11:00 AM - 1:00 PM
[SSS03-P01] Micro-seismicity in the northern part of the Nansei-Shoto subduction zone observed by ocean bottom seismic network
Keywords:OBS observation, Nansei-Shoto subduction zone, Regular and slow earthquakes
In the Nansei-Shoto (Ryukyu) subduction zone, seismic and aseismic activities associated with subduction of the Philippine Sea plate have been examined to understand their spatiotemporal variations. Although precise spatial relationships between regular and slow earthquakes have not been revealed due to the limited onshore seismic network, recent seafloor seismic observations in the northern part of the Nansei-Shoto subduction zone have identified low-frequency tremors possibly showing a complementary spatial distribution to active seismic areas.
In order to clarify the spatial patterns of the regular seismicity and the shallow low-frequency tremors, we have conducted dense seafloor seismic observations off the northeast of Kikaijima Island since 2019. The observation array consists of eight 1-Hz long-term ocean bottom seismometers (OBSs) with a station interval of ~20 km. The deployment and recovery of the OBS array have been repeated every year with shifting the observation area.
In this presentation, we used the second term data from July 2020 to April 2021 for the hypocenter determination of regular earthquakes. The observation area of this term is located around the expected epicenter of the 1911 M8.0 Kikaijima earthquake, which was the largest historical event in the northern part of the Nansei-Shoto subduction zone. We first determined initial hypocenters based on manual picking of P and S arrival times and a 1-D velocity structure. Here we selected the events whose S-P times at the center station of the array are less than 10 s. In order to correct travel-time delays due to unconsolidated sediments beneath each OBS, we applied station correction to P- and S-phase picks using PS phases converted at the sediment-basement boundary.
As a preliminary result, we obtained only ~200 hypocenters during the entire observation term, which means that the regular earthquake activity is relatively low in this area. Although the focal depths of the seismicity become ~20 km shallower after applying the station correction terms, the corrected hypocenters show that most earthquakes occur in the subducting plate. At the same time, our OBS array observed active shallow low-frequency tremors, which are widely distributed in the observation area. This spatial relationship between the low seismicity of the interplate earthquakes and the high activity of the tremors may reflect the seismic coupling along the plate interface.
Acknowledgements:
The OBSs were deployed and recovered by the T/S Nagasaki-Maru, Nagasaki University. We would like to thank all the crew members. We are grateful to the technical staff of Earthquake Research Institute, the University of Tokyo who worked on the preparation of the OBSs. We appreciate students at Kagoshima University, Kyushu University, and Kyoto University who participated in the research voyage. This work is supported by Earthquake and Volcano Hazards Observation and Research Program, the MEXT of Japan and by ERI JURP.
In order to clarify the spatial patterns of the regular seismicity and the shallow low-frequency tremors, we have conducted dense seafloor seismic observations off the northeast of Kikaijima Island since 2019. The observation array consists of eight 1-Hz long-term ocean bottom seismometers (OBSs) with a station interval of ~20 km. The deployment and recovery of the OBS array have been repeated every year with shifting the observation area.
In this presentation, we used the second term data from July 2020 to April 2021 for the hypocenter determination of regular earthquakes. The observation area of this term is located around the expected epicenter of the 1911 M8.0 Kikaijima earthquake, which was the largest historical event in the northern part of the Nansei-Shoto subduction zone. We first determined initial hypocenters based on manual picking of P and S arrival times and a 1-D velocity structure. Here we selected the events whose S-P times at the center station of the array are less than 10 s. In order to correct travel-time delays due to unconsolidated sediments beneath each OBS, we applied station correction to P- and S-phase picks using PS phases converted at the sediment-basement boundary.
As a preliminary result, we obtained only ~200 hypocenters during the entire observation term, which means that the regular earthquake activity is relatively low in this area. Although the focal depths of the seismicity become ~20 km shallower after applying the station correction terms, the corrected hypocenters show that most earthquakes occur in the subducting plate. At the same time, our OBS array observed active shallow low-frequency tremors, which are widely distributed in the observation area. This spatial relationship between the low seismicity of the interplate earthquakes and the high activity of the tremors may reflect the seismic coupling along the plate interface.
Acknowledgements:
The OBSs were deployed and recovered by the T/S Nagasaki-Maru, Nagasaki University. We would like to thank all the crew members. We are grateful to the technical staff of Earthquake Research Institute, the University of Tokyo who worked on the preparation of the OBSs. We appreciate students at Kagoshima University, Kyushu University, and Kyoto University who participated in the research voyage. This work is supported by Earthquake and Volcano Hazards Observation and Research Program, the MEXT of Japan and by ERI JURP.