Pn velocity and anisotropy beneath the Yishu fault zone and surrounding areas

The Yishu fault zone is located among the Luxi uplift, Jiaoliao uplift and Sulu fold belt, which is the most seismically active part of the Tanlu fault zone. The investigation of uppermost mantle velocity and anisotropy is of great significance for understanding the control of the Yishu fault zone on seismic activity. In this study, by integrating manually picked Pn arrival times with those in seismic observational bulletins, we obtain 26598 Pn travel-time data of 1665 earthquakes recorded at 290 seismic stations during 2008 to 2019, and invert these data for the velocity and anisotropy of the uppermost mantle beneath the Yishu fault zone and its surrounding areas. Our results show that there are significant lateral variations in velocity and anisotropy. The southern North China basin, Taihang orogenic belt, Luxi uplift and Jiaoliao uplift exhibit low-velocity anomalies. The North China basin shows significant lateral heterogeneities, and the strong high-velocity anomalies in the basin show obvious blocks. The Yishu fault zone shows obvious segmentation characteristics, and there are obvious differences in the velocity structure on both sides, indicating that the Yishu fault zone may extend down to the uppermost mantle. Most strong earthquakes occurred in the crust above the transition zones from high-velocity to low-velocity anomalies in the uppermost mantle, indicating that the occurrence and generation of these earthquakes could be related to lateral heterogeneities in the mantle. The fast direction of Pn propagation beneath the Taihang orogenic belt and Jiaoliao uplift area coincides with the extension direction of the geological structure, but it is different from the SKS splitting results, indicating that there may be local decoupling at certain depths of the lithosphere. The fast direction of Pn anisotropy in the South Yellow Sea basin is basically the same as the fault strike, suggesting that the partial faults in the South Yellow Sea formed by the northward extrusion of the Yangtze block reach the uppermost mantle and become lithospheric-scale faults.