Heterogeneities in the field of short period seismic wave attenuation in the lithosphere of central Tien Shan

Records of deep-focus Hindu Kush earthquakes in the depth ranges 70–110 and 190–230 km made by 45 digital and analogue seismic stations were analyzed to study the attenuation field of short period seismic waves in the lithosphere of central Tien Shan. The dynamic characteristics studied include the ratio of peak amplitudes in S and P waves (S/P) and the ratio of the S-wave maximum to the coda level in the range t = 400 ± 5 s, where t is the lapse time (S/c400) for 1.25 Hz. Comparatively high values of S/P are shown to prevail in most of the area, corresponding to lower S-wave attenuation. Upon this background is a band of high and intermediate attenuation in the west of the area extending along the Talas-Fergana fault in the south and afterwards turning north-northeast. The rupture areas of the two largest (M ≥ 7.0) earthquakes which have occurred in Tien Shan during the last 25 years are confined to this band. Abnormally high values of S/c400 were obtained for stations situated in the rupture zone of the August 19, 1992, magnitude 7.3 Suusamyr earthquake and around it. For two of the stations we found considerable time variations in the coda envelope before the earthquake. The effective Q was derived from compressional and shear wave data for the entire area, as well as for the band of high attenuation. Comparison with previous data shows that the attenuation field in the area has changed appreciably during 20–25 years, which can only be due to a rearrangement of the fluid field in the crust and uppermost mantle. It is hypothesized that a large earthquake is very likely to occur in the northern part of the attenuating band.