Description: Electromagnetic surveys such as magnetotellurics are known to be sensitive to the presence of smectite, which is abundant in subduction zones. However, the contribution of smectite to bulk conductivity is complex and still needs to be quantitatively understood better. Therefore, in this study, we investigated the effect of smectite on bulk conductivity by numerical modeling using the finite element method. In our simulations, we investigated the effects of four conditions: porosity, fraction of smectite in the quartz matrix, temperature, and NaCl concentration of pore water. Here, we randomly assigned each element's conductivity tensor of either material: smectite, quartz, or pore water. In addition, we introduced an anisotropy to the conductivity tensor of smectite between the directions parallel and perpendicular to the T-O-T layer. Meanwhile, the conductivity tensors of quartz and pore fluid were considered isotropic. The simulation results showed that the spatial arrangement of the materials significantly affected the bulk conductivity for the porosity greater than 0.01. In contrast, for smaller porosity, the effect of the spatial arrangement was almost negligible, and the volume fraction of smectite, pore water salinity, and temperature controlled the bulk conductivity. These results suggest that proper parameterization of the element arrangement patterns is essential when calculating the bulk conductivity of smectite-bearing rocks.

Description: The volcanic alteration zone at a depth of several hundred meters below the surface is a key feature to assess the degree of the imminence of phreatic eruption; Recent magnetotelluric images indicate that the conductive alteration zone prevents hypocenters of volcanic earthquakes from reaching the surface（Tsukamoto et al., 2018; Tseng et al., 2020; Gresse et al., 2021). A phreatic eruption could occur, when the hypocenters pass through the conducting layer. To approach the surface alteration zone and demagnetized area in the active volcanic edifice, a total intensity aeromagnetic survey using an unmanned aerial vehicle was carried out in 1×2.5 km over Oana Crater, with now on active fumaroles inside, of Mt. Azuma, NE Japan. The original magnetic anomaly distribution was obtained using Nakatsuka and Okuma (2006) with correcting the main field (IGRF-13) and the diurnal variations. The average magnetization intensity (AMI) was 2.06 A/m estimated by a statistical correlation method (Grauch 1987). After subtracting the contribution of the average magnetization from the original magnetic anomaly, the resultant magnetic anomaly data were inverted into a three-dimensional subsurface magnetization perturbation from AMI using the effective source volume minimization method (Nakatsuka and Okuma, 2014). The result shows a weakly magnetized region below the Tsubakurozawa Crater, where the past eruption occurred in 1893, 1896, juxtaposed with Oana Crater. The weakly magnetized location coincides with the demagnetization source location modeled by using the repeating total magnetic intensity observations (Japan Meteorological Agency, 2020).

Description: We conducted electromagnetic survey at Mt. Usu, one of the active volcanoes in Hokkaido, northern Japan. We succeeded in the first three-dimensional imaging of the resistivity structure beneath the summit crater field of the volcano. This study revealed the geometrical relationship between the subsurface structure and the sources of micro-earthquakes, remagnetization, and ground deformation. Mt. Usu is an active volcano that has repeatedly erupted about every 30 years since the 20th century. It is known from historical and geological records that the summit crater experienced repeated magmatic to phreatomagmatic eruptions in the past. Even in the present inter-eruptive period following the 2000 phreatomagmatic event on the western flank, remarkable ground deformation, micro-seismicity, and magnetic field changes have been observed in the summit crater field. Therefore, such geophysical monitoring is vital for evaluating the volcano’s activity level. On the other hand, although several electromagnetic surveys (e.g., Matsushima et al., 2001) were previously conducted in the past, detailed three-dimensional imaging has been awaited. To this end, we conducted audio-frequency band magnetotelluric surveys in 2021 and 2022 to unveil the shallow three-dimensional electrical structure. Our model recognizes a low resistivity anomaly (1-10 Ωm) extending westward from the Ginuma crater at approximately 200-500 m below sea level. Current micro-seismicity and continuous remagnetization (Hashimoto, 2022) occur slightly above this low resistivity anomaly. On the other hand, the center of the long-term ground deflation (Wang and Aoki, 2019) is located on another low resistivity patch on the northeast side of the crater field.

Description: Recently, continuous or periodic multivariate geophysical/geochemical monitoring at active volcanoes has become increasingly common worldwide. While it facilitates a comprehensive understanding of volcanic processes, day-to-day fluctuations in multiple data during a non-eruptive stage sometimes complicate distinguishing between unrest and background activity. Furthermore, different datasets often appear to contradict each other. Here, we face a challenge in quantitatively assessing unrest events based on multiple data of various kinds. This study has piloted the application of the volcanic unrest index (VUI) proposed by Potter et al. (2015) to one of the selected Japanese volcanoes on a research basis. First, we modified their template of the VUI worksheet in New Zealand to fit Mt Tokachidake in northern Japan. Then, we applied it to the monitoring records since 1964 to calculate the monthly index. Here, we introduced four categories: seismology, geodesy, geothermics, and geochemistry, with some specific entries for each. Parameter ranges were set to evaluate each time series based on a five-point scale from 0 to 4. The VUI is an integrated evaluation method for diverse data, developed as a communication tool with non-scientists and not for prediction purposes (Potter et al. 2015). One of the advantages of the VUI is its capability to assess the severity of a particular unrest event in an intuitive and transferrable way, i.e., how rare it is among all records at that volcano. Although we may need further tuning, our first trial looks reasonable, as the only magmatic eruptive event in 1989-90 follows the prolonged period of elevated VUIs.