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Seismic sources and Earth structure in the transition zone between Fore-Balkan unit and Moesian platform, NE Bulgaria

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Abstract

Investigations of hydraulically induced micro-seismicity are typically connected with magma injections, exploitation of geothermal fields and salt mines. Our study examines the seismicity and seismic sources in a peri-platform region with long salt production history. In addition, a very large site for extraction of aggregates and limestone is presented in the area. Increased seismicity in the region has been observed for several decades as during the study period of the last 12 years, more than 1000 events with magnitude \(M < 4.3\) have been registered. To study the complex seismic sources concentrated in such small area spatial distribution of seismicity and fault plane solutions are analyzed together with available geological and geophysical data, in order to connect registered earthquakes swarms with geological structures. Results show that several faults are outlined from geological information and confirmed by interpretation of geophysical data. Very different focal mechanisms calculated for the 13 earthquakes in the near region of the salt production site cannot propose a dominant fault mechanism and confirm the complexity of the seismic sources in the area. Our results suppose that activation of some of these structures is more likely due to water injected in the salt dome as part of the production process, which changes the stress equilibrium in the Earth crust.

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Redrawn from Yankova and Dimovski (2015)

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Acknowledgements

We thank our colleagues Iliana Popova, Blagovesta Babachkova, Sylvia Velichkova, Irena Aleksandrova who participated in the interactive processing of the local seismic events, as well as of all seismologists on duty. We thank our colleagues from the IRSM, Prague for the provided seismic data. We thank the editor and two anonymous reviewers for their valuable comments that helped us to improve the manuscript.

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Correspondence to Gergana Georgieva.

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Dimitrova, L., Georgieva, G., Trifonova, P. et al. Seismic sources and Earth structure in the transition zone between Fore-Balkan unit and Moesian platform, NE Bulgaria. Acta Geod Geophys (2020). https://doi.org/10.1007/s40328-020-00288-3

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Keywords

  • Moesian platform
  • Seismic sources
  • Induced seismicity
  • Crustal structure
  • Gravity anomalies
  • Magnetic anomalies