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Inferno Crater Lake, located in the Waimangu Geothermal Zone on the North Island of New Zealand, was formed by the eruption of Tarawera Volcano on June 10, 1886. Scott’s observations [1, 2] from 1972-1990 showed that the water level fluctuated on a 38-day cycle, and the water temperature fluctuated accordingly. As the water level rises or is high, the seismic noise level increases [3, 4], which has been interpreted as the two-phase region in the subsurface pushing up against the liquid-phase region above [5]. In addition, electrical resistivity surveys using the self-potential method during high and low water periods in both 2006 and 2007 identified a region of 40 times higher resistivity during the high-water period just below Inferno Crater Lake, suggesting the presence of the two-phase layer[6]. Monitoring and observing fluctuations in the gas-liquid two-phase layer is important for volcanic eruption forecasting research. In order to accurately observe the time-varying structure of the hydrothermal system, observations using the EM-ACROSS method are planned around the Inferno Crater Lake. In this study, we developed a forward analysis program for the EM-ACROSS method and investigated the response in the frequency band used for the observation. We first developed a program to calculate the electromagnetic field produced by finite-length dipole currents. We performed the calculations on a model for which the theoretical solution is known and compared the analytical and numerical solutions to confirm that they were sufficiently close. The tensor transfer function at an arbitrary observation point can be calculated from the observed electric and magnetic fields and the installed dipole moment. We then calculated the tensor transfer function and obtained the frequency response for a model mimicking Inferno Crater Lake. We found that the off-diagonal component of the tensor transfer function at points far from the inline is particularly sensitive to two-phase (high res

During the last magmatic eruption period of Aso volcano (November 2014 to May 2015), a controlled-source electromagnetic volcano monitoring experiment (ACTIVE) was conducted. Here, we interpret the temporal variations in the electromagnetic responses. The ACTIVE system installed at the first Nakadake crater, the only active crater of Aso, consisted of a transmitter located northwest of the crater and four (before the eruptions) or three (after the eruptions) vertical induction coil receiver stations. The ACTIVE system succeeded in detecting temporal variations in the resistivity structure during the latest magmatic eruption period. The response amplitude started to increase in November 2014, peaked in February 2015, and decreased slightly in August 2015. An unstructured tetrahedral finite element three-dimensional inversion that accounted for topographic effects was used to interpret temporal variations in the ACTIVE response. The 3-D inversion results revealed that temporal variations in the ACTIVE response are attributed mainly to (1) a broad increase in resistivity at elevations from 750 to 850 m, not only directly beneath the crater bottom but also outside the crater, and (2) a thin layer of decrease in resistivity at the elevation of ~ 1000 m on the western side of the crater. The increase in resistivity can be ascribed to a decrease in the amount of conductive groundwater in the upper part of an aquifer located below the elevation of 800 m, while the decrease in resistivity implies that enhanced fluid temperature and pressure changed the subsurface hydrothermal system and formed a temporal fluid reservoir at the shallow level during the magmatic eruption period.

Tsunami flow coupled with the geomagnetic field generates electric currents and associated magnetic fields. Although electromagnetic (EM) tsunami signals can be used for analysis and even forecasting tsunami propagation, the dynamically self-consistent effect of shoaling water depth on the fluid + electrodynamics has not been adequately clarified. In this study, we classify tsunami EM phenomena into three cases based on the ocean depth and find that the deeper ocean results in stronger self-induction due to the increase in both tsunami phase velocity and ocean conductance. In this deep-ocean case, the phase lead of the vertical magnetic variation relative to the sea surface elevation is smaller, while an initial rise in the horizontal magnetic component becomes observable prior to tsunami arrival. Furthermore, we confirm that the enhancement of tsunami height in shallower oceans shifts the ocean depth supplying maximum amplitudes of tsunami magnetic fields from approximately 2.0 km to 1.5 km.