Source parameters for 11 earthquakes in the Tien Shan, central Asia, determined by P and SH waveform inversion

Abstract

The Tien Shan mountain belt of central Asia was formed in late Paleozoic time and reactivated in Tertiary time following the collision of India with Eurasia, more than 1500 km to the south. To study the style and distribution of faulting occurring today in the Tien Shan, we digitized long‐period World‐Wide Standard Seismograph Network P and SH waveforms of 11 of the largest Tien Shan earthquakes between 1965 and 1982 and then used a least squares inversion routine to constrain their fault plane solutions and depths. Four of the earthquakes occurred near the southern edge of the Tien Shan, and two occurred in the intermontane Fergana Basin. These earthquakes occurred at depths of 10–20 km and are associated with thrust faulting on east‐west to southwest‐northeast striking fault planes that dip moderately (35°–55°). The other five earthquakes, in the northern Tien Shan, were mostly deeper (15–44 km), and the fault plane solutions of four of them are similar to the events farther south. The exception, an event on June 2, 1973, probably occurred on a gently northward dipping, east‐west striking fault plane. All events occur within the basement at depths of 10 km or greater. Waveforms for earthquakes near the edges of the Dzungarian, Fergana, and Kucha basins are fit best using velocity structures with thick sediment layers, implying that sediments from these basins have been and are being underthrust beneath the neighboring mountains. We examined only the relatively small (mb = 5.5–6.2) events after 1965 and did not study any of the largest events (Ms > 8) that occurred in the Tien Shan earlier in this century. Still, we can conclude that north‐south shortening is presently occurring in the Tien Shan, with the formation of basement uplifts flanked by moderately dipping thrust faults. The present‐day tectonics of the Tien Shan seem to be analogous to those of the Rocky Mountains in Colorado, Wyoming, and Utah during the Laramide orogeny in Late Cretaceous and Early Tertiary time.