Resistivity characterisation of Hakone volcano, Central Japan, by three-dimensional magnetotelluric inversion

Ryokei Yoshimura,Shingo Kawasaki,Tomoya Yamazaki,Hideaki Hase,Masato Kamo,Yojiro Yasuda,Tada-Nori Goto,Yohei Yukutake,Ryou Honda,Yoshiya Usui,Tetsuya Higa,Takeshi Suzuki,Masanori Tani,Masatake Harada,Shogo Komori,Wataru Kanda,Yasuo Ogawa

doi:10.1186/s40623-018-0848-y

Abstract

On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermal area on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of Hakone caldera, we carried out a three-dimensional inversion of magnetotelluric measurement data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh for the inversion code of the edge-based finite element method to account for the steep topography of the region during the inversion process. The main features of the best-fit three-dimensional model are a bell-shaped conductor, the bottom of which shows good agreement with the upper limit of seismicity, beneath the central cones and the Owakudani geothermal area, and several buried bowl-shaped conductive zones beneath the Gora and Kojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally altered zone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic activity may cause volcanic fluids to pass through the resistive body surrounded by the altered zone and thus promote brittle failure within the resistive body. The overlapping locations of the bowl-shaped conductors, the buried caldera structures and the presence of sodium-chloride-rich hot springs indicate that the conductors represent porous media saturated by high-salinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area may indicate several weak zones that formed due to these structural contrasts.

Highlights

Hakone volcano is located at the northern end of the Izu–Mariana volcanic arc in Central Japan, 100 km west of Tokyo
On 29 June 2015, a small phreatic eruption was observed in the Owakudani geothermal area, which is located on the northern slope of the central cones of Hakone volcano, forming several vents, there is no prior historical record of magmatic eruptions there
We first compare our model with the two-dimensional NW–SE image proposed by Ogawa et al (2011)

Summary

Introduction

The purpose of this study is to map the three-dimensional electrical resistivity structure, discuss the volcanic system and characterise the structure of Hakone caldera. Since resistivity is sensitive to temperature (including thermal alteration) and the presence of fluids, it is commonly applied to infer the structural setting in volcanic and geothermal environments (e.g. Nurhasan et al 2006; Kanda et al 2008; Komori et al 2013; Yamaya et al 2013; Gasperikova et al 2015; Seki et al 2015, 2016; Usui et al 2017). We obtained magnetotelluric (MT) data in the dense site array (Ogawa et al 2011; Yoshimura et al 2012, 2013) to infer the structure of Hakone volcano, with a focus on shallow caldera structures. The topographic effects due to the relief of Hakone volcano within the survey area are accounted for in the inversion by applying the code developed by Usui (2015), which treats the model as a series of unstructured tetrahedral elements, to achieve a realistic representation of surface topography and subsurface resistivity structures

Objectives

Findings

Conclusion