Based on field investigations, interpretations of high‐resolution UAV images, and analyses of available InSAR data, we mapped the fault geometry and surface ruptures of the 2021 Mw 7.4 Maduo earthquake that occurred on a low‐activity strike‐slip fault within the Tibetan Plateau. The results indicate that (a) the earthquake activated a fault that is ∼161 km long and has complicated structural geometry; (b) the surface rupture occurs over a distance of 148 km, but is separated into three distinct segments by two large gaps (38 and 20 km, respectively); (c) within the surface‐rupture segments, the horizontal and vertical displacements are typically 0.2–2.6 m (much lower than the InSAR‐based slip maximum of 2–6 m at depth) and ≤0.4 m, respectively. The two large gaps of the Maduo surface rupture represent the two largest surface‐rupture discontinuities of strike‐slip earthquakes ever documented, and coincide with structurally complicated fault portions and near‐surface soft sediments.

The Lenglongling fault (LLLF) is located along the northeastern margin of the Tibetan Plateau and forms a part of the Tianzhu seismic gap along the Qilian‐Haiyuan fault zone. Little is known about the recurrence of large earthquakes along the LLLF nor the associated seismic hazards of the gap. Here the six most recent surface rupturing paleoearthquakes of the LLLF are revealed by measurement of offset landforms, trench excavations, and radiocarbon dating. They are labeled E1–E6 from youngest to oldest, and their timings are constrained to the following time ranges: 636–498 to present, 2951–1155, 4016–3609, 5325–4476, 7284–6690, and 8483–7989 years BP, respectively. The LLLF displays evidence of fresh, recent surface rupture, and the trench sections reveal that the fault ruptured to the ground surface during the most recent event. Based on this fresh surface rupture and historical earthquake records, the latest event E1 was most likely the 1927 M8.0 Gulang earthquake. In conjunction with previous studies, Gulang earthquake might be a complicated event characterized by the combined rupture of both strike‐slip and thrust faults. The average recurrence interval of the six paleoearthquakes is 1643 ± 568 years, and the coefficient of variation is 0.34, indicating that the LLLF follows a quasiperiodic recurrence model. Based on this new understanding of the last event, the LLLF may not be a part of the Tianzhu seismic gap. However, an earthquake of up to MW7.6 could still rupture the gap sections composed of the Jinqianghe, Maomaoshan, and Laohushan faults.

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. However, the length of the rupture zone on the HSF is not consistent with the magnitude of the earthquake and damage distribution (Figure 1; Gu, 1983). Ai et al. (2017) mapped the Southern Wuwei Basin fault ©2020. American Geophysical Union. All Rights Reserved.

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