Deformation characteristics of co-seismic surface ruptures produced by the 1850 M 7.5 Xichang earthquake on the eastern margin of the Tibetan Plateau

摘要

The eastern margin of the Tibetan Plateau is bounded by a N-S-trending active fault zone, including the Zemuhe Fault, which is an important tectonic boundary in studies of continental dynamics and deformation modes. Here, we present the kinematic mechanism and deformation features of co-seismic surface ruptures caused by a large historic earthquake that occurred upon the Zemuhe Fault. Field investigations and interpretations of aerial photographs reveal a NNW-SSE-trending surface rupture zone, less than 100 m wide, defined by en echelon fractures, mole tracks, and fault scarps over a length of 60 km. Trench excavations and radiocarbon age data confirm that the surface rupture zone was produced by the 1850 M 7.5 Xichang earthquake. Gullies preserved on the youngest alluvial fans and the lowest terrace risers in the area record systematic left-lateral offsets of 1.4-6.0 m and vertical offsets of 0.2-0.65 m resulting from the 1850 earthquake. Topographic and geologic evidence reveals that the spatial distribution of the 1850 co-seismic surface rupture was controlled by pre-existing geological structures of the Zemuhe Fault. The present results confirm that the Zemuhe Fault plays an important role as a major strike-slip fault in the southeastward motion of the southeastern Tibetan Plateau, and that the southern segment of the Zemuhe Fault has high seismic potential.