Characteristics of geological hazards in the epicenter of the Jiashi MW6.0 earthquake on January 19, 2020

On January 19, 2020, an MW6.0 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 MW6.3 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(MS>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. © 2021, Editorial Office of Seismology and Geology. All right reserved.