FINDING PRINCIPAL TECTONIC STRESS DIRECTIONS IN TAIWAN FROM FOCAL MECHANISM SOLUTIONS

The collision of the Philippine Sea Plate with the Eurasian Plate on the eastern coast of Taiwan is the cause of many earthquakes on the island. The largest recent earthquake was the Mw7.6 Chi-Chi earthquake that occurred in 1999. Earthquake focal mechanism solutions for 700 earthquakes of magnitude greater than 3.0-5.5 recorded by the Broadband Array in Taiwan for Seismology (BATS) from 1995 to 2007 provide the location, strike, dip, and slip of fault planes (or the auxiliary plane). This information is combined with three-dimensional stress inversions to estimate the spatial distribution of principal stress directions in the Taiwan crust. The inversions reveal a pattern consisting of vertical principal compression below the highest topography in the eastern Central Ranges consistent with normal faulting, subhorizontal principal compression consistent with strike-slip faulting in the western Central ranges, and subhorizontal principal compression consistent with reverse faulting in the active fold and thrust belt. The inversions of data taken before 1999 are compared with those gathered after the Chi-Chi earthquake to see if the stress directions have changed after the major event.