2:45 PM - 3:00 PM
[SVC31-15] Imaging 3D resistivity structure under the seafloor of Kikai caldera volcano
Keywords:Kikai submarine caldera volcano, electromagnetic exploration, MT method
The Kikai submarine caldera volcano, located in the southern part of Kagoshima Prefecture, SW Japan, has repeatedly erupted in its giant caldera. The existence of lava dome inside the caldera suggests that the magma was supplied to this volcano even after the latest giant caldera eruption at 7.3 ka (Tatsumi et al., 2018). In addition, a low velocity anomaly region with width of ~30 km was inferred at depths of 3-11 km below the lava dome by seismic refraction survey (Nagaya et al., 2022). The purpose of this study is to better understand the magma supply system leading to giant caldera eruptions such as the one which formed the present Kikai caldera.
In this study, we explore the magma supply system by imaging the 3D resistivity structure beneath the seafloor of the Kikai caldera. Resistivity is more sensitive to the presence of fluid than seismic velocity, thus it is expected to provide another aspect in geophysical properties. We have been acquiring new seafloor magnetotelluric (MT) data by installing and recovering Ocean Bottom Electro-Magnetometers through cruises using T/S Fukaemaru of Kobe University, R/V Kairei and R/V Kaimei of JAMSTEC, and have been developing a numerical method to image the 3D structure from the obtained data.
Time-variations of electromagnetic fields with sampling rates of 8 Hz and 10 s, as well as instrumental tilts and temperature with sampling rates of 1 s or 10 s were observed at 24 sites. These sites cover not only densely an area near the lava dome but also a wide area over the caldera. The acquired time series data were denoised, de-trended, and rotated so that the measurement coordinate system at the seafloor was transformed into the geographic one. Trend components was removed from the data by fitting a cubic function at each 1.5-day interval. We obtained MT impedance at 24 sites from the analyzed data by using BIRRP (Chave & Thomson, 2004). Local noise in magnetic field observed at seafloor were excluded through remote reference processing with using magnetic field data from Japan Meteorological Agency.
We have developed a new MT inversion method to image 3D resistivity structures under the undulated seafloor. The new method models the seafloor topography by applying the Flattening Surface (FS) method (Baba & Seama, 2002) only to the seawater layer, and is incorporated into the inversion code of ModEM (Egbert & Kelbert, 2012; Kelbert et al., 2014); we named our new code ModEM+FS. Some images of the 3D resistivity structure beneath the seafloor of the Kikai caldera were obtained by the analysis of MT impedance using ModEM+FS. We carefully investigate the influence of initial models and model parameters on inversion results. We will present the inversion results of the 3D resistivity structure and their interpretations regarding the magma supply system.
In this study, we explore the magma supply system by imaging the 3D resistivity structure beneath the seafloor of the Kikai caldera. Resistivity is more sensitive to the presence of fluid than seismic velocity, thus it is expected to provide another aspect in geophysical properties. We have been acquiring new seafloor magnetotelluric (MT) data by installing and recovering Ocean Bottom Electro-Magnetometers through cruises using T/S Fukaemaru of Kobe University, R/V Kairei and R/V Kaimei of JAMSTEC, and have been developing a numerical method to image the 3D structure from the obtained data.
Time-variations of electromagnetic fields with sampling rates of 8 Hz and 10 s, as well as instrumental tilts and temperature with sampling rates of 1 s or 10 s were observed at 24 sites. These sites cover not only densely an area near the lava dome but also a wide area over the caldera. The acquired time series data were denoised, de-trended, and rotated so that the measurement coordinate system at the seafloor was transformed into the geographic one. Trend components was removed from the data by fitting a cubic function at each 1.5-day interval. We obtained MT impedance at 24 sites from the analyzed data by using BIRRP (Chave & Thomson, 2004). Local noise in magnetic field observed at seafloor were excluded through remote reference processing with using magnetic field data from Japan Meteorological Agency.
We have developed a new MT inversion method to image 3D resistivity structures under the undulated seafloor. The new method models the seafloor topography by applying the Flattening Surface (FS) method (Baba & Seama, 2002) only to the seawater layer, and is incorporated into the inversion code of ModEM (Egbert & Kelbert, 2012; Kelbert et al., 2014); we named our new code ModEM+FS. Some images of the 3D resistivity structure beneath the seafloor of the Kikai caldera were obtained by the analysis of MT impedance using ModEM+FS. We carefully investigate the influence of initial models and model parameters on inversion results. We will present the inversion results of the 3D resistivity structure and their interpretations regarding the magma supply system.