Abstract:

The nearly north-trending rift system and nearly east-west strike-slip faults are the major structures accommodating the east-west extensional deformation within the Qinghai-Xizang Plateau. The rift systems are roughly separated by the nearly east-west striking Indus-Yarlung suture zone into north and south parts. The eastern rift systems are a strongly active major seismic zone in Xizang, which is characterized by faulted landforms of alluvial fans, river terraces, and moraines with large magnitude earthquakes. The Yadong-Gulu rift experienced the 1411 Damxung M8 earthquake and the 1952 Gulu M7.4 earthquake. On January 7, 2025, a M_S6.8 earthquake occurred along the southern segment of the Xainza-Dinggye rift in Dingri County, Xigaze, which caused widespread concern about the seismic and Late Quaternary active behaviors along the rift systems. However, few studies on fault activity at the junction of the north and south segments of the rift systems were conducted along both side of the Indus-Yarlung suture zone, which greatly hinders us from understanding the active deformation process and seismic activity of the rift systems in southern Qinghai-Xizang Plateau.

The Xainza-Dinggye rift can be divided into the north and south segments, named the Xainza and Dinggye rifts, by the Indus-Yarlung suture zone. The Xietongmen and Dengmecuo segments are the northern and southern adjacent segments of Xainza-Dinggye rift, respectively. The Xietongmen segment as the south end of the Xainza rift intersect with the Indus-Yarlung strike-slip fault. There are no strong earthquakes of over M6 recorded near the Xietongmen segment, and the small and medium earthquakes are significantly less than those of other segments. The 2025 Dingri earthquake has caused great social concern about whether a larger earthquake will occur between the Dengmecuo and Xietongmen segment of the Xainza-Dinggye rift, especially in the densely populated area of Xietongmen County. A recent earthquake risk survey has not yet found evidence of Holocene activity along the Xietongmen segment. Meanwhile, previous studies suggest very low activity during the late Quaternary along the Dengmecuo segment, which is in stark contrast to current strong earthquake activities. Therefore, there are still great uncertainties about the fault geometry and activity of the Xainza-Dinggye rift on both sides of the Indus-Yarlung suture zone.

To determine the Late Quaternary activity of the Xietongmen to Dengmecuo segments of the Xainza-Dinggye rift, we used remote sensing interpretation, field survey, optically stimulated luminescence, and radiocarbon dating methods on the displaced landforms. We found new evidence of the latest activity on the eastern and western branches of the Xietongmen segment at the past millennium. The western branch of the Xietongmen segment has crossed the Yarlung Zangbo River southward, then terminates at the intersection basin with the latest active branches of southern Yarlung Zangbo fault. Our results also suggest the Dengmecuo segment had strong activity with several hundred meters width since the late Quaternary. The latest faulting has extended northward into the mountainous area according to the surface ruptures and positioning aftershocks of the M_S6.8 earthquake. The nearly east-west striking Indus-Yarlung suture zone constitutes a north-trending structural gap zone between the two segments. The fault geometries, displaced landforms, and seismic activity of these two segments reveal that they are probably approaching by cutting through this structural gap, and the seismic risk cannot be ignored in the future. Due to the limitations of existing data, further detailed field investigations, geodetic observations, and geophysical deep data are needed to verify and improve our speculation. Our results of the Late Quaternary activity of the Xietongmen to Dengmecuo segments of the Xainza-Dinggye rift provide scientific support for seismic risk assessment of national engineering projects and post-disaster reconstruction of the recent M_S6.8 earthquake in the Xigaze region.

Key words: Xainza-Dinggye rift, 2025 Dingri M_S6.8 earthquake, active tectonics, Late Quaternary activity, Qinghai-Xizang Plateau

摘要：

青藏高原近SN向的裂谷系与近EW向的走滑断裂是调节高原内部EW向伸展变形的主要构造。然而, 关于藏南裂谷系南、 北2段衔接部位雅鲁藏布江缝合带两侧的断层活动性研究工作较少, 极大阻碍了对裂谷系变形过程与地震活动的认识和理解。文中聚焦申扎-定结裂谷系南、 北2段相邻的谢通门段和登么错段, 采用遥感解译、 野外调查、 光释光与¹⁴C年代学等方法, 发现谢通门段东、 西支断裂均为全新世活动断裂, 最新一次事件发生在距今1ka以来; 谢通门西支断裂往S穿过了雅鲁藏布江缝合带的北缘断裂, 终止于缝合带晚更新世活动的南缘断裂; 登么错段晚第四纪活动强烈, 2025年定日6.8级地震的地表破裂和精定位余震揭示该段正在向北侧扩展。因此, 推测申扎-定结裂谷系可能正在持续扩展, 谢通门段与登么错段所夹持的断层空段未来的强震风险不容忽视。文中研究可为日喀则区域重大工程地震安全性评价和近期 M_S6.8 定日地震震区灾后重建提供断裂活动性数据支撑。

关键词: 申扎-定结裂谷系, 2025年定日6.8级地震, 活动构造, 晚第四纪活动性, 青藏高原