Three‐Dimensional Resistivity Structure of Iwo‐Yama Volcano, Kirishima Volcanic Complex, Japan: Relationship to Shallow Seismicity, Surface Uplift, and a Small Phreatic Eruption

Abstract

Iwo‐yama volcano, part of the Kirishima Volcanic Complex, has recently shown signs of unrest. We conducted a hypocenter relocation of shallow earthquakes and broadband magnetotelluric measurements around Iwo‐yama. Three‐dimensional inversion of magnetotelluric data revealed an electrically conductive layer that is interpreted as a hydrothermally altered clay‐dominated unit. Shallow earthquakes occur beneath this layer, suggesting that it controls the location of seismicity. The base of the layer corresponds to the depth of a pressure source identified by a leveling survey. These observations suggest that the supply of high‐temperature fluids has increased over time beneath Iwo‐yama, causing an increase in pore pressure beneath the clay‐rich layer and resulting in tectonic earthquakes and ground inflation. Increased upwelling of fluids through a fracture in the clay‐rich layer may have caused a vigorous liquid‐gas phase transition near the surface, which in turn might have led to the small phreatic eruption on 19 April 2018.

Significance of Electromagnetic Surveys at Active Volcanoes: Toward Evaluating the Imminence of Wet Eruptions

“…In the case of Iwo-yama in Kirishima Volcanic Complex, it is confirmed with a several lines of supporting evidence, such as the depth of the ground deformation and the epicenters of micro-earthquakes, that the low resistivity layer corresponds to the impermeable layer, which is rich in altered clay minerals [5]. However, this is not verified in the case of KVG, in which no obvious ground deformation of volcanic origin, which would be suggestive of an inflating pressure source, has been observed.…”

“…Pronounced low resistivity is often seen at shallow depths in geothermally active fields, as is reported in the recent MT surveys such as at Iwo-yama in Kirishima Volcanic Complex [5] and Hakone Volcano [36], both of which are based on 3D inversion modeling. As summarized in (3) and (4) in the previous section, similar low resistivity zones indicating 1−10 Ωm are identified in our study at KVG as C1 and C2.…”

“…One of these is to understand the locations of unrest phenomena in relation to the subsurface structure. Recent case studies have provided the general notion that shallow micro-earthquakes and ground deformation are tightly controlled by hydrological structure, especially by an altered clay-rich conductive layer that works as an impermeable seal around a vent region of volcanoes with repetitive wet eruptions (e.g., Kuchino-erabu-jima [39]， Kusatsu-Shirane [40] ， Hakone [36] ， Iwo-yama in Kirishima Volcanic Complex [5]，Jigokudani Valley in Mt. Tateyama [41]).…”

“…In some instances, eruption was thought to occur beforehand by the detection of so-called unrest phenomena, such as shallow micro-earthquakes and localized ground inflation, resulting in actual phreatic eruptions (Mt. Hakone in 2015 [4]; Iwo-yama in Kirishima Volcanic Complex in 2018 [5]). At Mt.…”

Electrical Conductive Fluidized Zones and Their Influence on the Earthquake Nucleation, Growth, and Arrest Processes of the 2016 Kumamoto Earthquake Sequence, Kyushu Island, Japan

“…3b and d). These triple-layered structures are constrained by the 200 °C and 400 °C isotherms, and the corresponding lithologies (C1 − C1N − C1Ntop, and C1 − C1S − C1Stop) are essentially the same as those in widely accepted resistivity models for geothermal zones and volcanoes (Fournier, 1999;Tsukamoto et al, 2018), and have also been proposed for active faults (Sibson, 2007).…”