SPLIT-S WAVE-FORMS OBSERVED IN NORTHERN TAIWAN - IMPLICATIONS FOR CRUSTAL ANISOTROPY

Short-period stations ENT and NSK in northern Taiwan have frequently recorded split S waveforms from earthquakes in the underlying subduction zone. To determine splitting parameters together with objective, straightforward error estimations, we employ a waveform cross-correlation method. The error for each measurement is estimated by translating the lower limit of the 95% confidence interval of the cross-correlation coefficient into azimuth and time. Only measurements with 95% confidence regions excluding zero delay time are accepted as splitting data. We also use an energy minimization method to constrain the solutions with minor splitting. We interpret the splitting as caused by stress-aligned cracks in the upper crust. The average delay times are 0.03-0.04 s for ENT and 0.07 s for NSK. The fast directions at the two stations are predominantly N-S, which roughly match the stress trajectories predicted by a model simulating the arc-continent collision in the Taiwan area. However, at ENT there is a subset of data with fast directions oriented in roughly E-W. The nearly 90-degrees rotation of the inferred crack orientation occurs in a horizontal distance of 10-15 km in the upper crust. Both the predicted heterogeneous stress regime and the local structural setting in northeastern Taiwan favor a lateral variation with a similar length scale.