The 2022 Lushan MS6.1 earthquake：A moderately strong earthquake on a blind back thrust fault
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摘要:
继2008年汶川MS8.0地震和2013年芦山MS7.0地震后，2022年6月1日在龙门山断裂带南段又发生了一次MS6.1强震，距离2013年芦山MS7.0地震震中位置仅10 km。为研究此次地震的发震断层及两次芦山地震的关系，对震后60天的余震序列进行重定位，获得了933个高精度定位结果，EW，NS和UD方向上的定位误差分别为0.15 km，0.13 km和0.23 km。余震序列在水平分布上沿北东—南西向略长，在深度上主要分布在12—20 km，10 km以浅余震很少。余震震源深度剖面显示发震断层面倾向南东，与2013年芦山MS7.0地震发震断层结构中的反冲断层倾向一致，两次芦山地震的发震断层结构相交为复式Y型断裂结构，此次芦山地震的发震断层为其中一条深度更深的反冲断层。此次地震没有产生地表破裂，推测发震断层为一条埋深较深的隐伏断层。两次芦山地震的余震震中分布区跨过了该区域的一条大型逆冲型断裂带，即双石—大川断裂带。深度剖面显示芦山MS7.0地震的南东倾向余震带穿过双石—大川分支断裂，大部分集中在断裂下方，但有少量地震发生在断裂上方，由于断裂带下方上地壳存在高速异常体，推测芦山MS7.0地震的反冲断裂向上逆冲滑动时受到断裂所在高速区的阻挡。此次芦山MS6.1地震的余震主要发生在双石—大川分支断裂下方，但在发震断层的浅部位置没有明显的高速体分布，推测双石—大川分支断裂可能对发震断层的滑动起到了控制阻挡作用，使其没有继续向浅部破裂。
Abstract:On June 1, 2022, a moderately strong earthquake of MS6.1, henceforth referred to as the Lushan MS6.1 earthquake, occurred in the southern segment of the Longmenshan fault zone, following the 2008 Wenchuan MS8.0 earthquake and the 2013 Lushan MS7.0 earthquake. Notably, the epicenter of the Lushan MS6.1 earthquake was situated approximately 10 km northwest of the epicenter of the 2013 Lushan MS7.0 earthquake, and the aftershock regions of these two events were intimately connected. Investigating the seismogenic structures of the Lushan MS6.1 earthquake, its relationship with surrounding fault systems, and its connection with the Lushan MS7.0 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 MS7.0 earthquake. The seismogenic fault structures of the two Lushan earthquakes intersect in a complex Y-shaped structure, comprising two back thrust faults. The Lushan MS6.1 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 MS6.1 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 MS6.1 earthquake. The depth profile indicates that the southeast-trending aftershock zone of the 2013 Lushan MS7.0 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 MS7.0 earthquake. However, the aftershocks related to the Lushan MS6.1 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.
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图 5 余震序列重定位结果分布图
（a） 余震震中位置分布图；（b，c） 震源深度沿经度和纬度的剖面图；（d，e） 震源深度沿AA′和BB′剖面的分布图及地形分布
Figure 5. Relocation results of the aftershock sequences
（a） Distribution of the aftershock epicenters；（b，c） Focal depth profiles along longitude and latitude；（d，e） Distribution of focal depth and topography along profile AA′ and BB′
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图 1 研究区地震台站和断裂分布图（a）及两次芦山地震的主震和余震分布（b）
F1：盐井—五龙断裂；F2：盐井—五龙分支断裂；F3：双石—大川主断裂；F4：双石—大川分支断裂；F5：新开店断裂；F6：名山断裂；F7：大邑断裂，下同
Figure 1. Distribution of seismic stations，faults in the studied area （a） and detailed distribution of mainshocks and aftershocks of the two Lushan earthquakes （b）
F1：Yanjing-Wulong fault；F2：Yanjing-Wulong branch fault；F3：Shuangshi-Dachuan main fault； F4：Shuangshi-Dachuan branch fault；F5：Xinkaidian fault；F6：Mingshan fault；F7：Dayi fault，the same below
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图 2 直角坐标系（左）和对数坐标系（右）下的震级-时间分布图（a）及每日余震数量统计图（b）
Figure 2. Magnitude-time plots of earthquake sequences （a） and daily number of aftershocks （b） with rectangular coordinate system （left） and logarithmic coordinate system （right）
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图 4 震相时距曲线（a）及双差重定位所使用的速度模型（Fang et al，2015）（b）
Figure 4. Travel time-distance curves of seismic phases （a） and velocity model used in the double-difference relocation （Fang et al，2015） （b）
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图 6 重定位前（a）、后（b）的震源深度分布图
Figure 6. Histogram of focal depth distribution before （a） and after （b） earthquake relocation
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图 7 2014年4月20日（芦山MS7.0地震一年后）至2022年5月31日（芦山MS6.0地震发震前）两次芦山地震震区的地震分布（数据来自四川省地震局余震观测报告）（a）及2022年芦山MS6.1地震前区域Ⅰ（b）和区域Ⅱ（c）的震级时间分布图。区域Ⅰ和区域Ⅱ分别为2022年芦山MS6.1和2013年芦山MS7.0地震研究区
Figure 7. Seismic distribution in the two Lushan earthquake areas between 20 April 2014 （one year after the 2013 Lushan MS7.0 earthquake） and 31 May 2022 （before the 2022 Lushan MS6.1 earthquake）（data from the China Earthquake Network Center）（a） and the magnitude-time plots in earthquake studied area Ⅰ （b） and Ⅱ （c） before the 2022 Lushan MS6.1 earthquake. Earthquake studied area Ⅰ and Ⅱ are the selected studied areas for the 2022 Lushan MS6.1 earthquake and the 2013 Lushan MS7.0 earthquake，respectively
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图 8 芦山MS6.1地震和MS7.0地震及其余震分布（a）以及沿CC′ （b）和DD′ （c）剖面的P波速度结构和震源深度剖面图。vP速度结构来自肖雨辰和吴建平（2022）的双差层析成像研究
Figure 8. Distribution of aftershocks of the Lushan MS6.1 earthquake and Lushan MS7.0 earthquake （a） and the distribution of the vP velocity structure （Xiao，Wu，2022） and focal depth along profile CC′ （b） and DD′ （c）
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