Relationship Between the Focal Depth Distribution of Earthquakes and Rheological Structure of Lithosphere in Tibetan Plateau

It has been shown that almost all earthquakes on the continents are confined within the crustal layer. However, earthquakes of the Tibet plateau occur not only in the upper part of the thickened crust, but also in the mantle portion of the lithosphere. The lower crust appears to be aseismic. It is still unclear regarding the distributions of the focal depth and mechanism of earthquakes in Tibetan plateau. To study the distributions of the focal depth of the earthquakes in Tibet, high-quality databases from International Seismological Centre (ISC), National Earthquake Information Centre (NEIC) and China Seismological Network (CSN) are used in the period from 1964 to 2006 including 7157 events. The statistical results show that earthquakes are not confined only in the upper part of the crust, but extends all the way down to the lower crust and about 400 km depth in the mantle. Our preliminary results also suggest that the peaks of earthquake frequency are concentrated at the depths of about 10 km, 15 km, 25 km, 30 km and 90~100 km, respectively. Based upon the geophysics structure, thermal structure and composition of the lithosphere, the rheological structure of the lithosphere of the Tibetan Plateau is calculated to elucidate the mechanism of earthquakes at different depths. Our results indicate that there are three significant low-viscosity layers located at the base of the middle crust, within the lower crust and at the bottom of the lithosphere, respectively. The magnitude of the viscosity varies from 1024 to 1021 Pa·s, 1022 to 1020 Pa·s and 1023 to 1019 Pa·s in these three low-viscosity layers, respectively. Combining with the results of the focal depth distribution of earthquakes, our results suggest that most earthquakes are in the upper part of the crust with high mechanical strengths. The occurrence of shallow-focused earthquakes (<25 km) is closely related to the brittle fracture and frictional sliding. The earthquakes concentrated at the depths of about 30 km may relate to the brittle-ductile transition zone. Earthquakes also occur within the lower crust and the bottom of the lithosphere with low viscosity. The occurrence of earthquakes in the lower crust may be caused by fluids, partial melts or dehydrated reactions. The mechanism of mantle earthquakes (>300 km) may relate to the mantle phase transformation.