Three-dimensional shear-wave velocity structure under the Weifang segment of the Tanlu fault zone in eastern China inferred from ambient noise tomography with a short-period dense seismic array

Abstract

In this study continuous waveforms are observed at a short-period dense seismic array which consisted of 302 stations in the Weifang segment of the Tanlu fault zone from August to October 2017. A total of 122,302 Rayleigh wave phase velocity dispersion curves at 1.0–7.5 s periods are extracted to infer a high-resolution shear-wave tomography model of the shallow crust under the region. Our model well depicts surface geological structural features and has strong lateral heterogeneities in and around the fault zone. Both Weibei and Weifang depressions are imaged as striking low-velocity anomalies above ~2–3 km depth, possibly reflecting the Quaternary sedimentary layers, but the deeper low-velocity anomaly under the Weifang depression could indicate a channel for hot material upwelling in the crust. Wenxian and Gongdanshan uplifts are imaged as high-velocity anomalies down to 8 km depth, possibly indicating the existence of the Paleozoic crystalline basement rocks. A continuous high-velocity anomaly appears under the Changle volcano, which may be attributed to a magma system solidified in the shallow crust. The western and eastern boundaries of the Tanlu fault zone, where surface mapped faults exist, are clearly imaged as low-velocity anomalies, further demonstrating that the fault zone serves as a channel for hot material upwelling into the shallow crust through the middle and lower crust from the mantle. The upwelling processes may be related to mantle dynamics of the big mantle wedge caused by stagnation and dehydration of the subducted Pacific slab in the mantle transition zone under the region.