Geophysical research: article

G. Pavlenkova
Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
Journal: Geophysical research
Tome: 20
Number: 1
Year: 2019
Pages: 65-79
UDK: 550.34
DOI: 10.21455/gr2019.1-6
Full text
Keywords: Earth crust, seismic velocity, longitudinal and transverse waves, Lesser Caucasus
Аnnotation: The paper presents the results of detailed processing of deep seismic sounding (DSS) materials performed in the 80s in the Lesser Caucasus in the area of the devastating Spitak earthquake. These materials were not published at that time, as the work along this profile was continued by the method of the exchange waves from earthquakes (MEWE), which seemed more detailed at that time. This profile is the only DSS profile in the Lesser Caucasus, and it was important to process the remaining experimental materials using modern methods of wave field interpretation and compare them with DSS data for the Great Caucasus. Processing was carried out on the observed travel-times curves of the longitudinal (P) and transverse (S) waves on the basis of the ray modeling method. It turned out that the crust of the Lesser Caucasus has the same high thickness (50-55 km) as the Great Caucasus, but is characterized by higher heterogeneity. In the upper part of the crust at a depth of 5-15 km, an extended block with an increased P-wave velocity (more than 6.6 km/s) and abnormally high S-wave velocity (4.0-4.1 km/s) is observed, in the lower crust velocities, on the contrary, are lowered (6.7-6.8 km/s relative to 6.7-7.2 km/s in the crust of the Great Caucasus). A distinctive feature of the Lesser Caucasus crust is also the presence at a depth of 30-35 km a low velocity layer, limited on both sides by clear reflective boundaries. All these data indicate a different type of crust of the Great and Lesser Caucasus and the different nature of the tectonic processes that formed them. The crust of the Lesser Caucasus is closer to the crust of young plates, the Great Caucasus crust - to the crust of the cratons. This confirms the assumption that this region is a zone of collision of two different age plates. As a result, in the Lesser Caucasus the processes of intrusion of deep material of different composition into the upper part of the crust were widely developed. The distinct reflective boundaries at the both sides of the low velocity layer in the middle crust suggests an important role of the horizontal motions (perhaps even thrusts) of the upper crust relative to the lower one at the formation of the Lesser Caucasus. From the nature of the velocity inhomogeneity of the crust and the sharp change of the M boundary depth in the center of the profile, a deep destruction zone was identified, to which the devastating Spitak earthquake was timed.