Evidence for Fluids at the Hypocenter of the 2017 Ms 7.0 Jiuzhaigou Earthquake Revealed by Local Earthquake Tomography

Abstract

AbstractThe seismogenic structure of the 2017 Ms 7.0 Jiuzhaigou earthquake is still under debate. To address this issue, we use arrival times of P‐ and S‐ waves from 1766 local earthquakes recorded by our deployed 18 temporary seismic stations to perform the local earthquake tomography and present the 3‐D distributions of the P‐ wave and S‐ wave velocities (Vp and Vs), Vp/Vs ratio and the locations of the relocated aftershocks in the source region of the 2017 Jiuzhaigou earthquake. The aftershocks occurred at depths of 2–10 km, and can be divided into a NNW‐SSE trending northern segment and a NW‐SE trending southern segment relative to hypocenter of the main shock. Combined with previous observations, it is proposed that the Jiuzhaigou earthquake occurred on an unmapped fault which is a branch of the easternmost segment of the eastern Kunlun fault zone. The source region of the Jiuzhaigou earthquake is characterized by high Vp and low Vs anomalies as well as high Vp/Vs ratios, suggesting the presence of fluids in the fractured rocks. These fluids may have contributed to the occurrence of the Jiuzhaigou earthquake by reducing the effective normal stress and thus causing the brittle failure of rocks.