Three-dimensional Community Velocity Model in Southwest China from Joint Body and Surface Wave Traveltime Tomography

Abstract:

Due to the ongoing collision between the Eurasian plate and Indian plate, southwest (SW) China has complicated tectonic deformation and strong seismic activities and is acting as a transition channel of material extrusion from the active Tibetan Plateau. Although various researches have been taken to study the structure in SW China, a community velocity model is still absent due to different focuses of those studies. This results in inaccurate focal mechanism, earthquake location, strong ground motion, etc. To build a high-resolution community velocity model (both Vp and Vs) of the crust and uppermost mantle in SW China, we applied a joint inversion method using 386,958P- and 372,662 S-wave first arrival times, augmented by nearly 7000 Rayleigh wave dispersion data with the period range from 5 to 50 s. To alleviate the non-linear effect, a smoothed version of Vp and Vs models derived from joint inversion of Rayleigh wave dispersion data, ZH ratio and receiver functions is used as the initial model. Additionally, multigrid strategy is applied in the joint inversion, with different grid intervals. The checkerboard resolution tests indicate that the spatial resolution of both Vp and Vs models can reach up to 0.5º horizontally and 10 km vertically. The obtained Vp and Vs models show two low Vp and Vs anomaly zones in the middle-lower crust and a prominent high Vp and Vs anomaly region in between them, which are generally consistent with previous studies. The models are further validated by P-wave arrival times from active sources and Rayleigh wave dispersion data. Our new velocity models can serve as a first step towards building multiscale community velocity models of SW China, which could be used for better constraining earthquake source parameters as well as for understanding earthquake hazards and the tectonic evolution.