In the epicenter of the Lushan M S 7.0 earthquake there are several imbricate active reverse faults lying from northwest to southeast, namely the Gengda-Longdong, Yanjing-Wulong, Shuangshi-Dachuan and Dayi faults. Emergency field investigations have indicated that no apparent earthquake surface rupture zones were located along these active faults or their adjacent areas. Only brittle compressive ruptures in the cement-covered pavements can be seen in Shuangshi, Taiping, Longxing and Longmen Townships, and these ruptures show that a local crustal shortening occurred in the region during the earthquake. Combining spatial distribution of the relocated aftershocks and focal mechanism solutions, it is inferred that the Lushan earthquake is classified as a typical blind reverse-fault earthquake, and it is advised that the relevant departments should pay great attention to other historically un-ruptured segments along the Longmenshan thrust belt and throughout its adjacent areas.Lushan earthquake, earthquake surface rupture zone, blind reverse-fault earthquake, Longmenshan thrust belt, Qinghai-Tibetan Plateau Citation:Xu X W, Wen X Z, Han Z J, et al. Lushan M S 7.0 earthquake: A blind reserve-fault event.

Denudational history inferred from bedrock low-temperature thermochronological data provides significant constraint to tectonic models for explaining the growth of the Tibetan Plateau. Here, we present new apatite fission-track ages from the Luoji Shan, bounded by the Anninghe and Zemuhe faults in the southeastern Tibetan Plateau. Cooling ages vary from c. 5 to c. 13 Ma from the bottom to the top of a vertical profile. The age–elevation relationship can be interpreted as either (1) a phase of river incision at a rate of c. 0.16 km Ma −1 from c. 13 to c. 5 Ma or (2) a phase of denudation of c. 0.40 km Ma −1 between c. 13 and c. 10 Ma, followed by a phase of extremely slow denudation ( c. 0.03 km Ma −1 ). The results suggest that initiation of river incision started before c. 13 Ma in the study area. Combined with previously published data from adjacent sites, our results indicate that denudation in this region displayed substantial spatial variation. This finding does not support the existing lower crustal flow model for Tibetan Plateau growth, which predicts uniform denudation. We further speculate that the vertical component of slip along the southern extension of the Anninghe fault might have significantly declined since late Miocene time, explaining the decreased rate of erosion c. 10 Ma.

In order to understand the crustal structure and tectonic background of the Changning–Gongxiang area, southeastern Sichuan Province, where a series of moderate‐to‐strong earthquakes occurred in recent years, we utilized the seismic phase data both from a local dense array and from the regional seismic networks; we used the tomoDD program to invert for the high‐resolution three‐dimensional velocity structure within the depth range of 0–10 km and for accurate hypocentral locations in this area. We analyzed the seismogenic structures for the events of XingwenM5.7 in 2018 and Gongxian M5.3 and Changning M6.0 in 2019. The results show that: (1) widespread lateral inhomogeneity exists in the velocity structure of the study area, and the location of the velocity anomaly is largely consistent with known structures. In the range of distinguishable depth, the inhomogeneity decreases with increasing depth, and the velocity structure anomalies in some areas are continuous in depth; (2) earthquakes occurred in clusters, showing the characteristics of zonal folding trends in the NW‐SE and NE‐SW directions; the focal depth in the area is generally shallow in both the sedimentary cap and the crystalline basement. The seismogenic structures of small earthquake clusters are different in size and occurrence in different sections, and the clusters occurred mostly in regions with high P‐ or S‐wave velocities; (3) synthesis of a variety of data suggests that the seismogenic structures of the Xingwen M5.7 and Changning M6.0 earthquakes are associated with slip faults that trend NW‐SE in, respectively, the south wing and the axis of the Changning–Shuanghe anticline, while that of the GongxianM5.3 earthquake is associated with thrust faults that trend N‐S in the Jianwu syncline region. The dynamic sources of the three earthquakes are all from the SE pushing of the Qinghai–Tibet block on the Sichuan basin; (4) the risk of future strong earthquakes in this area must be reevaluated in light of the facts (a) that in recent years, moderate‐to‐strong earthquake swarms have occurred frequently in southeast Sichuan; (b) that the complex structural area exhibits the easy‐to‐trigger characteristic, and (c) that the small‐scale faults in this area are characterized by the phenomenon of stress “lock and release”.

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