|dc.description.abstract||The NW moving Philippine Sea plate (PSP) collides with the Eurasian plate (EUP) near Taiwan. Simultaneously, it subducts toward the north along the Ryukyu trench at a rate of about 8.2 cm/yr in the direction of ~312˚N and produces a series of major subducting structures. The western termination of the Ryukyu subduction zones hit against eastern Taiwan. In such a complex tectonic environment, a large number of seismic activities is expected. However, restricted by the seismic station distribution, the seismotectonic setting of this area is still not well understood.
To obtain more information about the submarine tectonic structures offshore eastern Taiwan, we combined the data from 15 OBSs as well as 11 CWB (Central Weather Bureau) onland stations for more detail earthquakes analysis. The 15 OBSs were deployed during the ATSEE2011 (Across Taiwan-Strait Explosion Experiment) project, from August 15 to September 7, 2011, in the eastern offshore Ilan plain, including the area of the forearc basin, Ryukyu arc and Okinawa trough. Seismic events were detected manually with the Antelope software and the global velocity model iasp91was used for the initial localization. In total, 1058 events with a depth range of 0~125 km and magnitude range (Md) of 1.66~5.22 were located. However, a 1-D seismic velocity model is not sufficiently accurate for a precise hypocenter determination. Thus, the HypoDD software was used for the earthquake relocation. Finally, 443 events were relocated during a recording period of about 23 days.
Results show that the relocated earthquakes were concentrated mainly in three clusters. The first cluster is located to the south of Ilan plain (about 24.5N, 121.9E), at the boundary of Taiwan and the PSP. The earthquakes located in this cluster possess a relatively larger depth (~10 to 60 km) compared to the other two clusters. Located at the transition zone of the collision, subduction and backarc extension processes, this area has high and complex seismicity activity. In this region, two south-dipping seismic cluster were observed. The earthquakes is characterized by sinistral strike-slip faultings at shallow depth but becomes to be dominated by lateral compression mechanism at depth between 20~35 km. Another seismic cluster shows a westward dipping distribution which reveals the westward subduction of the PSP.
The second cluster occurs in the forearc basin area between 5 and 40 km in depth. According to the former studies, this area is affected simultaneously by both trench-parallel extension and trench-perpendicular compression. Most earthquakes in this region occur along the plate interface, characterized by low-angle thrusting effect. However some normal faulting type earthquakes are generated by local asperity subducting effect.
The last earthquake cluster is located at the Okinawa trough, with a depth range of 5 to 30 km. Earthquakes in this region are generally caused by the backarc volcanism and series of normal faults in consequence of extension of the back-arc basin opening. Some earthquakes located at the area of Cross Backarc Volcanic Trail (CBVT) are characterized by low-frequency waveforms, suggesting their volcanic origin. Much more events were recorded by our OBS network in comparison with existing seismic networks, which help to get more understandings about the tectonic environment in the northeastern offshore Taiwan.||en_US|