Three-dimensional resistivity structure of Asama Volcano revealed by data-space magnetotelluric inversion using unstructured tetrahedral elements

Usui, Yoshiya; Ogawa, Yasuo; Aizawa, Katsuo; Kanda, Wataru; Hashimoto, Takeshi; Koyama, Takao; Yamaya, Yusuke; Kagiyama, Tsuneomi

doi:10.1093/gji/ggw459

Cited by 56 publications

(35 citation statements)

References 37 publications

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“…Such a bell-shaped conductor beneath the central cone or geothermal/fumarolic area has been previously detected at other volcanoes (e.g. Nurhasan et al 2006;Kanda et al 2008;Komori et al 2013;Yamaya et al 2013;Usui et al 2017). With reference to the conceptual model of a geothermal system described by Pellerin et al (1996), we interpret that the bell-shaped conductor and the resistive body represent an impermeable layer consisting of altered clays and a bounded hydrothermal reservoir, respectively.…”

Section: Discussionsupporting

confidence: 68%

“…Here, we used a fixed value of β 2 = 0.01 since several reports had demonstrated that such a small trade-off parameter was sufficient for the MT inversion (e.g. Sasaki and Meju 2006;Avdeeva et al 2015;Usui et al 2017). However, we ran six inversions with various values of α 2 (100, 31.6, 10, 3.16, 1.0 and 0.316) and determined the performance of each model in terms of both its fit to the various parts of the dataset and the differences with the other models.…”

Section: Three-dimensional Inversion and Modelled Resistivity Structurementioning

confidence: 99%

“…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 2015Seki et al , 2016Usui et al 2017). In our study area, we obtained magnetotelluric (MT) data in the dense site array (Ogawa et al 2011;Yoshimura et al 2012Yoshimura et al , 2013 to infer the structure of Hakone volcano, with a focus on shallow caldera Fig.…”

Section: Introductionmentioning

confidence: 99%

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

Yoshimura

Ogawa

Yukutake

et al. 2018

Earth Planets Space

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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 threedimensional 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 highsalinity 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.

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Section: Discussionsupporting

confidence: 68%

Section: Three-dimensional Inversion and Modelled Resistivity Structurementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Yoshimura

Ogawa

Yukutake

et al. 2018

Earth Planets Space

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“…There are three measurement sites within the crater system, around which there is steep topography. Therefore, we performed 3-D inversion with a finite element code utilizing an unstructured tetrahedral mesh that can incorporate topography into a resistivity model (FEM-TIC; Usui 2015; Usui et al 2017). This code has been applied successfully to volcanic areas at Asama volcano (Usui et al 2017) and Hakone volcano (Yoshimura et al 2018).…”

Section: -D Inversionmentioning

confidence: 99%

Hydrothermal system of the active crater of Aso volcano (Japan) inferred from a three-dimensional resistivity structure model

Kanda

Utsugi

Takakura

et al. 2019

Earth Planets Space

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During the past two decades, studies of the Aso volcano in Japan have improved our understanding of the shallow hydrothermal system that exists beneath the active crater of this volcano. Detailed knowledge of the subsurface structure of this volcanic edifice is essential for developing a better understanding of the behavior of the volcanic fluids and of the triggering mechanism of volcanic eruptions. Here, we report a three-dimensional (3-D) electrical resistivity model for the active crater of the Nakadake central cone of Aso volcano using audio-frequency magnetotelluric (AMT) data obtained during 2004-2005. The AMT data were collected at 43 sites on a grid (distance between grid points: ~ 300 m) around the crater. However, as yet, only two-dimensional sectional resistivity models have been generated for this survey area. Using 3-D inversion, we obtain a resistivity model that shows similar characteristics to those of the 2-D models. A highly conductive zone is observed beneath the active crater down to a depth of approximately 300 m. Based on the recent findings regarding the shallow hydrothermal system of the volcano, we interpret this conductive zone to have been formed by highly conductive acidic fluids filling a fractured region. This view modifies the past interpretation made on the 2-D models and promotes understanding of fluid behavior beneath the active crater.

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“…Furthermore, in some other geoelectric and geomagnetic exploration methods, FEM using an unstructured tetrahedral mesh has been increasingly used for 3-D modelling owing to its capability of incorporating precise topography and bathymetry into a computational mesh with a practical number of elements (Günther et al 2006;Schwarzbach & Haber 2013;Usui 2015). For 3-D land MT problem, Usui (2015) demonstrated that the approximation error of topographic expression can be reduced sufficiently by using an unstructured tetrahedral mesh, and Usui et al (2017) estimated the 3-D resistivity structure of Asama Volcano with a data-space MT inversion code using mesh of that type. In addition, Schwarzbach & Haber (2013) demonstrated the applicability of an unstructured tetrahedral mesh to 3-D marine controlled source electromagnetics.…”

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confidence: 99%

Marine magnetotelluric inversion with an unstructured tetrahedral mesh

Usui

Kasaya

Ogawa

et al. 2018

Geophysical Journal International

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The finite element method using an unstructured tetrahedral mesh is one of the most effective methods for the 3-D modelling of marine magnetotelluric data that are strongly affected by bathymetry, because it enables us to incorporate both small-scale and regional-scale bathymetry into a computational mesh with a practical number of elements. The authors applied a 3-D inversion scheme using mesh of this type to marine magnetotelluric problems for the first time and verified its applicability. Forward calculations for two bathymetry models demonstrated that the results obtained with an unstructured tetrahedral mesh are close to the reference solutions. To evaluate the forward calculation results, we developed a general TM-mode analytical formulation for a 2-D sinusoidal topography. Moreover, synthetic inversion test results confirmed that a 3-D inversion scheme with an unstructured tetrahedral mesh enables us to recover subseafloor resistivity structure properly even for a model including a land-sea boundary as well as seafloor undulations. The verified inversion scheme was subsequently applied to a set of marine magnetotelluric data observed around the Iheya North Knoll, the middle Okinawa Trough. 3-D modelling using a mesh with precise bathymetry demonstrated that the data observed around the Iheya North Knoll are strongly affected by bathymetry, especially by the sea-depth differences between the depression of the trough and the shallow East China Sea. The estimated resistivity structure under the knoll is characterized by a conductive surface layer underlain by a resistive layer. The conductive layer implies permeable pelagic/hemipelagic sediments, which are consistent with a previous seismological study. Furthermore, the conductive layer has a resistive part immediately below the knoll, which is regarded as the consolidated magma intrusion that formed the knoll. Furthermore, at a depth of 10 km, we found that the resistor underneath the knoll extends to the southeast, implying that subseafloor resistivity under the Volcanic Arc Migration Phenomenon area is more resistive than the surroundings due to the presence of consolidated magma.

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Three-dimensional resistivity structure of Asama Volcano revealed by data-space magnetotelluric inversion using unstructured tetrahedral elements

Cited by 56 publications

References 37 publications

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

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

Hydrothermal system of the active crater of Aso volcano (Japan) inferred from a three-dimensional resistivity structure model

Marine magnetotelluric inversion with an unstructured tetrahedral mesh

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