Analysis of Seismic Activity in the Crust from Earthquake Relocation in the Central Tien Shan

Abstract

Using P - and S -wave arrivals from local earthquakes recorded by a temporary broadband seismic network (ghengis) and a Kyrgyzstan broadband seismic network (knet), we determined the source parameters of 1938 earthquakes occurring from 1997 to 1998 in the central Tien Shan and adjacent areas, based on one- dimensional and three-dimensional velocity models. The results indicate that seismic activity is not restricted to only central Tien Shan but also penetrates into the edges of the Kazakh platform and the Tarim basin to the north and south, respectively. It implies that most range-bounding faults in the central Tien Shan are active and they play a significant role in the tectonic activity of the mountain belt. However, seismic activity of the Talas-Fergana fault in the study area is different; its central segment seems more active than its eastern segment. We infer that this part of the fault may be activated by neighboring faults, whereas its eastern segment could be temporally locked. The earthquakes at the depths of 30–40 km beneath the east-central Tien Shan reveal an interaction between the upper and mid-lower crust. This suggests that brittle failure is possible to greater depth within the crust. Another possibility is that the seismic activity in the mid-lower crust is affected by a positive buoyancy force created by mantle upwelling of this area. These observations indicate that dynamic processes such as small-scale mantle convection or mantle upwelling, which resulted in lithospheric thinning or removal, still occur beneath the central Tien Shan.