Discovery of the surface rupture zone along the southern branch of the Longshoushan Fault Zone, NW China and its significance to the deep structures of the 1954 Shandan MS 7¼ earthquake

A detailed field-based remote sensing interpretation in this study revealed abundant recent seismic surface ruptures on the southern margin of the Longshoushan Fault, including fault scarps, mole tracks and displacements of river channels. The total length of the surface rupture zone is over 20 km. The most recent vertical and horizontal displacements of the late Quaternary geomorphic markers fall into the range of 0.35~4 m and 0.3~1.9 m, respectively. The southern Longshoushan Fault is a high-angle thrust fault, and it only exhibits left-lateral strike-slip on the west end. Revealed by profile and trench along the surface rupture, the southern branch of the Longshoushan Fault Zone has gone through several earthquake events since the Holocene, and the latest one happened around 3.96 ka. By comparing with the strongest earthquake recorded in this region, it is suggested that the 1954 MS 7¼ Shandan earthquake may have caused the newly discovered surface ruptures, which developed along both the southern and northern branches of the Longshoushan fault zone, presenting a positive flower structure at the shallow surface. This coseismic displacement distribution mostly is found in strike-slip earthquakes in previous studies, but it is found in a thrust earthquake this time. The discovery of coseismic surface rupture on the southern branch of the Longshoushan Fault Zone will throw light on the focal mechanism and rupture pattern of the 1954 Shandan MS 7¼ earthquake.