P波极性数据所揭示的台湾地区三维应力结构的初步结果

Preliminary result of Taiwan 3-D stress field from P wave polarity data

采用综合震源机制方法,利用503393个P波极性数据反演了台湾地区的三维应力结构.对于给定点,根据该点及邻近区域地震的所有P波极性数据采用地震综合震源机制进行估算,P波极性数据的权重由高斯距离函数给定.该方法可以采用大量P波极性数据,包括不能确定单个地震震源机制的P波极性数据.与传统的震源机制确定应力场的方法相比,该方法可以提供更好的震源机制的平均结果并覆盖更为广泛的地区.本文求得了台湾及邻区0.25°×0.25°×2.5 km的综合震源机制.初步结果表明,P轴和T轴方向在Moho间断面附近变化明显.P轴和T轴方向在15～45 km深度范围内与P波速度异常相关,描绘出菲律宾板块在台湾西北部的北西向俯冲消减.该三维应力场结果可为台湾地区的地球动力学过程提供新的约束.

We apply the composite focal mechanism method and use more than half million P wave polarity data to invert for the 3-D stress field in Taiwan region. At a given location, composite focal mechanism is estimated using all the P wave polarity data originated from its neighborhood, and the data are weighted using a Gaussian distance weighting function. This method has the advantage of allowing maximum amount of data input in the solution, including polarity data from small earthquakes whose focal mechanisms are undeterminable with too few observation data available. Comparing to traditional focal mechanism estimation methods, this method produces focal mechanism estimates covering a broader area, and provides better averages of local focal mechanism solutions. We present the focal mechanism result in a 0.25°×0.25°× 2.5 km grid in Taiwan and its adjacent areas. The preliminary result shows that the P and T axis directions change markedly across the Moho discontinuity. The P and T axis directions in the 15-45 km depth range also correlate with P wave velocity perturbations and delineate the northwestward subduction of the Philippine Sea plate in northeast Taiwan. This 3-D stress field result may provide new constraints on geodynamics process in Taiwan region.