Abstract
We relocated 6,173 shallow earthquakes and determined focal mechanism solutions of thirty-three M(subscript L)≥5.0 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 resuits 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 500). 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 strike- slip 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-type with strike about N25°E and dip 54° to the east.
Highlights
Taiwan is located at plate boundaries; the Philippine Sea Plate (PSP) subducts northward under the Eurasia Plate (EP) along the Ryukyu trench, and the EP subducts eastward under the PSP off the southern tip of Taiwan
The epicentral and hypocentral distributions of the CWB seismic network (CWBSN) and hypoDD results are shown in Figs. 3 and 4, respectively
Comparing with these two results, epicentral and hypocentral distributions of the CWBSN results are more scattered than the hypo-DD results
Summary
Taiwan is located at plate boundaries; the Philippine Sea Plate (PSP) subducts northward under the Eurasia Plate (EP) along the Ryukyu trench, and the EP subducts eastward under the PSP off the southern tip of Taiwan. The trilateration and leveling data, the GPS data, and paleomagnetic studies (e.g., Lee et al 1991) are all in agreement with a clockwise rotation of the Coastal range The interactions between these two plates (PSP and EP) have resulted in a very complicated crustal structure in the Taiwan region. This complexity and not well-determined seismic velocity structure can cause large uncertainties in earthquake location. The location uncertainty of routinely determined hypocenters (from a layered velocity model) is typically many times larger than the source dimension of the earthquakes themselves This limits the study of the fine crustal structure using the seismicity data, especially, for eastern Taiwan (which is a suture zone of the PSP and EP). In this study we will apply the hypo-DD technique to relocate eastern Taiwan earthquakes from 1991 to 2002 and determine the focal mechanism solutions of the larger events in order to study seismic patterns and the relationship between these two plates in eastern Taiwan
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