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

2018; American Geophysical Union; Volume: 45; Issue: 23 Linguagem: Inglês

10.1029/2018gl080202

1944-8007

Kaori Tsukamoto, Koki Aizawa, Keita Chiba, Wataru Kanda, Makoto Uyeshima, Takao Koyama, Mitsuru Utsugi, Kaori Seki, Takahiro Kishita,

Geophysical Methods and Applications

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.