Uppermost mantle anisotropic tomography in and around the Tanlu fault zone derived from PickNet automatically picked <i>P</i> n data

SUMMARY We efficiently extract high-quality Pn wave arrival times from seismograms recorded at recently deployed 120 portable seismic stations of TanluArray and 317 permanent stations of the Chinese provincial seismic network for 7231 local earthquakes (M &amp;gt; 2.0) using the PickNet automatic picking method. Then we use the Pn data to determine 2-D P-wave velocity and anisotropic tomography of the uppermost mantle in and around the Tanlu fault zone. Our Pn tomography reveals segmented features of the fault zone, which are well consistent with geological structural features. A continuous low-velocity anomaly parallel to the fault zone is revealed along the Bohai Bay-Weifang, Weifang-Tancheng and Tancheng-Mingguang segments, whereas the Mingguang-Wuxue segment exhibits a bead-like alternating high and low velocity belt. A similar segmented characteristic also appears in the Pn wave anisotropy in and around the fault zone. A majority of strong earthquakes are located in transitional zones between high and low Pn velocities, suggesting that structural heterogeneities in the uppermost mantle may affect crustal seismogenesis. The low Pn velocities may reflect upwelling of hot and wet material flows in the big mantle wedge due to mantle convection and dehydration of the flat Pacific slab in the mantle transition zone, which cause seismic anisotropy in the upper mantle in and around the Tanlu fault zone.