Background and Clustering Characteristics of Recent Seismicity in Southwestern China

This study analyzed seismicity in southwestern China (1 January 2008 to 30 June 2021) using the earthquake catalog compiled by the China Earthquake Network Center and four different space–time Epidemic-Type Aftershock Sequence models: the 2D point-source (PS) model, the 2D finite-source (FS) model, the 3D PS model, and the 3D FS model. Our objective was to understand the features of the background seismicity and the patterns of earthquake clusters to better evaluate the regional seismic hazard. We carefully investigated the aftershock sequences that followed 7 of the 10 MS≥6.0 earthquakes that have struck this region since the occurrence of the 2008 Wenchuan MS8.0 earthquake (i.e., the Panzhihua (31 August 2008; MS6.0), Yaoan (9 July 2009; MS6.0), Lushan (20 April 2013; MS7.0), Ludian (3 August 2014; MS6.5), Jinggu (7 October 2014; MS6.6), Kangding (11 November 2014; MS6.3), and Yangbi (21 May 2021; MS6.4) earthquakes). Our results revealed the following. (1) The background seismicity level for natural earthquakes is usually stable but can experience sudden change due to major events, such as the 2014 Ludian MS6.5, and the 2014 Jinggu MS6.6 events. Such changes in the background rate can reach 50%. (2) Reservoir-induced earthquakes substantially increase the level of regional seismicity, indicating that they cannot be ignored when analyzing natural seismicity and evaluating regional earthquake hazards. (3) Events triggered directly by the mainshock occur mostly in regions adjacent to areas with large coseismic slip, showing a pattern complementary to the mainshock ruptures.