Seismic Imaging of the North Xiaojiang Fault with Fault Zone Head Waves and Teleseismic P-Wave Arrivals

The Xiaojiang Fault Zone, situated in southeastern Tibet, is renowned for its intense seismic activity. Studies have identified areas of high slip rate and seismic gaps that could indicate future large earthquakes. In our latest research, we look at the detailed seismic structure of the northern Xiaojiang fault and its surroundings using data from 68 seismic stations of dense networks. By analyzing these data, we identify two types of fault zone head waves (FZHW) through a combination of automatic picking, manual selection, and horizontal particle motion analysis of seismic data of local events. The first type of FZHW is detected at four stations, indicating a 2%-4% P-wave velocity contrast across the fault. The second type of FZHW is recorded at eight stations from five clusters of earthquakes primarily in and around localized low-velocity zones.  To bolster our findings, we employed teleseismic P-wave arrival time delays between station pairs, which confirmed the FZHW analysis with a velocity contrast ranging from 1%-5%. A joint analysis of FZHWs and teleseismic P-wave arrival times reveals a sharp transition in velocity contrast across the fault near 26.7°N.To the north of this latitude, the west side of the fault exhibits lower velocities, suggesting the preferred rupture direction for a future earthquake would be from north to south. Conversely, to the south, the east side shows lower velocities, indicating a northward rupture direction. This structural variation suggests that an earthquake may be unable to rupture the entire NXJF, placing limits on the maximum size of potential earthquakes in the region.