Relocated Earthquakes Confined to the Upper Crust in the Southern Tibet

We have located a total of 202 local earthquakes, based on the data recorded by the Hi-CLIMB seismic stations from 2002-2005, in the southern Tibet. The focal depths of all relocated earthquakes, in the Lhasa terrane, are shallow than 30km, however, the depths can extend to 50km under Himalaya, although there are many earthquakes deeper than 80 km according to the catalogue.  The absence of the earthquakes of the lower crust, in Lhasa terrane, implying a brittle upper crust lying on a soft felsic lower crust. Moreover, the focal depths, in Himalaya, show a low angle (~12°) of subducted Indian lower crust. The focal mechanisms show that the normal faults are the main type of the crustal deformation, which indicate the dominant direction of the extension is approximately east-west, in Lhasa terrane. The strike-slip faults played a regulatory role between normal faults. The thrust faults are only occurred in the south of STDS. The calculated mechanisms correlate well with the surface geology features. Our new results suggest that the whole crust of the Himalaya is brittle and prone to triggering earthquakes under the northward convergence of the Indian plate. However, the lower crust of the Lhasa terrane may be soft, felsic and stable floating above the mantle, under a brittle upper crust which is easier to collapse.