P-wave upper-mantle tomography of the Tanlu fault zone in eastern China

The Tanlu fault zone is the most significant active fault in eastern China, which generated the great 1668 Tancheng earthquake (M 8.5). It is still unclear whether or not there is a link between the great earthquake generation and the upper-mantle structure. To address this issue, we study P-wave upper-mantle tomography beneath eastern China using 44,047 teleseismic P-wave arrival times. Our results show that at depths < 150 high-velocity (high-V) anomalies appear west of the Tanlu fault zone, whereas low-velocity (low-V) anomalies are visible east of the fault zone. Strong lateral heterogeneities are revealed along the fault zone. At depths of 230-470 km, northwest of the Tanlu fault zone, there are obvious low-V anomalies that may reflect hot and wet mantle upwelling, whereas to the east high-V anomalies are visible, which may reflect the detached Eurasian lithosphere (downwelling). In the mantle transition zone (MTZ), both high-V and low-V anomalies are revealed, and the widespread high-V anomalies may reflect the stagnant Pacific slab. Beneath the hypocenter of the 1668 Tancheng earthquake, intermittent low-V anomalies are revealed in the upper mantle down to the MTZ depth, which may reflect hot and wet mantle upwelling flow. Integrating the present results with previous findings, we deem that the Tancheng earthquake was affected by fluids from the hot and wet mantle upwelling associated with the lithospheric delamination. Complicated mantle convection, including both upwelling and downwelling flows, may occur under the Tanlu fault zone in the big mantle wedge above the stagnant Pacific slab in the MTZ.