Fine crustal structures of northeast margin of the Tibetan Plateau and structural features of Jiuzhaigou earthquake focal area constrained by the data from a high-density seismic array

We applied the double difference tomography method to simultaneously invert the seismic event locations and the crustal structure of the northeastern Tibetan Plateau with the observations of a high-density temporary seismic array and permanent seismic stations. In total 9644 seismic events are selected for inversion. The results indicate that the velocity structure in the northeastern Tibetan plateau has obvious lateral heterogeneity. Overall, this plateau has low-velocity anomalies, while the Ordos and Yangtze blocks have high-velocity anomalies. The velocity structures at different depths are variable, while the velocity structure of P-waves and S-waves at the same depth are also different. In the region with strong seismic activity enclosed by the faults, such as the West Qinling, Lintan-Dangchang and the Lixian-Luojiapu faults, the velocity structure of P-waves changes from low-velocity anomalies at depth 0km into alternating high and low anomalies at depth 5 km. And in this region, the S wave velocity structure is characterized by the distribution of alternating high and low velocity anomalies at depth 0 km near the surface, and almost low-velocity anomalies at depth 30km. There is a prominent anomaly of the P-wave velocity at depth 20km beneath the area surrounded by the Minjiang, Tazang and Xueshan faults where the Jiuzhaigou M7 earthquake occurred on 8 August 2017. The hypocenter of this event is located in a transition zone between the high- and low-velocity anomalies. In addition, the Min-Zhang M(s)6.6 earthquake of 22 July 2013 also took place in such a transition zone. Likely such a transition zone is relatively easy to accumulate stress, leading to occurrence of major earthquakes in this area.