Anisotropic tomography and seismotectonics of the central Tanlu fault zone in East China

SUMMARY We determine detailed 3-D velocity and anisotropy models of the crust beneath the central segment of the Tanlu fault zone (TLFZ) in East China using hand-picked arrival times of direct waves (Pg, Sg), Moho refracted waves (Pn, Sn), and the Moho reflected waves (PmP and SmS) of local earthquakes recorded at 55 portable seismic stations of our newly deployed TanluArray and 45 Chinese provincial stations. Our results show that the pattern of seismic velocity and anisotropy is in good agreement with surface geological and tectonic features in the study region. Along the TLFZ, obvious low-velocity (low-V) anomalies are visible in the lower crust in the north of the Cangni fault, whereas high-velocity (high-V) anomalies appear in the upper and middle crust. The 1668 Tancheng earthquake (M8.5) and small-to-moderate earthquakes occurred in the high-V zones but underlain by low-V anomalies. The low-V zones may reflect significant effects of fluids derived from hot and wet upwelling flow in the big mantle wedge. Under the Suqian seismic gap, low-V anomalies are clearly imaged in the middle crust, whereas high-V anomalies appear in the lower crust, suggesting the absence of fluids in the deep crust, which could explain why few earthquakes occurred there. The fast velocity direction (FVD) of P-wave azimuthal anisotropy changes with depth. The FVD is parallel with the TLFZ strike in the upper crust in and around the TLFZ, reflecting the strike-slip features of the fault zone. Our results provide new seismic constraints on the structure and tectonic evolution of the TLFZ.