Rupture process of the 19 August 1992 Susamyr, Kyrgyzstan, earthquake

The Susamyr earthquake of August 19, 1992 in Kyrgyzstan is one of the largest events (M-s = 7.4, M-b = 6.8) of this century in this region of Central Asia. We used broadband and long period digital data from IRIS and GEOSCOPE networks to investigate the source parameters, and their space-time distribution by modeling both body and surface waves. The seismic moment (M-0 = 6.8 x 10(19) N m) and the focal mechanism were determined from frequency-time analysis (FTAN) of the fundamental mode of long period surface waves (100-250 s). Then, the second order integral moments of the moment-rate release were estimated from the amplitude spectra of intermediate period surface waves(40-70 s). From these moments we determined a source duration of 11-13 s, major and minor axes of the source of 30 km and 10-22 km, respectively; and an instant centroid velocity of 1.2 km/s. Finally, we performed a waveform inversion of P and SH waves at periods from 5-60 s. We found a source duration of 18-20 s, longer than the integral estimate from surface wave amplitudes. All the other focal parameters inverted from body waves are similar to those obtained by surface waves (theta = 87 degrees +/- 6 degrees, delta = 49 degrees +/- 6 degrees, lambda = 105 degrees +/- 3 degrees, h = 14 +/- 2 km, and M-0 = 5.8 +/- 0.7 x 10(19) N m). The initial rupture of this shallow earthquake was located at the south-west border of Susamyr depression in the western part of northern Tien Shan. A finite source analysis along the strike suggests a westward propagation of the rupture. The main shock of this event was preceded 2 s earlier by small foreshock. The main event was almost immediately followed by a very strong series of aftershocks. Our surface and body wave inversion results agree with the general seismotectonic features of the region.