Systematic Search for Repeating Earthquakes Along the Haiyuan Fault System in Northeastern Tibet

Repeating earthquakes have been found at many faults around the world. The Haiyuan fault is a major left-lateral strike-slip fault along the northeastern boundary of the Tibetan Plateau. Two great earthquakes (1920 Haiyuan and 1927 Gulang) have occurred on and round 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 Laohushan section of the Haiyuan fault near 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 northeastern Tibet based on seismic data recorded by permanent stations in 10 years (2009-2018). Based on waveform cross-correlations, we find several repeating earthquake clusters at the Laohushan section. This is consistent with the shallow creep inferred from the geodetic data, indicating repeating earthquakes can be driven by nearby aseismic slip. Approximately 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 likely driven by long-term postseismic relaxation process associated with the 1920 Haiyuan and 1927 Gulang earthquakes. Plain Language Summary Repeating earthquakes usually have a similar magnitude and occur at the same fault patch, and they generate very similar seismic waves recorded by seismic stations. Repeating earthquakes are mostly found in fast-slipping faults near major plate boundary. In this study, we systematically search for repeating earthquakes along the Haiyuan Fault, a major left-lateral fault along the northeastern edge of the Tibetan Plateau. In the past 100 years, two great earthquakes (the 1920 Haiyuan and 1927 Guhang) with magnitude around 8 occurred on and around this fault. We find repeating earthquakes at the Laohushan section of the Haiyuan fault, which mark the boundary between the creep and locked regions, and the western end of the fault that ruptured in the 1920 great Haiyuan earthquake. In addition, we find several clusters of repeating earthquakes around the aftershock zone of the 1927 Gulang earthquake. We suggest that those repeating events are likely driven by long-term creep in the surrounding faults following both great earthquakes.