<p>The Tibetan Plateau is one of the regions with the most active neotectonic movement and frequent seismic activity in the world. The Bayan Har Block, located in the Northern Tibetan Plateau, has been the main area for clusters of major earthquakes with <i>M</i> ⩾ 7 on the Chinese mainland in the past 28 years. The Kunzhong fault (KZF) is an important branch of the East Kunlun fault zone (EKF), which forms the northern boundary of the Bayan Har Block. The KZF is crucial for understanding regional tectonic evolution and seismic activity, but research on its Late Quaternary tectonic activity remains limited. On the basis of satellite image interpretation, field investigations, and high-resolution unmanned aerial vehicle photogrammetry, this study identifies an ∼40 km-long seismic surface rupture zone well preserved in the eastern segment of the KZF. The rupture zone is characterized by left-lateral dislocated small gullies, cracks and scarps, with a maximum co-seismic horizontal displacement of ∼2.9 ± 0.1 m. This earthquake’s magnitude is estimated to be <i>M</i>7.2 ± 0.2 according to the statistical relationships between parameters of strike-slip seismic surface rupture zone and magnitude. Based on the surface rupture characteristics, seismogenic faults, and historical seismic records, this earthquake may have been a clustered event occurring shortly before or after the 1937 Tuosuo Lake <i>M</i>_S7.5 earthquake. Alternatively, it may also have been a joint earthquake involving the KZF and EKF that occurred simultaneously with the Tuosuo Lake earthquake. Combined with deep crustal magnetotelluric profiles, these findings suggest that the region between the KZF and EKF forms a diffuse transition boundary zone in the northern Bayan Har Block. In seismic risk assessments of active faults, greater attention should be given to clustered or joint rupture earthquakes occurring along both main block boundary faults and their branch faults.</p>