关键词: 2025年定日M_S6.8地震, 晚第四纪活动, 垂直滑动速率, 登么错断裂, 定结-申扎裂谷

Abstract:

On January 7, 2025, a significant earthquake occurred in Dingri, Shigatse, China. Both the China Earthquake Networks Center and the USGS provide focal mechanism solutions indicating that the earthquake was a normal fault event. The epicenter was located in the Dengme Cograben, part of the southern segment of the Dinggye-Xainza rift, with the seismogenic fault identified as the bounding normal fault of the Dengme Cograben, the Dengmecuo fault. The late Quaternary throw rate of this fault is crucial not only for regional seismic hazard assessments but also for understanding the east-west extensional deformation within the Tibetan plateau. Previous studies on the kinematics of the Dengmecuo fault report varying throw rates: (0.28±0.04)mm/a since ~56ka, (0.28±0.04)mm/a since ~50ka, and(0.09±0.03)mm/a since ~95ka. However, the observed maximum coseismic vertical displacement of ~3m during the January 2025 earthquake suggests that the maximum cumulative time for this displacement is approximately 37ka, based on the published late Quaternary throw rate. This is inconsistent with the ~5ka elapsed time since the most recent earthquake on this fault, highlighting the uncertainty in the late Quaternary throw rate and limiting our understanding of strain partitioning and seismic risk in the southern Dinggye-Xainza rift.

To resolve this uncertainty, we combine high-resolution remote sensing image interpretation with field geological and geomorphological surveys to determine the geometric distribution of the Dengmecuo fault. Two study sites were selected along the fault, each featuring clear faulted geomorphology suitable for dating. Low-altitude photogrammetry using an unmanned aerial vehicle(UAV)was combined with optically stimulated luminescence(OSL)and AMS ¹⁴C dating to refine the late Quaternary throw rate of the Dengmecuo fault and assess its seismic hazard.

Our results show that the Dengmecuo fault, which strikes NNW and dips NW or W, is approximately 58km long and can be divided into northern, central, and southern segments. The northern segment is defined by the Laangshuiku area, while the southern boundary is located at the edge of Pum Qu. In the northern study site, vertical offsets of 19.2(+3.5/-2.3)m on the T2 alluvial fan and 8.0(+0.9/-0.7)m on the T1 alluvial fan correspond to formation ages of(28.3±1.4)ka and(12.0±1.5)ka, respectively. By matching the vertical offsets with their respective formation ages, we estimate a throw rate of (0.7±0.1)mm/a. However, the throw rate for the T1 fan is uncertain, as its vertical offset is smaller than the cumulative displacement since its formation. At the southern study site, combining a vertical offset of 5.3(+0.3/-0.5)m with the formation age of the T1 terrace ((9.2±1.0)ka), we calculate a throw rate of(0.6±0.1)mm/a.

Overall, our results indicate late Quaternary throw rates of(0.7±0.1)mm/a since ~28ka and(0.6±0.1)mm/a since the Holocene. Additionally, using the relationship S=D/R_x, where S is the slip rate, D is displacement, and R_x is the recurrence interval, we estimate a slip rate of(0.5±0.1)mm/a based on the average value of the maximum displacement(2.5~3m)of 2025 Dingri M_S6.8(Shigatse)earthquake as 2.75m and the interval of paleoseismic events during the Holocene as(5500±1100)a. This result is consistent with the throw rate of(0.6±0.1)mm/a since the Holocene determined from faulted geomorphic surfaces.

Finally, combining the throw rate of (0.6±0.1)mm/a since the Holocene with the ~5ka elapsed time since the most recent earthquake, we conclude that the Dengmecuo fault had accumulated 2.5~3.5m of coseismic displacement before the 2025 Dingri M_S6.8 earthquake, corresponding to a magnitude of M_W6.9-7.0. These parameters align with the 2025 M_S6.8 earthquake, indicating that the Dengmecuo fault posed a significant seismic risk prior to the event.

Key words: 2025 Dingri M_S6.8 earthquake, Late Quaternary activity, throw rate, Dengmecuo fault, Dinggye-Xainza rift