Abstract:

At 2:09 on January 23, 2024(Beijing time), an earthquake of M_S7.1 occurred in Wushi County, Aksu region, Xinjiang Uyghur Autonomous Region. This earthquake is the first earthquake with a magnitude greater than 7 in the Tian Shan seismic belt after the 1992 Susamer M7.3 earthquake. This earthquake occurred near the range-front Maidan fault which is the boundary structure between the southern Tian Shan and Tarim Basin. In fault geometry, the Maidan fault shows a complex geometry. On the plane, the Maidan fault comprises multiple roughly parallel secondary faults. On the profile, the gently dipping mountain front fault controls the folding deformation of the late Cenozoic. The front and back mountain faults converge in the deep part. This causes the Tarim Plate to deeply subduct along a large subduction zone under the Tianshan. The thrust-nappe structural system is often composed of multiple fault zones that can generate strong earthquakes because these fault zones converge to the same detachment surface in the deep part. Because a fault zone triggers or suppresses strong seismic activity in adjacent faults after a strong earthquake.
Because the main earthquake did not generate obvious coseismic surface ruptures, there is still a great controversy and uncertainty on its seismogenic structure. Determination of the seismogenic structure is crucial for analyzing the potential location of the next major earthquake. It is of great significance for evaluating the future risk of strong earthquakes in the region and the stress loading and triggering relationships between different faults.
According to the analysis of Google’s high-precision satellite image interpretation results and field inspections, the geometric structure of Maidan is more complicated. The fault is roughly near the ancient stream of Yu Shangu Xi River, and it can be divided into two branches, east and west of the left steps. The faults all show clear signs of activity at the surface. In this study, we utilize the earthquake relocation results to determine the seismogenic structure of the 2024 Wushi event. Our results show that the main shock began to break in the deep and the aftershocks are extended from deep to the part. The deeper focal depth may be an important factor in preventing the coseismic sliding of the Wushi M_S7.1 earthquake from being transmitted to the surface. The seismic sequence is exhibited by the northeast-southwest. The long axis direction is about 55°, and the total length is about 85km. The aftershock sequence is divided into the northeast, middle section, and southwest section. The rupture range of the Wushi M_S7.1 earthquake is about 35km, which is the middle section of the aftershock sequence. The aftershocks in the northeast section are mainly distributed along F_1-1, the aftershocks in the southeast section are mainly distributed along F_2-2 and the aftershocks in the middle section are mainly distributed along F_1-2. The aftershocks of different sections are distributed on different branches, which means that the strong earthquake triggered the adjacent seismic activity, which belongs to a more complicated grade joint rupture earthquake. At the same time, the CAP waveform counter and method was adopted to obtain the focal mechanism. The nodal planes parameters of the best double-couple focal mechanisms are: strike 115°, dip 52° and rake 132° for nodal plane I, and strike 240°, dip 54° and rake 49° for nodal plane Ⅱ, the depth of the centroid is about 17km. Based on the inversion results of the focal mechanism and the spatial distribution characteristics of the earthquake sequence, we believe that nodal plane II is the seismogenic fault plane. Based on the spatial distribution characteristics of the earthquake sequence, the focal mechanism solution, and the geological structure data of the earthquake area, we suggest that the Biedieli-goukou fault is the piedmont branch of the Maidan fault. The upper part of the fault(4~5km)has no rupture, and it still has a strong risk of strong earthquakes in the future. The Wushi M_S7.1 earthquake triggered the Biedieli fault and the Aheqi fault. During the Late Quaternary period, these faults have repeatedly ruptured to the surface of strong earthquake incidents.

Key words: 2024 M_S7.1 Wushi earthquake, HypoDD localization, earthquake sequence, seismogenic structure

摘要：

北京时间2024年1月23日2时9分在新疆维吾尔自治区阿克苏地区乌什县发生 M_S7.1 地震, 此次地震发生在西南天山山前的迈丹断裂附近。迈丹断裂是西南天山与塔里木盆地的分界构造, 几何结构复杂。由于该次强震没有形成明显的同震地表破裂, 目前对乌什 M_S7.1 地震的发震构造还存在较大的争议和不确定性, 确定发震构造对于评估该地区未来强震风险及不同断裂之间的应力加载和触发关系具有重要意义。文中基于截至2024年3月31日的地震资料, 采用HypoDD方法对主震及余震进行了重新定位。结果显示, 地震序列整体呈NE-SW向展布, 长轴走向约为55°, 全长约85km; 同时, 采用CAP波形反演方法获得了主震的震源机制解: 节面I的走向为115°, 倾角为52°, 滑动角为132°; 节面Ⅱ的走向为240°, 倾角为54°, 滑动角为49°; 矩心深度为17km。综合地震序列空间分布特征、 震源机制解及震区活动构造资料等分析认为, 此次地震的发震构造为迈丹断裂东段的山前分支(别迭里沟口断裂), 破裂长约35km。85km长的余震条带是主震触发相邻分支断裂活动的结果, 属于一次较为复杂的级联破裂地震。该次地震的震源深度较深, 同震破裂主要发生在地表5km以下的深度, 别迭里沟口断裂未来仍具有较高的强震风险。另外, 乌什 M_S7.1 地震触发了别迭里断裂(F_1-1)和阿合奇断裂(F_2-2)活动, 后续也需要进一步关注这2条断裂的强震风险。

关键词: 乌什M_S7.1地震, HypoDD定位, 地震序列, 发震构造