Abstract:

As the dynamic hub between collisional orogenic belts and foreland basins, the formation and propagation of foreland fold-and-thrust belts are often accompanied by destructive earthquakes. In this process, the detachment, which is regarded as an incompetent layer, plays a significant controlling role in the propagation of the thrust system and the occurrence of strong earthquakes. Therefore, research focused on the structural patterns and physical property architecture of foreland fold-and-thrust belts is conducive to analyzing their activity characteristics and seismogenic patterns, thereby providing a basis for regional seismic hazard assessments.
The western Kunlun orogenic belt is situated at the Qinghai-Xizang Plateau’s northwestern margin and the Tarim Basin’s southwestern edge. Governed by the convergence of the Indian and Eurasian plates, the compressive collision between the Qinghai-Xizang plateau and the Tarim Basin has led to the formation and propagation of the Western Kunlun foreland fold-and-thrust system, triggering intense and frequent tectonic and seismic activities within the region. A M_W6.4 earthquake occurred in Pishan on July 3, 2015 in the front of western Kunlun, which was one of the most destructive seismic events that happened in Xinjiang in the recent decade. The epicentral area of this earthquake experienced an intensity of VIII, with many regions feeling strong tremors. The disaster area was vast, affecting a large population, and was accompanied by hazards such as sand liquefaction, ground fissures, and collapses. Studies on this event will provide an opportunity to better understand active structures, stress state, and future seismic hazard of the front of western Kunlun and the adjacent region.
Synthesizing geology, geomorphology and seismology studies, Slik Anticline, located on the western Kunlun foreland fold-and-thrust belt, is regarded as the seismogenic fault of this event. The Slik Anticline is located within the Guman fold belt of the Hotan thrust belt, which lies in the eastern segment of the western Kunlun foreland. The Slik ramp, situated beneath the Slik Anticline, is approximately at a depth of 10 to 15km, with a ramp angle of about 20 degrees. The Pishan earthquake ruptured blind ramp under the Slik Anticline. From north and shallow to south and deep level, the fault is revealed with the structural style of lower flat, ramp and upper flat and may merge with other thrust in a deeper part near to the hinterland. The upper and lower flats are corresponded to regional detachment of lowest Paleogene and inner Cambrian incompetent strata, respectively. And the mainshock occurred at the Slik ramp which connecting these two detachment, which implicates that the Pishan earthquake is an event caused by independent structural unit. The postseismic deformation mainly occurred on the lower flat as afterslip, and accumulated gradually over time. The moment released after a slip in 5 months corresponds to a sub-M_W6.3 event, and is equivalent to the moment released by the mainshock.
Worldwide, earthquakes exhibiting similar characteristics include the 2015 Gorkha M_W7.8 earthquake in Nepal, the 2005 Kashmir M_W7.6 earthquake, the 1999 Chi-Chi M_W7.6 earthquake in Taiwan, China, and the 1995 Colima-Jalisco M_W8.0 earthquake in Mexico, among others. Conducting detailed analyses of these seismic events provides a valuable entry point for clarifying the structural patterns, stress states, and seismogenic features of foreland fold-and-thrust fault zones. Furthermore, it aids in the in-depth analysis of regional seismogenic models and predicting future seismic trends.
Focusing on the Pishan earthquake, expect settling slip by the folding strata above the upper flat; this phenomenon may also depend on both matter properties and gravitational differences between the upper and lower flats. 1)During the fault deformation, a detachment with low competence and low friction coefficient is unable to accumulate significant stress, resulting in the absence of notable stress drops during frictional sliding and instead manifests as a state of after-slip activity. 2)When the detachment, especially the shallow one, has an undersized gravitational load, it may fail to effectively play a role in detachment, leading to a geological process where fault activity may be altered or inhibited. Occurring at the lower flat, the afterslip of the Pishan earthquake cooperated these two situations. 1)The deeper detachment, located on the Cambrian gypsum, may have a lower friction coefficient compared with the upper detachment. 2)And apparently, the deeper detachment possesses more gravitational load. 3)Plus, stress cannot accumulate to a high level at a flat part of a seismogenic fault. These points may lead to the afterslip release of accumulated stress, mostly on the lower flat, as the postseismic deformation. Therefore, the risk of a major earthquake occurring at the lower flat of the fault may be relatively low.
On the one hand, this phenomenon may cause stress loading on the lower fault ramp or the root fault at a deeper level below the fault flat, increasing the seismic risk in the deep structures. On the other hand, if an earthquake was trigged on the lower ramp, the lower flat above it, which had undergone a slip and released a significant amount of energy, would limit the extent of the rupture.
Furthermore, Cambrian gypsum layers are widely present at depth, which can serve as deep detachment surfaces and lower fault flats at the scale of the study area and its adjacent region. Therefore, whether the seismic activity and energy release pattern of the Slik anticline can represent the seismogenic patterns in the front of western Kunlun, and whether such a pattern can effectively provide a basis for regional seismic hazard assessment, deserves further attention and research.

Key words: Western Kunlun, Pishan earthquake, seismogenic fault, ramp, afterslip

摘要：

2015年7月3日, 西昆仑山前皮山地区发生了皮山 M_W6.4 地震, 该地震是新疆近十年来造成灾害较为严重的一次地震。针对此次地震开展研究, 有助于分析西昆仑山前及邻区的构造活动特征、 应力状态及未来地震趋势等。在结合地质、 地貌、 地震剖面、 震源机制解和精定位等数据开展综合研究的基础上, 西昆仑山前斯力克背斜下伏盲断坡被确定为皮山地震的发震构造, 此次地震沿盲断坡发生了破裂。发震断层结构由深至浅主体呈现出下断坪-断坡-上断坪的构造样式, 在更深部通过下断坡逐渐与山根附近的逆冲断裂汇合。断层的上、 下断坪分别对应于区域古近系底部和寒武系内部的2套滑脱层, 主震则发生在连接二者的斯力克断坡。震后, 在斯力克断坡下倾方向的下断坪处发生了较为明显的震后余滑, 这部分变形随时间推移逐渐累积, 在5个月内释放的地震矩接近 M_W6.3 地震的释放量, 与此次主震的释放量几乎相当。这可能与断层上、 下断坪之间的物性差异及重力加载不同等有关; 也可能指示了断层在深部和浅部分别以震后余滑和发震的形式释放累积应力的特征, 对揭示斯力克背斜, 乃至西昆仑山前的孕震模式具有一定意义, 值得进一步研究。

关键词: 西昆仑, 皮山地震, 发震断层, 断坡, 震后余滑