogeol 35:23–30. https://doi.org/10.52321/igh.35.1.23

Unsworth M, Bedrosian PA (2004) On the geoelectric structure of majorstrike-slip faults and shear zones. Earth Planet Space 56(12):1177–1184

Unsworth M, Rondenay S (2013) Mapping of the distribution of the fluids in the crust and lithospheric mantle utilizing geophysical methods. In: Harlov DE, Austrheim H (eds) Metasomatism and the chemical transformation of rock, lecture notes in earth system sciences. Springer-Verlag Berlin Heidelberg. https://doi.org/10.1007/978-3-642-28394-9_13

Valev G, Rainov G, Vassileva K (2016) Geodetic measurements in the area of Kozloduy Nuclear Power Plant. Coordinates XII 8:37–40

Vangelov D, Pavlova M, Gerdjikov I, Kounov A (2016) Timok fault and Tertiary strike-slip tectonics in part of Western Bulgaria. Annu Univ Min Geol St Ivan Rilski 59:112–117

Vanyan L, Tezkan B, Palshin N (2001) Low electrical resistivity and seismic velocity at the base of the upper crust as indicator of rheologically weak layer. Surv Geophys 22:131–154. https://doi.org/10.1023/A:1012937410685

Varentsov IM (2007) Joint robust inversion of MT and MV data// Electromagnetic sounding of the Earth’s interior. In: Spichak V (ed) Methods in geochemistry and geophysics 40. Elsevier, pp 189–222

Volvovsky BS, Starostenko VI (eds) (1996) Geophysical parameters of the lithosphere of the southern sector of the Alpine Orogene. Kiev-Naukova Dumka (in Russian)

Wieland M, Griesser L, Kuendig C (2000) Seismic early warning system for a nuclear power plant. In: 12th IWCEE,p 1781

Wynn J, Mosbrucker A, Pierce H, Spicer K (2016) Where is the hot rock and where is the ground water - using CSAMT to map beneath and around Mount St. Helens J Environ Eng Geophys 21:79–87

Yaneva M, Shanov SB (2015) Sedimentological Model of Lom Lignite Basin (North Bulgaria) – Integrated Use of Geophysical and Geological Data. In: 8th Congress of the Balkan Geophysical Society, EAGE, p 26678. https://doi.org/10.3997/2214-4609.201414133

Zafrir H, Barbosa S, Levintal E, Weisbrod N, Horin YB, Zalevsky Z (2020) The impact of atmospheric and tectonic constraints on radon-222 and carbon dioxide flow in geological porous media - a dozen-year research summary. Front Earth Sci 8:559298. https://doi.org/10.3389/feart.2020.559298

Zagorchev I (2009) Geomorfological formation of Bulgaria. Principles and state of the art. Comptes Rendus De L’acade’mie Bulgare Des Sciences 62(8):981–992

Zhan W, Pan L, Chen X (2020) A widespread mid-crustal low-velocity layer beneath Northern China revealed by the multimodal inversion of Rayleigh waves from ambient seismic noise. J Asian Earth Sci 196:104372. https://doi.org/10.1016/j.jseaes.2020.104372

Zhao DP, Ochi F, Hasegawa A, Yamamoto A (2000) Evidence for the location and cause of large crustal earthquakes in Japan. J Geophys Res Solid Earth 105(B6):13579–13594. https://doi.org/10.1029/2000jb900026

Zorin ZA, Mordvinova VV, Turutanov EK, Belichenko BG, Artemyev AA, Kosarev GL, Gao SS (2002) Low seismic velocity layers in the Earth’s crust beneath Eastern Siberia (Russia) and Central Mongolia: receiver function data and their possible geological implication. Tectonophysics 359:307–327

Download references

Acknowledgements

This work was carried out as part of the implementation a scientific project «Research on Partial Differential Equations and their applications in Modelling of non-linear processes», funded by Bulgarian National Science Fund, contract KP-06N42/2 and partially supported by scientific project 0117U000117 «Deep processes in the crust and upper mantle of Ukraine and formation of mineral deposits» funded by National Academy of Sciences of Ukraine. We thank the editor and the reviewers for their helpful comments and suggestions.

Open access publishing supported by the National Technical Library in Prague. The authors declare that no other funds, grants, or other support were received during the preparation of this manuscript and they have no financial or other non-financial interests.

Author information

Authors and affiliations.

Institute of Geophysics, Academy of Sciences Czech Republic, Bocni II/1401, 4-14131, Praha, Czech Republic

S. Kovacikova

Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Sofia, Bulgaria

G. Boyadzhiev

S.I. Subbotin Institute of Geophysics, National Academy of Sciences of Ukraine, Kiev, Ukraine

I. Logvinov

You can also search for this author in PubMed   Google Scholar

Corresponding author

Correspondence to S. Kovacikova .

Ethics declarations

Conflict of interest.

The authors have not disclosed any competing interests.

Additional information

Publisher's note.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

I. Logvinov: deceased

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ .

Reprints and permissions

About this article

Kovacikova, S., Boyadzhiev, G. & Logvinov, I. Geoelectric studies in earthquake hazard assessment: the case of the Kozlodui nuclear power plant, Bulgaria. Nat Hazards (2024). https://doi.org/10.1007/s11069-024-06867-9

Download citation

Received : 20 December 2022

Accepted : 02 August 2024

Published : 02 September 2024

DOI : https://doi.org/10.1007/s11069-024-06867-9

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

• Kozlodui nucler power plant
• Moesian plate
• Seismic risk
• Magnetotellurics
• Find a journal
• Publish with us
• Track your research