Seismic Array Study of the Tectonic of the Tianzhu Seismic Gap and the Deep Characteristics of the Laohushan and Maomaoshan fault

The Gulang Ms 8.0 strong earthquake that occurred in 1927 claimed the lives of more than 40,000 people in the surrounding areas. Geological studies have shown that the occurrence of the Gulang earthquake did not reduce the seismic hazard of the Laohushan, Maomaoshan and Lenglongling faults, which are the westward extensions of the Haiyuan Fault. Based on the seismic moment accumulation rate, the existence of the Tianzhu Seismic Gap has been proposed. This seismic gap is potentially at risk of producing earthquakes of magnitude 7.0 or higher. Coupled with the frequent occurrence of small earthquakes in this area in recent years, it is regarded as being of considerable seismic danger. In the junction area of the Tianzhu Seismic Gap and its surrounding faults, we observed and collected dense broadband seismic array data for a period of more than 7 years. Through long-term continuous observations, the seismicity and the crustal S-wave velocity structure of the study area was obtained. The research results show that the current seismicity in the Gulang Seismic Zone distributes along the Wuwei-Tianzhu Fault with a southwestward trending feature, and does not extend to the Lenglongling Fault. This indicates that the seismogenic fault of the Gulang Earthquake may not include the Lenglongling Fault and the Jinqianghe Fault. In the Tianzhu Seismic Gap, seismicity distributes linearly along the Laohushan-Maomaoshan Fault, exhibiting obvious strike-slip fault characteristics. In terms of depth, seismic activities around the Maomaoshan Fault are concentrated in two intervals: the shallow layer above 10 km and the deep layer below 20 km, which also delineates the strong locking feature at the depth of 10–20 km beneath the Maomaoshan Fault. Obvious weak seismicity is also observed in the western segment of the Haiyuan Fault. The velocity structure results demonstrate that at the depth of the upper and middle crust, there are significant velocity differences on both sides of the Laohushan Fault, Maomaoshan Fault and Wuwei-Tianzhu Fault, and the seismic distribution is highly consistent with the boundary zones of these velocity differences. Beneath the Maomaoshan Fault and in the middle segment of the Laohushan Fault (at the upper and middle crust depth), there exist high-velocity anomalies distributed on both sides of the faults. These anomalies are inferred to be asperities that impede fault rupture, with a length of approximately 50 km and a width exceeding 20 km along the fault plane.