ACTIVE FAULTS AND SEISMIC HAZARDS OF THE TIEN SHAN, KYRGYZSTAN

This report is devoted to the active tectonics of the Tien Shan and Dzungaria (Chu, Naryn, Atbashin, Dzhungar and other depressions) in the northern plate boundary zone of the India-Eurasia collision. This intracontinental orogeny developed at a distance >1000 km from the Himalayan front, relatively late in the 55-Ma collision. A key observation for understanding the geodynamics of the orogen is the spatial distribution of slip rates of active faults in the late Quaternary. GPS geodesy indicates the current shortening rate across the Tien Shan is about 20 mm/yr, almost half the total convergence between India and Eurasia. Geologic slip rates are critical for resolving how this elastic deformation is partitioned into permanent strain along active faults in the Tien Shan over the past ~140 ka. Within the studied area of the Tien Shan and Dzungaria, "fault-related fold" methods are used to calculate displacement rates for the eight most active faults in the late Quaternary. The dip-slip velocities on individual faults range from 0.2 to 3.1 mm/yr and together accommodates half of the sub-meridional shortening. The highest rates of late-Quaternary shortening are in the Kochkor and Naryn basins in the central Tien Shan with lower typical for the southernmost and topographically high intermountain Aksay depression. The displacement velocity of the Alma-Ata fault in the late Pleistocene-Holocene is 1.6 mm/yr. Paleoseismic data provides the link between the long-term displacement velocity in fault zones, the rate of interseismic strain from geodetic measurements, and hazard applications. Paleoseismic data obtained by our investigations in different parts of the Tien Shan and Dzhungaria show that strong seismic events occurred with an average recurrence interval of 1-5 ky, with average surface displacement of 2-4 m and estimated earthquake magnitude ranges from M = 6.9-8.2.