Abstract: On January 19, 2020, an M_W6.0 earthquake occurred in Xikeer town, Xinjiang, northwest China. This earthquake was another strong earthquake event that occurred on the Kepingtage fold-and-thrust belt(FTB)after the 2003 Bachu-Jiashi M_W6.3 earthquake. The Kepingtage FTB is bounded by the southern Tian Shan area to the north and the Tarim Basin to the south. The Kepingtage FTB is ~300km long from east to west and 60~140km wide from north to south. It is composed of a series of monoclinal or anticlinal mountains(fold-and-thrust)with a near east-west direction and parallel distribution. Combined with the focal mechanism, seismic reflection profiles, and interferometric synthetic aperture radar coseismic deformation, we can reveal the seismogenic structure of this earthquake. The Jiashi event was mainly of a horizontal compression movement; the slip distribution was concentrated at a depth of 4~6km, and the fault-slip angle was~15°. Our results show that the seismogenic structure of the Jiashi event is the Keping thrust fault at the leading edge of the Kepingtage FTB. We carried out detailed field surveys, measurements, and drone aerial photography of the earthquake area after the earthquake. In the magistoseismic area(Ⅷ degrees), a lot of seismic geological disasters were found, including ground fissures, sand liquefaction, and collapse. This paper summarizes and describes the characteristics of geological hazards from four observation points. In observation point 1, a large area of ground fissures were developed in the area of Xikeer overpass. According to the statistics of ground fissures of this area, the dominant direction of the ground fissures is NEE, the south-north extrusion uplift is 0.1~0.15m, and the horizontal displacement is 0.05~0.1m. In observation point 2, the earthquake caused serious damage to the Xikeer dam, creating the tensile fissures at the dam crest, with a maximum depth of 4m and a maximum length of 900m. In observation point 3, a series of ground fissures were observed parallel to the road in the west of Xikeer town, and the length of ground fissures is~500m. A large area of sand liquefaction developed along the ground fissures, and the liquefied material was gray brown argillaceous silty. In observation point 4, a series of large and huge fresh rock collapses developed in the Shankou gully north of the epicenter. The largest single collapse is 50~100m³, and the largest collapse range is about 200~300m². According to the field investigation and dynamic calculation results, the maximum horizontal deformation is 29.8cm, located downstream of the dam crest. The horizontal deformation upstream of the dam crest is 22.35cm. Because of the sand liquefaction that occurred behind the dam, local settling of the foundation behind the dam also occurred. The horizontal deformation upstream and downstream of the dam crest are inconsistent, which produced the longitudinal fissures on the dam crest. We collected a large amount of strong-motion earthquakes data from the 2020 Jiashi earthquake. By combining the fault strike and upper and lower wall effects, the PGAs of the foreshock, main shock, and aftershocks were fitted, and isoseismal lines were generated. The Xikeer dam is located at the region where the vibration intensity of the Jiashi event was the highest. The effects of the aftershocks were also superimposed mainly in this area. Notably, sand liquefaction and most of the fissures were caused by the main shock, while the aftershocks(M_S>4.0)exacerbated this damage. However, in this study, we could only determine the extent of the damage caused by the main shock, because our detailed field investigation and drilling were conducted in April 2020, after the main shock and aftershocks.

Key words: The 2020 M_W6.0 Jiashi earthquake, seismic geological hazard, distribution characteristics, superposition effect

摘要： 2020年1月19日新疆伽师M_W6.0地震发生在南天山柯坪塔格前陆冲断带前缘的柯坪逆断裂(KPT)上, 是该区域自2003年巴楚-伽师M_W6.3地震后发生的最大地震。 震后对震区进行了详细的实地调查、 测量和无人机航拍, 在极震区(Ⅷ度区)内发现了大量地震地质灾害, 主要包括地裂缝、 砂土液化和崩塌等。 文中对震区的4个观察点进行了总结, 并描述了地质灾害的特征: 1)在西克尔互通立交(观察点1)区域内发育了大量纵横交错的地裂缝, 对这部分地裂缝进行统计发现, 其优势走向为NEE, SN向挤压抬升量为0.1~0.15m, 水平位错量为0.05~0.1m; 2)地震对西克尔大坝(观察点2)造成了严重的破坏, 在坝顶形成最大深度为4m、 长约900m的拉张性地裂缝, 坝后也出现一系列砂土液化, 喷砂锥的最大直径达3m; 3)西克尔库勒镇以西(观察点3), 路面上发育了多段与公路平行、 长约500m的地裂缝, 同时沿地裂缝出现大面积砂土液化, 液化物质为灰褐色泥质粉砂; 4)震中北部山口沟(观察点4)沿线发育了一系列大型、 巨型的新鲜岩质崩塌, 最大单体崩塌物的体积为50~100m³, 最大崩塌倒石堆的范围为200~300m²。 综合分析上述资料发现, 由于观察点4位于KPT上盘, 在震后未能及时对该观察点进行调查, 导致2020年1月19日伽师M_W6.0地震北部的烈度影响范围被低估, 尤其是Ⅷ度圈。 结合现场调查和数值模拟分析西克尔大坝的破坏机理, 认为坝后的砂土液化导致坝基出现不均匀沉降, 使得坝前坡和坝后坡出现了不等的水平位移(坝前坡为22.35cm, 坝后坡为29.8cm), 导致大坝向下游方向旋转, 故在坝顶形成了拉张性的贯通地裂缝。

关键词: 2020年伽师地震, 地震地质灾害, 分布特征, 叠加效应