High-resolution velocity structure and seismogenic potential of strong earthquakes in the Bamei-Kangding segment of the Xianshuihe fault zone

On September 5, 2022, a strong M S 6.8 earthquake struck the Luding area in the Kangding-Moxi segment of the Xianshuihe fault zone, which is the northern boundary of the Sichuan-Yunnan rhombic block, causing considerable casualties. The Bamei-Kangding segment of the Xianshuihe fault zone, which is located only tens of kilometers away from the Luding earthquake, has hosted frequent moderate to strong earthquakes in history and is a dangerous earthquake-prone zone. Therefore, it is critical to investigate the regional seismogenic environment for strong earthquakes and to evaluate the impact of the Luding earthquake in this area. For this purpose, we deployed a dense seismic array comprising over 200 short-period nodes in this region from July to August, 2022 and acquired seismic ambient noise for over 30 days. Using the collected data, we conducted surface wave tomography and obtained a high-resolution 3-D shear wave velocity model for the regional shallow crust down to 8 km in depth. The key findings include: (1) the Bamei-Kangding segment of the Xianshuihe fault zone exhibits widespread stripped low-velocity anomalies, suggesting shear movements at a relatively high temperature of the Xianshuihe fault zone; the Zheduoshan granitic pluton situated between the Zheduotang and southern Selaha faults shows a distinct low-velocity anomaly, which may be attributed to the localized high-temperature anomaly resulted by a deep magmatic heat source and the recent rapid uplift of the Zheduoshan area; (2) a ten-kilometer-wide high velocity body found below 4 km in depth near the Zhonggu area in the Bamei segment coincides with the seismic gap of moderate to strong earthquakes in this region, suggesting that the high velocity body may act as a seismic barrier; (3) the heterogeneity of the velocity structure along the Bamei-Kangding segment of the Xianshuihe fault zone corresponds to the regional changes in temperature, which reveals the reason for the spatially varying seismogenic potential in this segment; especially, the Selaha and Zheduotang faults which are located along the boundaries between the high and low velocity anomalies may possess considerable seismogenic potential; (4) the Coulomb failure stress calculations indicate that the Luding earthquake has imposed nontrivial stress loading in the Bamei-Kangding segment, and may shorten the earthquake recurrence intervals of the southern Selaha fault, the Zheduotang fault, and the Xuemenkan segment of the Xianshuihe fault zone. Thus, the Luding earthquake may potentially pose threats to the Sichuan-Xizang railway passing through this region.