Seismic evidence for influences of deep fluids on the 2019 Changning Ms 6.0 earthquake, Sichuan basin, SW China

Abstract

• Velocity models and earthquake locations are determined in Changning source area. • The Changning mainshock is underlain by low-Vp, low-Vs and high Vp/Vs anomalies. • Mantle fluids could play a key role in stress concentration of seismogenic layers. An Ms 6.0 earthquake struck Changning county, Sichuan basin, SW China on 17 June 2019, which caused huge casualties and economic losses. Four Ms greater than 5.0 events subsequently occurred around the Changning source area, three of which occurred within one week. In order to better understand the mechanism of these moderate-sized earthquakes, we determine 3-D high-resolution velocity models around the source area simultaneously relocating earthquakes using double-difference tomography. In the present study, we use a total of 53,487 P-wave and 52,527 S-wave arrival times from 8818 events recorded at 39 seismic stations. Our results show that focal depths of the Changning mainshock and most aftershocks are ~5–10 km, and they form a fault plane with a steep dip angle. Most earthquakes are underlain by the zone with low Vp, low Vs, and high Vp/Vs anomalies, reflecting the existence of fluids there. These results suggest that the Changning mainshock and other moderate-sized earthquakes might be associated with the influence of fluids that could decrease effective normal stress on the fault planes. These fluids might be related to the hot and wet mantle upwelling in the big mantle wedge due to the deep subduction of the Indian plate down to the mantle transition zone. A clear high-to-low velocity transition zone is observed at ~10 km depth beneath the Gongxian and Xingwen swarms, which matches well with the detachment layer revealed by deep seismic soundings in the area. All these results suggest that the structural contrast could control the mainshock generation and aftershock extension.