Coseismic Surface Deformation Characteristics of the 1915 M7.0 Sangri Earthquake in Tibet

The spatial distribution and deformation characteristics of coseismic surface rupture zones are fundamental to understanding the rupture behavior of strong earthquakes. They provide critical insights for predicting the extent, scale, and degree of deformation of future events, which is of great significance for assessing the magnitude of potential seismic hazards.

The December 3, 1915, M7.0 Sangri earthquake in the Woka Graben (northern Cona-Woka Rift) is the region’s most recent major seismic event. Historical records place the epicenter near Zangga, identifying the Eastern Boundary Fault (EBF) as the primary seismogenic structure. However, its remote, high-altitude location and coarse legacy satellite imagery have left details undocumented and source parameters poorly constrained. To address this, we integrated UAV-derived centimeter-scale Digital Surface Models (DSM), orthomosaics, and field investigations. This enabled multi-scale, multi-perspective analysis of fault traces, surface rupture geometry, and coseismic deformation.

Refined mapping reveals that the seismogenic EBF manifests as a continuous, single-branch structure with a total length of approximately 60 km. The fault trace is well-defined and can be divided into northern and southern segments by the Delimuqu River. The northern segment extends ~29km in a nearly N-S direction with a westward dip, while the southern segment extends ~31 km with a NNE strike and a NW dip. A distinct coseismic surface rupture zone, ~35 km in length, developed primarily along the entire northern segment and the northern part of the southern segment of the EBF. Field measurements revealed a maximum coseismic vertical displacement of ~2.1m.

Furthermore, we utilized a MATLAB-based displacement measurement program to perform quantitative extraction of cumulative offsets and Cumulative Offset Probability Density (COPD) analysis across 225 investigation sites, yielding an average coseismic vertical displacement of ~0.79 m. Additionally, a fault scarp diffusion age modeling program was employed to constrain the extent of the coseismic surface rupture based on morphological degradation. Analysis of 362 measurement sites via COPD indicated an average diffusion age of 2.05 ± 0.88 kt for the coseismic scarps. The integration of spatial distributions for minimum mean diffusion ages and cumulative vertical displacements allowed us to quantitatively define the coseismic surface rupture length to ~32 km. This result is in excellent agreement with the ~35 km length derived from remote sensing interpretation, validating the reliability of the estimated rupture scale. Using empirical scaling relationships based on the obtained rupture length and the average/maximum vertical displacements, we re-estimated the earthquake magnitude to be Mw 6.71~6.84, highlighting the high seismic potential of the EBF. This study fills a critical gap in the detailed investigation of the coseismic surface rupture of the 1915 Sangri earthquake and underscores the significant utility of high-resolution topographic data in active tectonics research.