13:45 〜 15:15
[SVC34-P03] Seafloor seismic observation in the north of Izu-Oshima island
キーワード：伊豆大島、広帯域海底地震計、火山活動
Izu islands consist of several volcanic islands. For example, Izu-Oshima is located at the northernmost part and thirty-six years have been passed since the last medium-scale eruption (VEI3) in 1986 (e.g., Japan Meteorological Agency, 2013). Miyake-jima island is located approximately 60 km south of Izu-Oshima, which erupted in 2000 (e.g., Kumagai and Ohminato, 2001). Since most of the main bodies of volcanoes are below sea level, seafloor observations are needed to detect precursory volcanic activities that may reflect the rapid intrusion of the magma.
Thus, we are planning seafloor seismic observation to investigate the shallow earthquakes around the northern Izu islands where many earthquakes occur (Figure 1). These earthquakes are possibly related to the volcanic activities such as magma intrusion below the seafloor. However, the accuracy in the hypocenter depth was probably insufficient due to small number of offshore seismic stations. Constraining the hypocenter depth is crucial to discuss the magma activities. Our main aim is to determine the precise hypocenter locations of earthquakes, and to detect small earthquakes which cannot be observed from land stations. As the first step of our project, we conducted a feasibility observation for ten months at off north coast of Izu-Oshima island and off the eastern coast of the Izu Peninsaula. Swarm of shallow earthquakes in this area has been frequently observed by the existing land-based seismic observation (e.g., Okada and Yamamoto, 1991; Hayashi and Morita, 2003; Morita et al., 2005). We will present the outline of our observation and the preliminary results of seismic activities.
We used 4 broadband ocean bottom seismometers (BBOBS) which were equipped with differential pressure gauges (DPG). BBOBS is useful not only for studying earthquakes but also for the long-period crustal activities such as volcanic tremors and low-frequency earthquakes. We deployed BBOBSs from 5th to 6th October, 2021 using the R/V Kairei (KR21-16 cruise) (Figure 1). They were installed on the seafloor at depths of 1000–1500 m with an average spacing of approximately 15 km. Although all BBOBSs were successfully deployed on the seafloor, the broadband sensor could not be activated at one site (S3) because the leveling unit is unable to correct for the inclination of the seafloor where it has settled (approximately 30 degrees). Only DPG data is available at the site. The recovery was carried out by the R/V Kaiyo-maru No. 3 (Kaiyo Engineering Co. Ltd., chartered by JAMSTEC) in 2nd August, 2022. All BBOBSs were successfully recovered with full of data for ten-month period.
Nine hundred one earthquakes have been detected by the Japan Meteorological Agency (JMA) just below or near the BBOBS array, in the region of latitude range between 34.7°N and 35.1°N, and longitude range between 139.0°E and 139.5°E, during the observation period. The magnitudes of these earthquakes ranged from −0.8 to 3.3. Currently, we have analyzed 187 of those earthquakes with magnitude larger than or equal to 1.2 through manually picking the P- and S-wave arrivals, and determining hypocenters using a one-dimensional velocity structure with reference to the results by Morita et al. (2021). We will analyze smaller earthquakes as well in the future. In addition to the earthquakes, we plan to search volcanic tremors and low-frequency seismic activities from the continuous seismograms. Furthermore, we succeeded to observe about 100 teleseismic events, which are expected to contribute to understanding of the deep structure beneath this region.
Thus, we are planning seafloor seismic observation to investigate the shallow earthquakes around the northern Izu islands where many earthquakes occur (Figure 1). These earthquakes are possibly related to the volcanic activities such as magma intrusion below the seafloor. However, the accuracy in the hypocenter depth was probably insufficient due to small number of offshore seismic stations. Constraining the hypocenter depth is crucial to discuss the magma activities. Our main aim is to determine the precise hypocenter locations of earthquakes, and to detect small earthquakes which cannot be observed from land stations. As the first step of our project, we conducted a feasibility observation for ten months at off north coast of Izu-Oshima island and off the eastern coast of the Izu Peninsaula. Swarm of shallow earthquakes in this area has been frequently observed by the existing land-based seismic observation (e.g., Okada and Yamamoto, 1991; Hayashi and Morita, 2003; Morita et al., 2005). We will present the outline of our observation and the preliminary results of seismic activities.
We used 4 broadband ocean bottom seismometers (BBOBS) which were equipped with differential pressure gauges (DPG). BBOBS is useful not only for studying earthquakes but also for the long-period crustal activities such as volcanic tremors and low-frequency earthquakes. We deployed BBOBSs from 5th to 6th October, 2021 using the R/V Kairei (KR21-16 cruise) (Figure 1). They were installed on the seafloor at depths of 1000–1500 m with an average spacing of approximately 15 km. Although all BBOBSs were successfully deployed on the seafloor, the broadband sensor could not be activated at one site (S3) because the leveling unit is unable to correct for the inclination of the seafloor where it has settled (approximately 30 degrees). Only DPG data is available at the site. The recovery was carried out by the R/V Kaiyo-maru No. 3 (Kaiyo Engineering Co. Ltd., chartered by JAMSTEC) in 2nd August, 2022. All BBOBSs were successfully recovered with full of data for ten-month period.
Nine hundred one earthquakes have been detected by the Japan Meteorological Agency (JMA) just below or near the BBOBS array, in the region of latitude range between 34.7°N and 35.1°N, and longitude range between 139.0°E and 139.5°E, during the observation period. The magnitudes of these earthquakes ranged from −0.8 to 3.3. Currently, we have analyzed 187 of those earthquakes with magnitude larger than or equal to 1.2 through manually picking the P- and S-wave arrivals, and determining hypocenters using a one-dimensional velocity structure with reference to the results by Morita et al. (2021). We will analyze smaller earthquakes as well in the future. In addition to the earthquakes, we plan to search volcanic tremors and low-frequency seismic activities from the continuous seismograms. Furthermore, we succeeded to observe about 100 teleseismic events, which are expected to contribute to understanding of the deep structure beneath this region.