Shear wave anisotropy beneath the Taiwan orogen

Shear wave splitting in regional and teleseismic shear waves (ScS and S) has been analyzed for fast polarization directions and delay times at 33 short-period and 17 broad-band permanent and temporary seismic stations in the Taiwan region. The orientations of anisotropy for the Taiwan stations are generally parallel to the strike of the mountain belt, with a clockwise rotation following the trend of regional geologic fabric in north Taiwan. The delay times, on the other hand, vary greatly from 0.5 to 2.1 s; the largest values are from the southwestern Taiwan foothills, and an abrupt change in delay times is found in south Taiwan. The main source of shear wave splitting in Taiwan is compatible with an anisotropic region between 25 and 230 km depths. The observed splitting parameters are consistent with upper mantle anisotropy resulting from the collisional tectonics that built the Taiwan orogen.