Evolution of Views on the Structure of Sources of Strong Earthquakes at the End of XX and Beginning of XXI Centuries

The evolution of scientific views on the structure of the sources of strong earthquakes at the end of the 20th and beginning of the 21th century in Russia is considered. The lack of a clear, consistent understanding of the structure of sources of the strongest seismic events was initially typical of the scientific concepts that emerged in the main developed countries. In the 1950s, at the Schmidt Institute of Physics of the Earth of the USSR Academy of Sciences, G.A. Gamburtsev formulated a hypothesis about the stability of the seismic regime of a system of seismic sutures over a long period of time (a few hundred years). The seismic sources of the recently studied earthquakes are located in the regions of large faults. With the increase in magnitude, they become more extended and structurally complex. In the considered cases, there are sources that are relatively simply to reconstruct, which encompass the fault planes of the large existing faults (Spitak source, M = 6.8), as well as sources that are more complex, formed in the disjunctive nodes, or those that encompass the crustal blocks. For example, the seismic source of the Altai earthquake (M = 7.3) is characterized by a volumetric structure and is developed along the boundaries of the large seismogenic blocks. The source of the Wenchuan earthquake (M = 7.9) is most complicated. It appears as a three-dimensional (3D) structure composed of a few crustal blocks framed by two extended northeast striking faults and separated by a transverse fault of the northwestern orientation. The sources having a different focal structure differently manifest themselves in the structure of seismic dislocations on the surface and in the distribution of aftershock hypocenters at depth. The anomalously low velocity “pockets” that were identified by the method of local seismic tomography in the source areas of the Spitak and Altai earthquakes and that accompany the main and secondary faults at depth are likely to be the zones of the dynamic influence of these faults. The damaged near-fault zones, with abundant cracks and fractures, are the severely destroyed inclusions in the crustal rocks, and they hamper the propagation of seismic waves. Therefore, within these pockets, the P-waves propagate at a lower velocity than the undamaged geological medium. The results of the paleoseismological study of seismic faults in the trenches showed that strong earthquakes also occurred in the same sources in the past, and the period of recurrence of the strongest seismic events ranges from a few hundred to the first thousand years. Thus, the integrated studies of the source zones of the strongest earthquakes that were conducted in the past decades in different regions of Eurasia have shown that the hypothesis of Gamburtsev has remained relevant.