High-Resolution Lithospheric Shear Velocity Structure of the Suqian Segment of the Tanlu Fault Zone From Ambient Noise Tomography

Abstract

To understand causes of the Suqian seismic gap and the 1668 M8.5 Tancheng earthquake, we determine a new 3-D S-wave velocity (Vs) model of the lithosphere under the Suqian segment of the Tanlu fault zone in eastern China using ambient noise tomography. Rayleigh wave phase velocity dispersions at periods of 5–33 s are collected from seismograms recorded at our newly deployed TanluArray portable stations and permanent provincial seismic stations in and around the Suqian area. The addition of the TanluArray data leads to much higher tomographic resolution in the study region. Our model well depicts surface geological features and reveals strong lateral heterogeneities in and around the fault zone. Beneath the Suqian seismic gap, the upper crust and uppermost mantle show low-velocity anomalies, whereas the middle and lower crust exhibit high-velocity anomalies. Such a structure in the Suqian area may hinder the accumulation of tectonic stress, leading to the formation of a seismic gap. The Tancheng earthquake occurred in a transition zone between high- and low-Vs anomalies in the middle crust, which is prone to stress accumulation and earthquake nucleation. The Tanlu fault zone serves as a channel for hot and wet mantle material upwelling into the crust, where fluids reduced the threshold for fault rupture and so triggered the Tancheng earthquake. The mantle upwelling may be related to dynamic processes in the big mantle wedge associated with stagnation and dehydration of the subducted Pacific slab in the mantle transition zone under East Asia.