Inversion for the focal mechanisms of the 2017 Jiuzhaigou M7. 0 earthquake sequence using near-field full waveforms

In this paper, we apply the full waveform inversion to determine the focal mechanisms of the 2017 Jiuzhaigou M7. 0 earthquake sequence using near-field waveforms from Sichuan Seismic Network. Focal mechanism solutions show that the Jiuzhaigou M7. 0 mainshock' s moment magnitude is 6. 36 and the depth is about 22 km. The two nodal planes' parameters are: strike= 150 degrees, dip=80 degrees, rake = - 20 degrees; and strike = 244 degrees, dip = 70 degrees, and rake= - 169 degrees. The aftershock sequences are located in the depth range of 14 similar to 22 km, and focal mechanisms of the Jiuzhaigou event sequence are dominated by strike-slip faulting, of which 2 are normal, 2 are thrust, 24 are strike-slip, and 8 are mixed faulting. Strikes of fault planes are mostly in SSE direction; consistent with the trend of the Tazang fault and Huya fault, but different from the strike of southernmost part of the Tazang fault. The dip angles are in the range of 60 degrees similar to 80 degrees, and rake angles are largely close to 0 degrees, indicating the Jiuzhaigou earthquake sequence is under the control of the SEE-striking, approximately vertical left-lateral strike-slip fault. The azimuths of P-axes are mostly in SEE direction, and the plunges of P-axes are mostly less than 30 degrees, generally coincide with the regional tectonic stress field. Focal mechanisms and stresses vary between various segments in the space. We infer that the seismogenic fault of the Jiuzhaigou earthquake sequence is a blind fault extending northward from the Huya fault. However, we can not rule out the possibility that the rupture of the southern segment of the Tazang fault and the blind fault to the north of Huya fault may both contributed to the Jiuzhaigou earthquake.