Possible Source of the Deep Earthquake of October 15, 2018 in the Sea of Azov

Abstract

Earthquakes in the Sea of Azov are quite commonplace. Until now, the idea of ​​their nature has focused on tectonic processes in the Earth’s crust. In our work, using morphotectonic analysis, we examined on a lithospheric scale the geodynamic situation in the collision zone of the East Black Sea Plate and the margin of the East European Platform, in order to show that not only in the crust, but also in the lower lithosphere, the necessary tectonic prerequisites exist for the accumulation of seismic energy with subsequent release in the form of earthquakes. Based on a case study of an earthquake that occurred on October 15, 2018, in the Sea of Azov, we showed the possible causes of its occurrence for two options of solving the seismological problem, obtained by the Geophysical Service of the Russian Academy of Sciences and including in the solution according to V.Yu. Burmin’s method. The following focal parameters are published on the Geophysical Survey website: latitude, 46.36; longitude, 37.18; depth, 5 km; mb= 4.3. The focal parameters were obtained for the solution using the Burmin method: latitude, 46.42564; longitude, 37.24274; H = 133.4 km. The mechanisms of two solutions were constructed; for a source with a depth of 5 km, the focal mechanism was obtained: a reverse fault, with the source confined to the intersection of Kalmius-Dzhiginsky and the Main Azov faults; for 133.4 km, a strike-slip fault, with the source confined to the intersection of the Gubkinsky-Central Azov, Kalmius-Dzhiginsky, and the Main Azov faults. The constructed model of collision of the continental lithosphere of the East European Platform and the subducting East Black Sea Plate shows that under conditions of weak compressive forces from the side of the Arabian indenter, a pseudosubduction situation is formed, as a result of which shear stresses occur in the deformable frontal part of the continental lithosphere of the East European Platform, transitioning to a reverse fault along the line of shear. During this process, seismic energy is accumulated, accompanied by weak and moderate earthquakes, of a reverse-fault type for the earth’s crust in the region of the Azov Rise, and a normal-fault type in the lower part of the wedge of the continental lithosphere as a result of its destruction. Regional morphotectonic analysis did not allow us to choose the most probable focal mechanism for the earthquake. However, it is noteworthy that the results of our research have shown that the occurrence of deep earthquakes has all the necessary tectonic prerequisites, despite the well-established opinion of seismologists about the exclusively crustal location of earthquakes in this region.