2:45 PM - 3:00 PM
[SVC36-05] 3-D resistivity structure beneath the Kikai submarine caldera volcano
Keywords:Kikai caldera submarine volcano, magnetotelluric survey, resistivity structure, magma supply system, mantle wedge
This study aims to understand the current magma supply system leading to giant caldera eruptions. The Kikai submarine caldera volcano is located on the Southwest Japan Volcanic Front and lies within southern Kagoshima Prefecture. This volcano is known for its 7.3 ka giant caldera-forming eruption, the most recent giant caldera eruption in Japan. Topographic and petrological studies suggest that a new magma supply led to the formation of the central lava dome even after the giant caldera eruption (Tatsumi et al., 2018), which has been constrained to have occurred after 3.9 ka (Hamada et al., 2023). While geological studies provide insights into past magmatic activity, the present state of magma supply can be constrained by investigating the current structure beneath the caldera volcano using geophysical methods. A seismic structure provided by seismic tomography has identified a low-velocity zone within the mantle wedge beneath this region (Yamamoto et al., 2024; VSJ 2024 Fall Meeting). In this study, we present the resistivity structure obtained from an MT survey, which provides an additional constraint on the current structure beneath the caldera volcano independent of seismic velocity data.
We estimated the MT (magnetotelluric) response function using the robust least-squares estimator BIRRP (Chave and Thomson, 2004) from the data acquired by Ocean Bottom Electro-Magnetometers at 32 sites. The magnetic data from two land-based stations in Kagoshima Prefecture (Kanoya and Haraigawa) was used as remote reference data. The coordinate system was defined such that the x-axis is oriented parallel to the trench axis. We examined the power spectral density (PSD) of the electric field and found that the PSD in the x-direction was smaller than the y-direction. As a result, the MT response function exhibited the following features: the apparent resistivities in the xx- and xy-components tended to be generally low, and the associated uncertainties tended to be large. These characteristics were observed consistently across all survey sites, rather than being limited to specific areas.
We estimated the three-dimensional resistivity structure beneath the seafloor in the Kikai Caldera area using the 3-D MT inversion code FEMTIC (Usui 2015; Usui et al., 2018). The model domain covered approximately 2700 x 2700 x 2000 km, and a flexible hexahedral mesh (Usui et al., 2024) was utilized to easily and appropriately handle the effect of the seafloor topography. The initial model was based on a 1-D structure estimated from the average MT response function obtained at the sites outside the caldera rim and incorporates the Ryukyu slab, referring to previous research on the regional tectonic setting (e.g., Baba et al., 2010). Data selection was performed using statistical quality criteria and visual inspection. To assess the robustness of the inferred resistivity structure, we examined multiple inversion settings. Specifically, we tested models where the resistivity of the slab was fixed at a constant value, as well as models where smoothing constraints along the slab surface were relaxed. We will present the results of these variations and discuss their implications for the subsurface structure of the Kikai Caldera.
We estimated the MT (magnetotelluric) response function using the robust least-squares estimator BIRRP (Chave and Thomson, 2004) from the data acquired by Ocean Bottom Electro-Magnetometers at 32 sites. The magnetic data from two land-based stations in Kagoshima Prefecture (Kanoya and Haraigawa) was used as remote reference data. The coordinate system was defined such that the x-axis is oriented parallel to the trench axis. We examined the power spectral density (PSD) of the electric field and found that the PSD in the x-direction was smaller than the y-direction. As a result, the MT response function exhibited the following features: the apparent resistivities in the xx- and xy-components tended to be generally low, and the associated uncertainties tended to be large. These characteristics were observed consistently across all survey sites, rather than being limited to specific areas.
We estimated the three-dimensional resistivity structure beneath the seafloor in the Kikai Caldera area using the 3-D MT inversion code FEMTIC (Usui 2015; Usui et al., 2018). The model domain covered approximately 2700 x 2700 x 2000 km, and a flexible hexahedral mesh (Usui et al., 2024) was utilized to easily and appropriately handle the effect of the seafloor topography. The initial model was based on a 1-D structure estimated from the average MT response function obtained at the sites outside the caldera rim and incorporates the Ryukyu slab, referring to previous research on the regional tectonic setting (e.g., Baba et al., 2010). Data selection was performed using statistical quality criteria and visual inspection. To assess the robustness of the inferred resistivity structure, we examined multiple inversion settings. Specifically, we tested models where the resistivity of the slab was fixed at a constant value, as well as models where smoothing constraints along the slab surface were relaxed. We will present the results of these variations and discuss their implications for the subsurface structure of the Kikai Caldera.