A New Tectonic Model for the 1927 M8.0 Gulang Earthquake on the NE Tibetan Plateau

The rupture patterns of large earthquakes in transpressional systems can provide important information for understanding oblique motion and strain partitioning between tectonic blocks. The 1927 M8.0 Gulang earthquake occurred on the transpressional boundary between the Tibetan and Gobi‐Alashan blocks. Our results, combined with those of previous studies show that the Lenglongling fault (LLLF) and Southern Wuwei Basin fault (SWBF) have both ruptured during the Gulang earthquake, but they exhibited different motions. An ~120‐km‐long surface rupture zone formed along the LLLF, with a left‐lateral strike‐slip motion and a coseismic horizontal offset of ~2.4–7.5 m. Bending, bifurcation, and change of the slip sense occurs at both ends of the fault. An ~42‐km‐long rupture zone formed along the SWBF, with a coseismic vertical offset of ~0.6–2.8 m. Thus, the Gulang earthquake is a complex rupture event where strike‐slip and thrust faults ruptured simultaneously. Analysis of deep and shallow structures and three‐dimensional finite‐element modeling reveal that the north dipping LLLF and the SWBF may converge downward to a low‐angle décollement, accommodating the strike‐slip and thrust motions during the earthquake, respectively. This pattern of deformation partitioning is similar to some other earthquakes where oblique block convergence is generally partitioned into strike‐slip motion on steeply dipping faults and vertical motion on gently dipping faults. Our study also suggests that the strain partitioning pattern in the NE Tibetan Plateau may be controlled by changes of the regional principal compressive stress directions and fault geometry at depth. Key Points The Gulang earthquake is a complex rupture event where strike‐slip and thrust faults ruptured simultaneously The north dipping Lenglongling fault and the Southern Wuwei Basin fault may converge downward to a low‐angle décollement Oblique convergence is usually partitioned into strike‐slip motion on steeply dipping faults and vertical motion on gently dipping faults