Earthquake distribution in southern Tibet and its tectonic implications

A temporary 37‐station seismic array was operated in southern Tibet from June 2004 to August 2005 close to Xigaze along a traverse from Tangra Yum Co rift in the west to Yadong‐Gulu rift in the east. This lateral array of the international Hi‐CLIMB project recorded a total of 885 local earthquakes during the 14‐month deployment. Hypocenter locations of these events were obtained using HYPOINVERSE2000, and about half of them were relocated by the joint hypocenter determination method. Lateral variations in crustal seismic P and S wave velocities beneath the array were obtained simultaneously as part of the station corrections during the relocation procedure. While earthquakes were scattered in the region, more than 250 earthquakes were clustered within a small area at about 50 km north of the Indus‐Yalu Suture and west of the Pumqu‐Xianza rift. Crustal and uppermost mantle earthquakes are concentrated beneath the Himalayan crest. Projection of all earthquakes to a north–south profile shows that the subducting Indian plate passes the Indus‐Yalu Suture below the Lhasa Terrane. While earthquakes are concentrated in the upper crust and upper mantle beneath the Himalayas, seismicity is mostly restricted to the upper crust to the north, which is consistent with a ductile middle/lower crust beneath southern Tibet. About 20 deep crustal and uppermost mantle events (at depths of 50–70 km) were observed to be associated with the subducting Indian lithosphere. Our relocation results of these local earthquakes in southern Tibet support the popular “jelly‐sandwich” model for the rheology of continental lithosphere in which the strong seismogenic layers of upper curst and uppermost mantle are separated by a weak lower crust.