Repeating earthquakes have been found at many faults around the world, and they provide valuable information on diverse faulting behavior at seismogenic depth. The Haiyuan fault is a major left-lateral strike-slip fault along the northeastern (NE) boundary of the Tibetan Plateau. Two great earthquakes (1920 Haiyuan, 1927 Gulang) have occurred on this fault system, but the section between the ruptures of the two earthquakes, also known as the Tianzhu seismic gap, remains unbroken. Shallow creep has been observed from geodetic data at the eastern end of the seismic gap. However, the driving mechanism and depth extent of shallow creep are not clear. Here we conduct a systematic search for repeating earthquakes in NE Tibet based on seismic data recorded by permanent stations in ten years (2009-2018). Based on waveform cross-correlations and subsequent relocations, we find several repeating earthquake clusters at Laohushan section. This is consistent with the shallow creep inferred from the geodetic data, indicating repeating earthquakes can be driven by nearby aseismic slip. ~300 repeaters were found within clusters of intense seismicity near the rupture zones of the 1927 M8.0 Gulang and 2016 M6.4 Menyuan earthquakes. Relocation of events in the cluster near the Gulang earthquake delineates two possible unmapped faults orthogonal to the Haiyuan fault. In addition, we also identify several repeating earthquakes generated by mining activities with different waveforms and occurrence patterns. Our study suggests that repeating earthquakes around the Haiyuan fault are mostly driven by postseismic relaxation process associated with 1920 Haiyuan and 1927 Gulang earthquakes.

The selected rotation angle and deconvolution time window during S-wave receiver function (SRF) calculations and the final SRF quality control may introduce artificial interference. Here we overcome these problems by proposing a new strategy, the GC_SRF strategy, which employs grid search and correlation analysis to obtain reliable SRFs. Extensive tests using synthetic and real data suggest that the GC_SRF strategy is a suitable approach for constraining lithospheric thickness, and reproducible and robust depth estimations of the lithosphere–asthenosphere boundary (LAB) are obtained. Specifically, this GC_SRF strategy can restore the weak Sp phases from full wavefield synthetic seismograms. Similar, but clearer, discontinuity patterns that contain no artificial interference compared with those obtained in previous studies of southeastern Tibet are produced here. The post-stack migrated SRFs reveal that the Chuandian region has a thicker crust and either a poorly defined or unclear LAB, whereas a thinner crust and clear LAB is present at ~160 km depth beneath the Sichuan Basin. The absence of a continuous LAB in the Chuandian region may suggest lithospheric regrowth due to mantle plume processes.