Magmatic-related earthquakes in the southern-most Okinawa Trough

Located offshore northeastern Taiwan, the Southern Okinawa Trough (SOT) is a back-arc basin, which belongs to the Ryukyu subduction system. Originated from the subduction of Philippine Sea Plate to the Eurasia Plate, the SOT is characterized by a very high density of active volcanic and earthquake activities and has a very high geohazard potential for northern Taiwan area. However, as the SOT is in the offshore area, it is difficult to carry out oceanographic observations to study the tectonic structures in the area. Therefore, by using the onshore seismic waveform and frequency analysis which collected by the onshore stations data collected from Central Weather Bureau(CWB) , this study aims to understand the activity of the volcano related earthquakes in the SOT, and assess the potential impact to Taiwan area. Previous studies shown that, the waveforms of the volcano-related earthquakes are usually characterized by unclear P and S wave arrival, and relatively lower frequency with regard to most of the tectonic earthquakes. Based on these criteria, we manually analyzed the waveform of 2263 earthquakes collected from 1991 to the end of 2013 to select the possible volcanic events. To insure that low frequency waveform content resulted from the source effect, instead of the ray path, we also identify several tectonic earthquakes from the same source area and recorded by the same station as the low frequency events, In total, 67 possible volcano related seismic earthquakes were identified. Most of these events are located in the central axis of the SOT and in the southeastern Kueishan Island. The depths of the events in the SOT are mostly shallower than 20 km, and become deeper when the hypocenters get closer to the central axis of SOT, However, all the events near Kueishan Island are shallower than 15 km. The magnitude of most volcanic events magnitude is lower than 3.5. By comparing the position of volcanic earthquakes and the available geophysical data in the area, we suggest that the major volcanic activities in northeast Taiwan offshore area are related to the extension of SOT. However, we found that some fundamental problems still need to be solved when using onshore station to observe offshore seismic events, such as the location accuracy and the capability for the signal detection.