The 2022 Lushan M_S6.1 earthquake：A moderately strong earthquake on a blind back thrust fault

On June 1, 2022, a moderately strong earthquake of M_S6.1, henceforth referred to as the Lushan M_S6.1 earthquake, occurred in the southern segment of the Longmenshan fault zone, following the 2008 Wenchuan M_S8.0 earthquake and the 2013 Lushan M_S7.0 earthquake. Notably, the epicenter of the Lushan M_S6.1 earthquake was situated approximately 10 km northwest of the epicenter of the 2013 Lushan M_S7.0 earthquake, and the aftershock regions of these two events were intimately connected. Investigating the seismogenic structures of the Lushan M_S6.1 earthquake, its relationship with surrounding fault systems, and its connection with the Lushan M_S7.0 earthquake holds pivotal scientific significance for comprehending earthquake mechanisms, seismic interactions, and assessing future seismic hazards.　　In this study, we relocated the mainshock and the aftershock sequences that occurred within 60 days, and obtained 933 high-precision relocation results using the double-difference relocation method （HYPODD）. The root mean square （RMS） residual for all earthquakes decreased from 0.16 s to 0.05 s. Employing a method based on singular value decompositions for error estimation, we determined that the average location errors were 0.15 km, 0.13 km and 0.23 km in the EW, NS and UD directions, respectively. These results distinctly show that the double-difference relocation method significantly improved the precision of earthquake location.　　The aftershock epicenters exhibit a slightly longer spread in the NE-SW direction. The focal depths are mainly located within the range of 12−20 km, with only a limited number of aftershocks occurring above the 10 km. The depth profile of the aftershocks reveals a seismogenic fault plane with a southeastward dip, consistent with the tendency of the back thrust fault in the seismogenic fault structure of the 2013 Lushan M_S7.0 earthquake. The seismogenic fault structures of the two Lushan earthquakes intersect in a complex Y-shaped structure, comprising two back thrust faults. The Lushan M_S6.1 earthquake’s seismogenic fault corresponds to the deeper one of these back thrust faults. Given the absence of surface rupture, we presume that this seismogenic fault is a deeply buried blind fault. The Lushan M_S6.1 earthquake and its aftershock sequences occurred within a high-velocity zone characterized by stable rocks, making them less susceptible to rupture and sliding. Over time, as stress and strain gradually accumulated, the back thrust fault experienced rupture and sliding, culminating in the occurrence of the Lushan M_S6.1 earthquake. 　　The depth profile indicates that the southeast-trending aftershock zone of the 2013 Lushan M_S7.0 earthquake intersected with the Shuangshi-Dachuan branch fault, with the majority of aftershocks concentrated beneath this fault. It is hypothesized that high-velocity anomalies in the upper crust beneath the fault zone impeded the upward slip of the back thrust fault associated with the Lushan M_S7.0 earthquake. However, the aftershocks related to the Lushan M_S6.1 earthquake predominantly occurred beneath the Shuangshi-Dachuan branch fault, without obvious high-velocity zone in the shallow region of the seismogenic fault. We speculate that the Shuangshi-Dachuan branch fault may have played a role in obstructing the slip process of the seismogenic fault, impeding its rupture towards shallower depths.