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

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, earthquake 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 degree x 0.25 degree x 2.5 km grid in Taiwan and its adjacent areas. The preliminary result shows that the P and T axis directions change markedly across moho discontinuity, suggesting certain amount of mechanical decoupling between the crust and mantle. The P and T axis directions at 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 and the eastward subduction of the Eurasia plate in southern Taiwan, respectively. This 3-D stress field result may provide new constraints on geodynamics process in Taiwan region.