Seismicity of central Tibet from Project INDEPTH III seismic recordings

As part of Project International Deep Profiling of Tibet and the Himalaya 111, a 400-km-long, densely spaced array of 57 broadband and short-period seismic stations was deployed in central Tibet from August 1998 through May 1999. Although originally designed to image the lithosphere with teleseismic events, the array also recorded numerous local and regional seismic events. More than 900 local and regional events were detected on at least 10 stations during the 1-year deployment, and we were able to locate 267 local earthquakes. A substantial number of the events were found to cluster in or near large grabens and along known strike-slip faults, while other events show no obvious correlation with known structures. In addition to spatial clustering, at least one of the large clusters also exhibits temporal clustering that may be associated with magmatic or geothermal activity in the upper crust. The average V-p and V-s are estimated to be 5.85 and 3.35 km/sec for the upper crust and 7.0 and 3.9 km/sec for the lower crust, respectively. The 50 focal mechanisms computed from this set of events are consistent with north-south shortening and east-west extension; there are no clear indications of significant local perturbations in the regional stress field induced by the collision between India and Eurasia. The majority of the focal mechanisms indicate normal and strike-slip faulting. At least six of the newly computed focal mechanisms, however, indicate thrust faulting, which is a phenomenon not well documented previously. Ninety-nine percent of the local earthquakes have focal depths shallower than 25 km, and the locations of the few deeper events are poorly constrained. The shallow earthquake focal depths are consistent with high temperatures and proposed ductile or aseismic behavior in the middle to lower crust of central Tibet.