Earthquake source parameters in the western Tarim basin and the Tien-Shan

Distribution of the continental lithospheric strength is currently debated (e.g., “jelly sandwich” vs. “crème brulee” models). A key argument in this debate is the presence (or absence) of earthquakes in the lower crust and the upper mantle. The Tarim basin is commonly regarded as a relatively rigid block that lies between two actively deformed regions (the Tien-Shan in the north, and the Tibetan plateau in the south). However, earthquake catalogs suggest intense seismicity in some parts of the basin (e.g., the Jiashi earthquake sequence). Using regional moment tensor inversion and data from temporary/permanent seismic networks, we investigate earthquake source parameters along the Tien-Shan orogenic belt and the western Tarim basin. Selection of seismic events was made using earthquake catalogs of the China Earthquake Administration. We search for earthquakes with local magnitude over 4 to ensure reasonable signal-to-noise ratios. Preliminary results show that thrust-faulting focal mechanisms with nearly vertical T-axes dominate along the Tien-Shan, implying uplift and possibly crustal thickening of the Tien-Shan. Within the Tarim basin focal mechanisms show a combination of strike-slip and thrust faulting, suggesting a more complex deformation regime. While most events we investigated are in the upper crust, we found two earthquakes at unusual depths. One, an oblique thrust mechanism with Mw=4.8, is located at 44 km depth beneath the Tarim basin. Active source studies and our own work on receiver function analysis suggest crustal thickness on the order of 50 km in the focal area, placing this earthquake near the bottom of the Tarim-basin crust. Another event, a 37-km deep oblique strike-slip with Mw=4.4, is at the southwestern end of the Tien-Shan, close to the surface trace of the Talas-Fergana fault. This event is also in the lower crust. One nodal plane of the best fitting focal mechanism is nearly parallel to the fault trace, suggesting that deformation associated with the motion along the Talas-Fergana fault extends to the lower crust. Overall, our results document pervasive earthquake activity within the northwestern Tarim basin, and show that the style of associated deformation differs from that of the Tien-Shan orogenic belt. Also, our results suggest brittle behavior of the lower crust in both northwestern Tarim and the western Tien-Shan.