Strain Partitioning on the Western Haiyuan Fault System Revealed by the Adjacent 2016 Mw5.9 and 2022 Mw6.7 Menyuan Earthquakes.

The 1,000 km‐long Haiyuan fault is the largest strike‐slip fault system in the northeastern Tibetan Plateau, accommodating part of the plateau's eastward extrusion. However, few large earthquakes have been instrumentally recorded, hindering our understanding of strain partitioning across the fault system. Here, we use synthetic aperture radar images to investigate fault geometries and slip distributions of the adjacent 2016 Mw5.9 and 2022 Mw6.7 Menyuan earthquakes that occurred 35 km apart along the western Haiyuan fault system. The purely strike and purely thrust slips of the 2022 and 2016 events indicate that strain was released separately on shallow steep fault and low‐angle fault at depth. We propose that such strain partitioning is controlled by the ratio between interseismic shear and normal velocities and the branching fault structure beneath the Lenglongling segment. Seismic hazard due to both focal mechanisms has to be considered along the western Haiyuan fault in northeastern Tibet. Plain Language Summary: Apart by 35 km, two earthquakes occurred on 20 January 2016 and 7 January 2022 with the moment magnitude of Mw5.9 and Mw6.7 along the western Haiyuan fault system, near Menyuan county, China. They are the largest events on the Lenglongling (LLL) mountain in past decades and also the only ones with clear surface deformation captured by radar imaging geodesy. Here, we use Synthetic aperture radar images to measure the surface deformations caused by these two events and derive their fault geometries and slip distributions. For the 2022 event, our modeling shows that the rupture propagated on both the LLL and the Tuolaishan (TLS) fault, supporting that the Haiyuan fault system may extend westward through LLL to the TLS fault. The purely thrust‐slip and purely strike‐slip mechanisms of the 2016 and 2022 events indicate the strain partitioning that the shear and normal strain can be released by the strike‐slip event and thrust event separately. Such strain partitioning reminds us to consider seismic hazards due to both shallow strike‐slip and deep thrust earthquakes along the western Haiyuan fault system. Key Points: Slip distributions of the 2016 Mw5.9 and 2022 Mw6.7 Menyuan earthquakes are derived from Sentinel‐1 interferograms and pixel offsetsThe 2022 Menyuan earthquake suggests the Tuolaishan fault may as the continuation of the Haiyuan fault west of the Lenglongling faultCoseismic slip partitioning along the western Haiyuan fault agrees with the interseismic shear‐to‐normal strain ratio and fault geometries [ABSTRACT FROM AUTHOR]