This study focuses on the results of an audio-frequency magnetotelluric (AMT) survey across the Jigokudani valley, Tateyama volcano, Japan, to investigate the spatial relationship between the distribution of electrical resistivity and geothermal activity and to elucidate the geologic controls on both its phreatic eruption history and recent increase in phreatic activity. The AMT data were collected at eight locations across the Jigokudani valley in September 2013, with high quality data obtained from most sites, enabling the identification of an underground 2D resistivity structure from the transverse magnetic (TM) mode data. The data obtained during this study provided evidence of a large conductive region beneath the surface of the Jigokudani valley that is underlain by a resistive layer at depths below 500 m. The resistive layer is cut by a relatively conductive region that extends subvertically toward the shallow conductor. The shallow conductive region is divided into an uppermost slightly conductive section that is thought to be a lacustrine sediment layer of an extinct crater lake containing hydrothermal fluids and a lower section containing a mix of volcanic gases and hydrothermal fluids. The low permeability of the clay zone means that the uppermost clayey sediments allow the accumulation of gases in the lower section of the conductive region, suggesting the existence of a cap structure. The deep resistive layer likely consists of units similar to the granitic rocks that are widely exposed throughout the Jigokudani valley. We suggest that the relatively conductive zone that separates these granitic rocks represents a high-temperature volcanic gas conduit, given that the most active fumarole in the Jigokudani valley lies directly along the trajectory of this path.
The Atotsugawa fault is one of the most active faults in Japan, and the strain accumulation at the fault is considered to be caused by an aseismic shear zone in the fluid‐rich lower crust. To identify the shear zone and investigate the origin of the aqueous fluid in the lower crust, we deployed a Network‐MT survey in addition to a conventional wideband‐MT survey around the fault and performed an inversion combining both the MT data sets. In the inversion, by modifying a conventional inversion algorism, we accurately represented kilometer‐scale dipoles of the Network‐MT measurement to provide constraints on the electrical resistivity structure. In the lower crust under the study area, there are localized conductive anomalies below the Atotsugawa fault, the Ushikubi fault, and the Takayama‐Oppara fault zone. Comparing our electrical resistivity structure with the seismic velocity structure, we interpreted that the lower‐crustal conductors are localized ductile shear zones with highly connected fluid. We considered that the localized ductile shear zones are responsible for the strain accumulation along the respective active faults. In addition, in the mantle wedge above the subducting Philippine Sea slab and its downward extension, a highly conductive portion is detected, which may be attributed to the fluid dehydrated from the Philippine Sea slab and/or the Pacific slab. The existence of the large conductive area supports the suggestion of previous seismic and geochemical studies that the fluid of the lower crust around the Atotsugawa fault originated from subducting slabs.
SUMMARYObservations of geoelectric potential have been conducted since 1997 at Ohtawa (Kamioka town, Gifu prefecture) for the purpose of testing the VAN method (short-term earthquake prediction), as collaborative research of Toyama University and RIKEN-IFREQ.The data from Ohtawa station are obtained in excellent condition for geoelectric potential study because of small artificial electric noises. A relatively strong change of geoelectric potential is observed. This is clearly correlated with the timetable of Tateyama railway. That is, the geoelectric potential change is caused by the leakage current from DC train of Tateyama railway. The current travels a distance of more than 16 km.The geoelectric potential changes caused by the leak current of Tateyama railway may be useful to study the electrical resistivity of the soil and its time variation around the area. Time variation of the largest amplitude of the rail leak current has been analyzed since April 1998 to obtain the variation of apparent resistivity.So far, there has not been sensible correlation for apparent resistivity variation and an inert seismic activity. In contrast, it is remarkable that the resistance changes gently in a cycle of 1 year. The gentle variation of resistivity is thought to be caused by an annual or seasonal phenomenon. The leakage of the current from DC train is a powerful tool to monitor temporal variation of resistivity structure around the area.
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