Application of Ocean Bottom Seismometer: Study of the 4 Aug 2021 southernmost Okinawa Trough M6.1 event.

The Southern Okinawa Trough (SOT) is a back-arc-basin and characterized by an active normal faulting system and magmatic activity. Most seismic activities beneath the Southern Okinawa Trough at shallow depths (<30 km) are located about 50 km east of Ilan Plain (122.15°E - 122.55°E).  The seismic rate has increased significantly after May 2021. An earthquake struck offshore area of Ilan Plain on 4 August (0804 earthquake). Its magnitude of 6.1 at 7.0 km made it one of the rare, extremely powerful quakes ever before in the study area. However, the cause and mechanism are still unclear and worthy of further investigation. Furthermore, the previous study shows that submarine landslides have occurred in the northern continental margin of SOT. The frequent earthquakes will raise the risk of slope failure and may generate a local tsunami causing damage around the northeast coast of Taiwan. In order to explore the generation of 0804 earthquake, we have deployed an Ocean Bottom Seismometer (OBS) network to capture the seismicity around the study area.  In total, 852 events have been relocated and most of them are located in the high positive magnetic anomaly zone. The distributions of the earthquake show an NW-SE trending direction and may be related to the magmatic activity.