Crustal Seismotectonic Deformations in Central and Western Tien-Shan

This study is concerned with Western and Central Tien-Shan. It is an area of intracontinental collision and is of great interest for the study of the geodynamic processes that are occurring in the crust. The area of study was investigated using the method of seismotectonic deformation (STD). The STD was calculated on the basis of the approaches proposed by Yu.V. Riznichenko and S.L. Yunga. We used the ISC (International Seismological Centre, London) catalog for estimating the seismicity distribution, for calculating the mean annual rate of STD (STD intensity) IΣ, and the parameter of concentration of earthquake-generating ruptures KСР. The catalog includes over 84 000 earthquakes for the period 1902–2019. The distribution of the parameters mentioned above was calculated for three depth ranges: 0–5, 5–25, and over 25 km. We identified areas of intensive seismotectonic deformation, seismic activity, and high concentration of earthquake-generating faults. The study of the entire earthquake-generating layer gave the result that the maximum STD intensity IΣ = ~9 × 10–8 yr–1 was obtained for the junction zone between Southern Tien-Shan and Northern Pamirs. As to the north part of the area of study, high values of STD intensity were obtained for the western Terskey Alatau: IΣ = ~2 × 10–9 yr–1. At all the depths studied here, the maximum rate of earthquakes occurs in the Gissar-Kokshaal earthquake-generating zone. In the northern part, high seismicity is characteristic for the mountain ranges that encircle the Issyk Kul Basin (Terskey Alatau, Kungei Alatau, and Zailiisky Range). The area of study typically exhibits a high level of the concentration of earthquake-generating ruptures, with most of these lying at depths of 0–5 km. Our study of STD directivity is based on data for focal mechanisms of 11 376 earthquakes occurring in 1949–2020. We plotted diagrams showing the distribution of azimuthal directions for the principal stress axes. The azimuth of the compression axis for most events falls within the sector 300°–360°. The resulting STD maps were inspected to determine the directions of shortening and lengthening axes, and to note that the deformation settings show a great diversity in the area of study. The STD tensors obtained for the depths 5–25 km (the earthquake-generating layer) were used to find the distributions of the Lode–Nadai coefficient με, of the sum of the horizontal components (ХХ + YY), and of the vertical component (ZZ). We determined areas that show simple compression and maximum shortening. The models of crustal deformation derived by the STD method and from GPS data were compared to find a fairly good consistency.