Relocation of Eastern Taiwan Earthquakes and Tectonic Implications

We relocated 6,173 shallow earthquakes and determined focal mechanism solutions of thirty-three M 5.0 L ≥ events in the eastern Taiwan region (bounded by 22.7°N, 120.8°E; 24.5°N, 121.5°E; 24.3°N, 122.3°E and 22.5°N, 121.6°E) in order to investigate their tectonic implications.Our results suggest that the distribution of the relocated earthquakes generally agrees with the fault plane determined from the focal mechanism solutions.The Longitudinal Valley (LV) is an east-dipping seismic suture zone that separates two different tectonic regions.On the left-hand side of the LV, shallow earthquakes and normal fault-type focal mechanism were found in the Central Range region.But on the right-hand side of the LV, most events that occurred in the Philippine Sea Plate (PSP) were the thrust-type, especially for the southern portion and a few normal and strike-slip focal mechanism events were observed in the northern portion.The tectonics of eastern Taiwan can be classified from south and north into two regions, and there may exist a structure or boundary at about latitude 23.5°N with a strike of N110°E.In the northern portion of eastern Taiwan, the plate boundary appears to be an east-dipping plane defined by the earthquake distribution.The focal mechanism solutions determined in this zone show thrust type faulting and agreed with this boundary plane (strike N20°E and dip about 50°).In this region, the Eurasia Plate (EP) dips to the east, and the PSP bends and dips to west.Thus, shallow earthquakes with normal and strikeslip focal mechanisms were observed in the offshore area of this region.On the other hand, the southern part of eastern Taiwan exhibits simple tectonics conditions.The EP subducts to the east under the PSP.The LV is a suture zone dipping to the east, and most events have occurred on the righthand side of the LV.Focal mechanism solutions of these events were thrust-