Yoshiya Usui

The University of Tokyo | Todai · Earthquake Research Institute

A 3-D magnetotelluric (MT) inversion code using unstructured tetrahedral elements has been developed in order to correct the topographic effect by directly incorporating it into computational grids. The electromagnetic field and response functions get distorted at the observation sites of MT surveys because of the undulating surface topography, and...

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 wide...

The electrical conductivity of subsurface rocks is generally anisotropic. The anisotropy of the subsurface electrical conductivity provides important information on the stress-strain state and geodynamics. To quantitatively interpret anisotropic conductivity structures revealed by electromagnetic surveys, it is essential to use a mixing model consi...

The solution of the remote reference method, a frequently used technique in magnetotelluric data processing, can be viewed as a product of the two-input-multiple-output relationship between the local electromagnetic field and the reference field at a remote station. By applying a robust estimator to the two-input-multiple-output system, one can sup...

Intraplate earthquakes occur more frequently in the Japanese islands than in other regions. Large intraplate earthquakes in the island arc preferentially occur in strain concentration zones detected by geological and geodetic studies. Crustal heterogeneity plays a crucial role in generating large intraplate earthquakes and strain concentrations. Th...

This study presents a 2-D magnetotelluric inversion code tailored for unstructured triangular meshes, developed using Julia, a high-level, high-performance programming language designed for scientific and numerical computation. The forward modeling engine utilizes a node-based finite element method to solve electromagnetic fields throughout the mod...

Magnetotelluric data from Mount Tongariro has been analysed using an unstructured tetrahedral finite-element inversion code that incorporates topography, which was not included in previous analysis of these data. Incorporating topography adds information, which stabilises the resistivity inversion modelling, and for the first time allows details of...

Many active volcanoes and various types of seismic activities exist in the southern part of the Northeast Japan subduction zone. One of the geologically most interesting features in this area is the sequential explosive eruptions of a group of volcanoes. The group consists of Mt. Azuma and Mt. Adatara on the volcanic front line, Mt. Bandai west of...

The southern part of Tohoku, Northeast Japan, is an area with significant in-land activities owing to the ongoing subduction mechanism. Among these are active volcanoes distributed on the volcanic front and back-arc, active faults throughout the area, and a recently observed swarm of shallow earthquakes on the fore-arc side. As fluids play an essen...

I tested the forward-calculation part by using the model of the seafloor with a 2-D sinusoidal undulation. Schwalenberg & Edwards (2004) and Usui et al. (2018) showed an analytical formulation for this model. Originally, Schwalenberg & Edwards proposed, and later, Usui et al. modified the formulation. I calculated the apparent resistivity, the phas...

The combined inversion using distortion-free response functions is an effective approach to robustly estimate the 3-D electrical resistivity structure against the distortions caused by near-surface resistivity anomalies. However, previous combined inversion analyses have presented a significant dependency of the inversion results on initial and pri...

Topographic distortion is a major problem in magnetotelluric modeling and interpretation. Especially, observed data of ocean magnetotelluric surveys can be affected by large-scale bathymetry and land-ocean distribution as well as smaller-scale seafloor undulations around observation sites. Some modeling approaches have been proposed to overcome the...

We aimed to perform three-dimensional imaging of the underlying geothermal system to a depth of 2 km using magnetotellurics (MT) at around the Yugama crater, the Kusatsu-Shirane Volcano, Japan, which is known to have frequent phreatic eruptions. We deployed 91 MT sites focusing around the peak area of 2 km × 2 km with typical spacings of 200 m. The...

In FBR plants the head plate constitutes a part of the boundary of the containment vessel (CV), therefore, it is a very important issue if the function as the boundary is maintained or not in the case of severe accident (SA). Buckling and post-buckling behaviors due to pressure loading must be affected by the thickness distribution of the head plat...

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 observati...

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 au...

Asama Volcano is an andesitic composite volcano and one of the most active volcanoes in Japan. In order to reveal electrical resistivity structure beneath the volcano accurately, we performed a 3-D inversion of dense magnetotelluric survey data. In order to prevent misinterpretation of the subsurface resistivity due to the steep topography around A...

The finite element method using unstructured tetrahedral elements is one of the most effective methods to correct topographic distortions in the marine magnetotelluric data since it can precisely incorporate the bathymetry into computational grids without using too many elements. However, in relation to marine magnetotelluric problems, the use of h...

For the purpose of confirming failure modes and safety margin, some studies on the ultimate strength of thin-walled piping components for Sodium-cooled Fast Reactors (SFRs) under extreme loading conditions such as large earthquakes have been reported these several years. Nonlinear finite element analysis has been applied in these studies to simulat...

A resistivity section based on magnetotelluric data was obtained for the Shonai Plain fault in northeastern Honshu, Japan. Faults in this area were created as normal faults during the opening of the Japan Sea in the Miocene but are now reactivated as high-angle reverse faults under compressional tectonics. Geological interpretations of the resistiv...