||Located at the arc-continental collision region between the Eurasian (EU) and Philippine Sea Plate (PSP), Taiwan is characterized by a complex tectonic environment, particularly the eastern part of the island. Based on geodetic, geological and geophysical data, the tectonic structures in the eastern Taiwan have been well studied by several former works. However, the seismotectonic structures in the offshore area of eastern Taiwan remains poorly understood, because most seismic stations are inland and the earthquakes occur offshore cannot be located accurately. For understanding the seismic activities and the structures in the eastern Taiwan, we located the earthquakes from OBS (Ocean Bottom Seismometer) data, which was deployed by ATSEE project (Across Taiwan Strait Explosion Experiment) during Jul. 9th~ Aug. 3rd, 2012, and the data from 14 seismic stations of CWB (Central Weather Bureau) by using the program Antelope and the global velocity model iasp91. For improving the precision of location, we relocated the earthquakes with the Hypo-DD relocation method which could minimize errors owing to the velocity structure without the use of station corrections. Additionally, the SIMUL2000 program and the flexible gridding method were applied to build a 3-D Vp velocity model from the 1-D initial model CWB1D.|
In the 25- day data, 979 earthquakes were recorded while 333 events were relocated. The features of onshore and offshore seismicity are quite different, especially in the north of 23.3⁰N. With the northward subduction of the PSP, offshore seismicity deepens continuously while there is a seismic gap onshore. Along the collided Luzon arc, the high gravity anomaly (volcanic arc) progressively becomes negative (trench) between 23.3⁰N and 23.7⁰N, and there is a significant difference of the surface velocity field near 23.7⁰N from GPS data, which indicates a stress change in the shallow part. As the PSP subducted, the collision zone deepens northward, resulting in an onshore seismic gap. With the northwestward subduction of the PSP north of the seismic gap, normal and strike-slip earthquakes present at the depth shallower than 35km. Taking together all these observation, we suggests a tectonic transition zone from collision to northwestward subduction at the latitude of 23.3~ 23.7⁰N. Also, the Ruisui earthquake series, located in the seismic gap, may provide an evidence of the southward propagation of the PSP subduction.
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