Abstract
The 3 February 2020 MS 5.1 Qingbaijiang earthquake, southwestern China, is the closest recorded MS ≥ 5.0 event to downtown Chengdu City to date, with an epicentral distance of only 38 km. Here we analyze seismic data from the Sichuan and Chengdu regional seismic networks, and employ a multi-stage location method to relocate the earthquakes that have occurred along the central and northern segments of the Longquanshan fault zone since 2009, including the MS 5.1 Qingbaijiang earthquake sequence, to investigate the seismogenic structure of the region. The relocation results indicate that the seismicity along the central and northern segments of the Longquanshan fault zone has occurred mainly along the eastern branch since 2009, with the hypocentral distribution along a vertical cross-section illustrating a steep, NW-dipping parallel imbricate structure. The terminating depth of the eastern branch is about 12 km. The distribution of the MS 5.1 Qingbaijiang earthquake sequence is along the NE–SW-striking Longquanshan fault zone. The aftershock focal depths are in the 3–6 km range, with the mainshock located at 104.475°E, 30.73°N. Its initial rupture depth of 5.2 km indicates that the earthquake occurred above the shallow decollement layer of the upper crust in this region. The hypocentral distribution along the long axis of the aftershock area highlights that this earthquake sequence occurred along a fault dipping at 56° to the NW. Our surface projection of the inferred fault plane places it near the eastern branch of the Longquanshan fault zone. We infer the MS 5.1 mainshock to be a thrust faulting event based on the focal mechanism solution via the cut-and-paste waveform inversion method, with strike/dip/rake parameters of 22°/36°/91° and 200°/54°/89° obtained for nodal planes I and II, respectively. We identify that the seismogenic fault of the MS 5.1 Qingbaijiang earthquake lies along the eastern branch of the Longquanshan fault zone, and nodal plane II represents the coseismic rupture plane, based on a joint analysis of the event relocation results, mainshock focal mechanism, and regional geological information. Our study provides vital information for assessing the seismic hazard of the Longquanshan fault zone near Chengdu City.
Highlights
The ongoing Cenozoic collision between the Indian and Eurasian plates has contributed to the uplift of the Tibetan Plateau (Molnar and Tapponnier, 1975; Houseman and England, 1986; Yin and Harrison, 2000; Tapponnier et al, 2001)
We provide three depth profiles (A–A’, B–B’, and C–C’ in Figure 5A) to further understand the seismogenic structure associated with the 2020 Qingbaijiang earthquake
We suggest that the distribution of these earthquake clusters, which are located just below the surface expression of the eastern branch, reflects the geometry of this branch, which is consistent with regional geological and geophysical survey results (Huang and Tang, 1995)
Summary
The ongoing Cenozoic collision between the Indian and Eurasian plates has contributed to the uplift of the Tibetan Plateau (Molnar and Tapponnier, 1975; Houseman and England, 1986; Yin and Harrison, 2000; Tapponnier et al, 2001). We analyze seismic phase and waveform data from the Sichuan and Chengdu seismic networks, and relocate the earthquakes that have occurred along the central and northern segments of the Longquanshan fault zone since 2009, which include the MS 5.1 Qingbaijiang earthquake sequence. The eastern branch is a back-fault associated with the entire fold system, similar to the back-fault on the main fault plane of the 2013 MS 7.0 Lushan earthquake (Long et al, 2015) Another possibility is that the Longquanshan fault zone and LFTB are components of the same structural system, such that the Longquanshan fault zone forms the southeastern boundary of the LFTB nappe foreland basin in western Sichuan (Chen et al, 1994; Zhang et al, 2003; Liu, 2007). Origin, tectonic setting, and seismic hazard of the Longquanshan AUTHOR’S NOTE fault zone need to be undertaken to resolve these discrepancies
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call