Shear wave splitting analysis on Nias Island: Preliminary results for crustal anisotropy in the Sumatran-Forearc

We perform investigation of crustal anisotropy in the Sumatran forearc through shear wave splitting analysis. We employ high quality splitting measurements from around 59 local events with depth less than 25 km recorded at a seismic station in Nias Island. The result shows an average splitting time of 0.19 ± 0.01 s and the fast directions aligned parallel to the strike of the Sumatran Trench or perpendicular to the absolute plate motion. In comparison with the direction of regional strain rates and fast directions obtained from the previous mantle anisotropy studies, the fast orientations of our result are nearly perpendicular to the results of the previous studies. This suggests that anisotropy in the shallow depth around the island is either due to structural induced anisotropy associated with the regional tectonic activity or the fossil anisotropic feature created from the past tectonic events.