Delineating fluid distribution beneath the southern Tohoku, Northeast Japan, with a joint inversion of magnetotelluric and geomagnetic transfer functions

Abstract

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 essential role in arc magmatism and the associated seismicity, this study aims to understand the deep fluid distribution beneath southern Tohoku to clarify the origin of the activities. A magnetotelluric survey delineating the subsurface electrical resistivity structure was used as the bulk resistivity is sensitive to the composition and connectivity of fluids. Using a newly developed joint inversion code, we estimated the resistivity structure using the inter-station horizontal magnetic field transfer function (HMTF) in addition to the conventional magnetotelluric response functions. Joint inversion with HMTF improved the recovery of low-resistivity anomalies owing to the sensitivity of the HMTF to electrical current concentration, resulting in a model with smaller data misfits. The main feature of the resulting resistivity structure is that, instead of under the volcanic front, a deep conductive body is found under the back-arc side in a position closer to a back-arc volcano (Mt. Numazawa) and a swarm of low-frequency earthquakes. Petrological studies indicate that the deep source of fluids supplying to Mt. Numazawa may be the same as that of Mt. Azuma and Mt. Adatara on the volcanic front. Magmatic fluids ascend from the upper mantle to the upper crust via different branches, resulting in multiple eruption centers. Thus, we inferred that the conductor reflects the fluid path to Mt. Numazawa. The high conductivity, especially in the uppermost mantle, may be caused by flux melting, where water or other volatiles released from the subducting slab reduce the solidus of high-temperature basaltic rocks.

Two-dimensional magnetotelluric inversion using unstructured triangular mesh implemented in Julia

“…After solving eq (9) for 𝒙, or the field value at all interior nodes, response functions at the observation stations can be calculated. The code can calculate the MT impedance, vertical magnetic field transfer function (or the Tipper), and inter-station horizontal magnetic field transfer function.…”

“…Due to the wide spectrum of the EM source fields, the MT method can probe depths spanning hundreds of meters to hundreds of kilometers. As a result, MT has been used for various purposes, such as local-scale geothermal investigation in volcanic areas (e.g., [3][4] [5][6]) and regional-scale imaging of tectonically active areas (e.g., [7][8] [9]).…”