Revisiting surface ruptures of the 1955 Zheduotang earthquake (M ~7.5) in eastern Tibet: kinematic implications on the southern Xianshuihe fault zone

Surface ruptures associated with large historical earthquakes provide critical insights into earthquake magnitudes and the kinematics of their seismogenic faults. In 1955, a major earthquake occurred along the Zheduotang fault, a segment of the southern Xianshuihe fault zone in eastern Tibet. The magnitude of this earthquake has been a subject of debate, with estimates ranging from M6.6 to M7.5, primarily due to conflicting interpretations of its associated surface ruptures. This study reviews previous research on the surface ruptures of the 1955 Zheduotang earthquake and presents new field data, including unmanned aerial vehicle (UAV)-based topographic surveys, trench excavations, and lichenometry in the epicentral region. Evidence from the freshness of ground ruptures, dating of faulting events from trenching, and lichen size measurements supports a ~55 km long surface rupture zone, corresponding to a moment magnitude (Mw) of ~7.1 for the 1955 earthquake. Analysis of offset glacial interfluves reveals a late Quaternary left-lateral slip rate of ~2.5–3.0 mm/yr in the southern segment of the Zheduotang fault, lower than ~3.4–4.8 mm/yr previously observed in the northern section. Deformed landforms and surface ruptures indicate that the fault trends NWN and exhibits predominantly left-lateral strike-slip motion in its northern section, while the southern segment trends NW and includes a notable normal faulting component. Our findings suggest that the Zheduotang fault delineates the southwestern boundary of the Bamei-Kangding releasing stepover zone within the southern Xianshuihe left-lateral strike-slip fault zone. These results enhance understanding of seismic hazards and the tectonic kinematics along the eastern boundary of the Tibetan Plateau.