[S-VC32] Active Volcanism

[SVC32-P13] Estimation of density and magnetization structure of Kusatsu-Shirane Volcano using joint inversion

*Hiroki Nagaike¹, Wataru Kanda² (1.Graduate major in Earth and Planetary Sciences, Department of Earth and Planetary Sciences, School of Science, Institute of Science Tokyo, 2.Multidisciplinary Resilience Research Center, Institute of Integrated Research, Institute of Science Tokyo)

Keywords:density structure, magnetization structure, joint inversion, Kusatsu-Shirane Volcano

Kusatsu-Shirane Volcano, where multiple phreatic eruptions have occurred in recent years, has been studied using various geophysical methods. Makino et al. (2004) conducted microgravity surveys in the summit area of the volcano and estimated the two-dimensional basement structure based on the significant negative gravity anomaly to the east of the Yugama crater lake. Koyama et al. (2021) conducted aeromagnetic surveys using an unmanned helicopter after the 2018 eruption at Mt. Motoshirane. They estimated a three-dimensional magnetization intensity distribution from the measured magnetic anomaly data and showed its correspondence with the resistivity structure model presented in previous studies. However, the problem with the inversion of potential fields such as gravity and magnetic anomalies is that the solution is non-unique, and multiple models may fit the observed data with the same accuracy. To avoid this non-uniqueness as much as possible, it is necessary to impose constraints on the data and/or model. One method for this is joint inversion, which estimates a model that simultaneously explains two or more kinds of data.

In this study, we conducted a three-dimensional structural analysis of the broad area around Kusatsu-Shirane Volcano using the Gravity Database of Japan (Geological Survey of Japan, 2013) and the Aeromagnetic Database of Japan (Nakatsuka and Okuma, 2009). We conducted joint inversion on the two kinds of data and obtained a three-dimensional density and magnetization structure model. Here, we assumed that the structures generating the gravity and magnetic anomalies are similar, and a cross-gradient term is added to the objective function of the joint inversion to impose a constraint that the spatial variations in the density and magnetization structures are correlated. In this presentation, we will discuss the differences between the model obtained by the joint inversion and the inversion results obtained from each data set individually. In addition, Matsunaga et al. (2022) estimated a three-dimensional resistivity structure of Kusatsu-Shirane Volcano and pointed out a correlation between the resistivity distribution at a depth of 0 km above sea level and the Bouguer anomaly. We plan to compare and discuss the density and magnetization structures obtained in this study with the resistivity structure model proposed by Matsunaga et al. (2022).