The 2022, Ms 6.9 Menyuan earthquake: Surface rupture, Paleozoic suture re-activation, slip-rate and seismic gap along the Haiyuan fault system, NE Tibet

The January 8, 2022, Ms 6.9 Menyuan earthquake occurred near the junction between the western Leng Long Ling (LLL) and eastern Tuolai Shan (TLS) faults, along the Haiyuan fault system, further illuminating the geometry of that system. Based on unmanned aerial vehicle (UAV) high-resolution images and field investigation, we mapped the fault geometry and surface ruptures, and measured co-seismic offsets in detail. Our results indicate that the event ruptured both a NWW-striking, ∼ 22 km-long segment of the west LLLF and a ∼ EW-striking, ∼ 4 km-long eastern segment of the TLSF, with maximum left-lateral displacements of ∼ 3.4 and 0.95 m, respectively. Both ruptures isolate a 2.8 km-wide, left-stepping, step-over/pull-apart, rather than a releasing bend as previously inferred. However, the two faults may connect continuously at depth, as suggested by both the aftershock distribution and the interferometric synthetic aperture radar (InSAR) data inversion, following the deep Paleozoic Qilian Shan suture, which is marked at the surface by discontinuous, elongated outcrops of ultramaphic rocks. Much larger, longer-term offsets of up to ∼ 71 and 55 m along the TLS and LLL, respectively, may have accrued since the end of the last glacial maximum (LGM, ∼20 ka) and the beginning of the Holocene (∼10 ka), respectively. This may be taken to imply average slip-rates since such times of ∼ 5 ± 0.5 and 3.5 ± 0.4 mm/yr along the LLLF and TLSF, respectively, consistent with extant InSAR/GPS rates. Based on our new UAV-based field survey and recent trenching results (Shi et al., 2019), we infer that neither the LLL nor the Gulang faults ruptured during the 1927, M 8 Gulang earthquake. The co-seismic Coulomb stress increased east of the 2022 Menyuan rupture. This, and the fact that the east-central LLLF does not appear to have ruptured in the last several hundred years may enhance the risk of occurrence of a Mw 7.4 earthquake along it. This might also imply that a large (M 8?) earthquake might rupture that segment, together with the ∼200 km-long Tianzhu seismic gap along the Haiyuan fault farther east, during a pending cascading event.