SEISMOGENIC STRUCTURE OF THE M4.9 AND M5.1 LITANG EARTHQUAKES ON 23 SEPTEMBER 2016 IN SOUTHWESTERN CHINA

On 23 September 2016, two earthquakes with magnitude of M4.9 and M5.1 occurred successively near Litang city in Sichuan Province, southwestern China. These two events are located between two large-scale fault zones, i.e., the Jinshajiang and Litang faults, in the northwest of the Sichuan-Yuannan active block, eastern Tibetan plateau. Based on the phase data and waveform data from the Sichuan regional seismic network, the M4.9 and M5.0 mainshocks and 390 aftershocks have been relocated using the multi-step locating method, and the focal mechanism solutions and centroid depths for the two mainshocks were calculated by the CAP waveform inversion method. From the spatial distribution of the relocated aftershocks and fault plane solutions of the two mainshocks, combining with the seismic intensity map and tectonic setting, we suggested that the two earthquakes were generated by the E-W trending northward dipping Hagala fault. The nodal plane consistent with the strike and dip of the Hagala fault is interpreted as the coseismic rupture plane with a dip angle of 44° for both the M4.9 and M5.1 earthquakes. And we inferred that the M4.9 and M5.1 earthquakes may be resulted from the nearly E-W striking Hagala normal faulting in the upper crust between the Litang and Batang regions due to the continuous eastward extrusion of the material of the Qiangtang block in the west.