台灣及鄰近地區地體動力學研究(Ⅴ)-子計畫：南沖繩海槽岩漿活動及地震特性調查

The Southern Okinawa Trough (SOT) is a back-arc-basin, which propagated westward into to offshore area of Ilan Plain. SOT is characterized by active normal faulting system and magmatic activity. The number of earthquakes in this area with magnitude larger than 3 is about 150 each year. Although the seismic rate in SOT is relatively high, it’s rare to generate earthquake with magnitude larger than 6 in shallow depth (< 30 km). It’s noted that the seismic rate has increased significantly in 2021. About 500 earthquakes with magnitude range 3.0 - 4.61 occurs within 122.32° - 122.55°Ｅ and 24.95 -24.84°N during the May and June 2021, and then an earthquake of magnitude 6.1 at 7.0 km struck offshore area of Ilan Plain at 4 August. (hereafter termed as 0804 earthquake). The aftershocks of 0804 earthquake was located in 122.16°- 122.37°Ｅ and 24.74 -24.82° N, just to the southwest of the earthquake swarm during the May and June. In addition, there are more than ten earthquakes generate (ML > 3.0) around Guishan Island during this period. Almost all of events are located in the area where positivity magnetic anomalies appear. The observation may refer to magmatic activity become ever more frequent. The mechanism between them is still not clear. Furthermore, previous study used geophysical data to shows the submarine landslides occurs in the northern continental margin of SOT. Such frequent earthquake will increase the risk of slope failure and may generate a local tsunami and affect the northeast coast of Taiwan. Because the lack of seismic data, it’s hard to reveal the tectonic below this region. For instance, to the north of SOT is also characterized by a high magnetic anomaly, however, very few events were discovered in that area. This may cause by the small magnitude of volcanic activities for that area may be too small to be record by onshore station. We will benefit from the deployment of Ocean Bottom Seismometer (OBS) network to clarify the seismogenic of the SOT. This project is scheduled in three years and aims to explore the relationship between magmatic activity and generation of earthquakes. This study also provided us a good opportunity to investigate geological development of SOT.