Variation in the On‐Fault Coseismic Offset Associated With the 2008 Mw 7.9 Wenchuan Earthquake in Eastern Tibetan Plateau and Its Implications for Active Thrust Fault Deformation

The characteristics of coseismic surface ruptures associated with large earthquakes are important for understanding fault deformation mechanisms and seismic rupture behavior. We revealed that the variation in the coseismic vertical offset at a short wavelength scale produced by the 2008 Wenchuan Mw 7.9 earthquake occurred in the Longmen Shan fault zone along the eastern Tibetan Plateau. Field investigations, including topographic surveys, trenching, and geophysical surveys, revealed that the coseismic vertical offset gradient ranged from −5 to −24 m/km at the Shenxigou site, whereas the corresponding amount reached −100 m/km at the Pingxi site. Both sites show a strong relationship between the coseismic vertical offset and near‐surface sediment. Furthermore, we used a two‐dimensional discrete element numerical simulation to reveal that sediment properties, including strength and thickness, are the first‐order contributors to the along‐strike high‐frequency coseismic offset variation within tens of meters. Moreover, we suggest that near‐surface sediment might directly affect off‐fault deformation, which then affects fault deformation patterns and offset amounts. These results have significant implications for understanding the spatial heterogeneity of coseismic deformation as well as for probabilistic seismic hazard analysis for active thrust faults.