A new insight view to the strong (М7.2 and М7.8) earthquakes of 1904 in Bulgaria
Boyko Ranguelov
entre ofNatural Sciences, Bulgarian Academy ofSciences andArts, Sofia, 1421, Bulgaria
Emil Oynakov
Department ofSeismology and Seismic Engineering, National Institute ofGeophysics, Geodesy and Geography, Bulgarian Academy ofSciences, Sofia, 1113, Bulgaria
Irena Alexandrova
Department ofSeismology and Seismic Engineering, National Institute ofGeophysics, Geodesy and Geography, Bulgarian Academy ofSciences, Sofia, 1113, Bulgaria
DOI: https://doi.org/10.59429/ear.v2i2.7044
Keywords: strongest earthquakes in continental Europe, seismicity, tomography, Microseismic Sounding Method
Abstract
The Kresna-Kroupnik seismic source (SS K-K) is under deep investigation. This source is famous with the occurrence of two very strong earthquakes (M7.2 and M7.8) on 4th April 1904 in a time domain of 20 minutes, thus considered to be a “doublet”. A short review of the European seismicity shows that during the last 200 years there is not so strong seismic event on the territory of continental Europe. Besides its power this “doublet” demonstrated a numerous peculiarities which do not fit the recent knowledge about the surface expression of such powerful seismic event. The very short length of the surface dislocation (less than 40 km), the relatively small area of high intensities (up to X-XI EMS), the large area of felt effects, the large area of liquefaction, etc. do not coincide with the recent observations of similar earthquakes (for example Kahramansharah-Gasientep on 6th February 2023). All these strange peculiarities triggered a wide interest to seismologists to study in depth the behavior of the SS K-K. We started with deep insight view to the low velocity layer established by us earlier at depths of 50 to 150 km. Then proceed up by seismic tomography for the depth interval 0-50 km, considered as a thickness of Moho and finally reach the recent GNSS measurements in the area to reveal how and why the SS K-K produced such effects by the earthquakes of 1904. Following the concept of the protrusion of the low velocity body just under the source, than the logic of existence of vertical blocks at shallow depths with different geophysical characteristics (lower and higher P and S waves velocities), thermal and Bouguer gravity anomalies and finally expanding our knowledge by the GNSS displacements of the surface layers which show the behavior of the recent stress field. Thus we were able to create an algorithm explaining the geodynamic environment of the SS K-K observed peculiarities without visible logical contradictions
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