3-D isotropic and anisotropic shallow crustal structure on Pingtan Island, Fujian, southeastern coast of China

Abstract

Pingtan Island is located on the southeastern coast of China between the Changle-Zhao'an and Binhai fault zones. We use P- and S-wave traveltime data from marine large-volume airgun sources for 3-D velocity structure inversion, and the spatial resolution is high at depths of 1–3 km. The isotropic inversion results show that the velocity in the island is higher in the midwestern area, which consists of sedimentary plains. The velocity is lower in the southern and eastern areas, which consist of eroded hills. Then we perform azimuthally anisotropic inversion using the obtained isotropic model as the initial model. The fast directions in the island are predominantly NE-SW and NNE-SSW, consistent with the main fault structures along the southeastern coast of China. The magnitude of anisotropy gradually decreases from south to north, consistent with the decreasing intensity of fault activity from south to north. On the southeastern coast of China, the fast direction in the upper crust is nearly orthogonal to that in the upper mantle from previous studies, which is nearly parallel to the absolute plate motion in the lower crust and upper mantle. The faults along the Fujian coastal area and Taiwan formed at different time period. But the fast directions in these two areas are similar. Therefore, the fast direction is mainly dominated by the trends of faults and fractures in the upper crust. And it is dominated by aligned olivine crystals induced by plate motion in the lower crust and upper mantle.