Anisotropic upper crust above the aftershock zone of the 2013 Ms 7.0 Lushan earthquake from the shear wave splitting analysis

We have conducted a systematic shear wave splitting analysis using 1000 selected aftershocks with M > 2 from the 2013 Ms 7.0 Lushan earthquake along the Longmenshan fault system in southwest China. Polarization directions of fast shear waves show a bimodal distribution with one dominant direction approximately parallel to the fault strike and the other close to the regional maximum horizontal compressive stress direction. This indicates that in this area mechanisms causing crustal seismic anisotropy are both stress induced and fault zone structure controlled. Delay times between fast and slow shear waves do not show a clear trend of increase for deeper events, suggesting the anisotropic zone is mostly above the aftershocks, which are generally located below 8 km. We further applied a shear wave splitting tomography method to measured delay times to characterize the spatial distribution of seismic anisotropy. The three-dimensional anisotropic percentage model shows strong anisotropy above 8 km but low anisotropy below it. The mainshock slip zone and its aftershocks are associated with very low or negligible anisotropy and high velocity, indicating that the zones with high anisotropy and low velocity above 8 km are mechanically weak and it is difficult for stress to accumulate there. The main and back reverse fault zones are associated with high anisotropic anomalies above ∼8 km, likely caused by shear fabric or microfractures aligned parallel to the fault zone.