[S-CG55] Ocean Floor Geoscience

[SCG55-18] Magnetic anomaly on the Sofu Seamount, Izu-Ogasawara (Bonin) Arc

*Toshiya Fujiwara¹, Noriko Tada¹, Kenta K. Yoshida¹, Aki Ito¹, Kentaro Imai¹, Masayuki Obayashi¹, Koichiro Obana¹, Gou Fujie¹, Shigeaki Ono¹, Shuichi Kodaira² (1.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, 2.Japan Agency for Marine-Earth Science and Technology)

Keywords:Izu-Ogasawara (Bonin) Arc, Sofu Seamount, Submarine volcano, Magnetic anomaly

In response to the earthquakes and tsunamis that occurred near Torishima-Sofugan, Izu-Ogasawara Arc, in early October 2023 (until October 9), we conducted a research cruise by the R/V Kaimei in November 2023 in the area around the Sofu Seamount, where a submarine volcanic eruption was presumed to have occurred. During the cruise, in addition to the installation of ocean bottom seismographs (Obana et al., JpGU 2024) and the bathymetric survey (Fujiwara et al., GRL 2024), the magnetic survey was conducted using a surface-towed cesium magnetometer and a shipboard three-component magnetometer. The aim of the magnetic survey is to understand the geological background of the Sofu Seamount and to investigate changes in magnetization caused by the submarine volcanic eruption. In an area extending from Torishima to Sofugan back-arc rifts centered on the Sofu Seamount, eight magnetic total force survey tracks aligned in a north-south direction at 1.5 or 3 mile intervals and 10 to 25 miles long and six east-west tracks at 1 mile intervals and 17 to 18 miles long were acquired. Magnetic anomalies were calculated by subtracting the International Geomagnetic Reference Field IGRF-13. The RMS of the crossover error was reduced from 27.4 nT to 13.8 nT by correcting for time variation using geomagnetic data from the Chichijima Station of the JMA Geomagnetic Observatory. The Sofu Seamount is about 25 km long from east to west, 17 km long from north to south, and has a relative height of about 2000 m. The water depth of the central cone in the caldera is about 900 m. Corresponding to the seamount body, a positive anomaly maximum of about +750 nT is observed above the caldera, and a negative anomaly minimum of about -450 nT is observed on the northeast side of the seamount. The general trend of magnetic anomalies is negative north and positive south of the seamount in the central to eastern part of the seamount. This indicates that the seamount is magnetized almost in the direction of the Earth's current magnetic field. However, the western part of the seamount is disrupted in its positive/negative dipole pattern. Preliminary analysis of the magnetization structure (assuming constant magnetization in the depth direction) shows that a positive magnetization area extending in a NNW-SSE direction parallel to the strike of the rift extends from the Sofu Seamount to the Torishima Rift. The western part of the seamount is weakly magnetized or inversely magnetized, and the weakly (inversely) magnetized area extends in the NNW-SSE direction. The central cone is strongly positively magnetized, but there may be a tendency for slightly lower magnetization in the NNW-SSE direction through the center of the central cone.