Three-dimensional electromagnetic imaging of fluids and melts beneath the NE Japan arc revisited by using geomagnetic transfer function data

Wataru Kanda,Yasuo Ogawa

doi:10.1186/1880-5981-66-39

Abstract

The three-dimensional distribution of fluids and melts under the NE Japan arc was imaged using its resistivity structure, modeled with geomagnetic transfer functions. The data were collected at 37 stations located on a 20-km grid, at periods ranging from 16 to 256 s. In spite of the narrow period band nature, these periods turn out to be sensitive to conductors in the deep crust and upper mantle. The geomagnetic transfer functions represent lateral resistivity variations, which yield inherently nonunique model results when using the geomagnetic transfer functions alone. However, by fixing the resistivity structure of the surrounding seawater distribution, the intrinsic nonuniqueness is alleviated. In this study, we show an inversion result using a 100-Ωm uniform Earth with fixed resistivity of surrounding oceans. As a result, it was found that the features of the short period transfer function require shallow conductors in the upper crust, which is suggested to represent the northern Tohoku conducting belt of a previous study. The final model is characterized by a highly conductive zone along the quaternary volcanic arc in the depth range of the lower crust to the upper mantle. The conductor, which is obtained mainly from the features of longer-period data, is particularly clear beneath the Sengan geothermal area. The deep crustal conductor implies the existence of partial melt and/or high-salinity fluids.

Highlights

Ogawa (1987a) qualitatively interpreted geomagnetic transfer function data at 37 sites located on a 20 × 20-km grid using induction coils in the period range of 16 to 256 s, which was shorter than the periods used in the preceding studies because of the need to clarify the structure of deep crust but was narrow as compared with present-day wideband magnetotelluric measurements
The sites on the northernmost profiles (L0 and L1) show large northward components (Figure 2c), but their lengths are shorter near the volcanic front. These data were qualitatively interpreted in terms of conductors along the volcanic front that were reducing the lengths of the northward arrows (Ogawa 1987a)
Modeling, we approximated the standard deviation of the transfer functions (TFs) as half of the TF errors, which are listed in Ogawa

Summary

Introduction

Geomagnetic transfer functions (TFs) represent the ratio of vertical to horizontal magnetic variations and are robust to the effects of near-surface local anomalies. Ogawa (1987a) qualitatively interpreted geomagnetic transfer function data at 37 sites located on a 20 × 20-km grid (as shown in Figure 1) using induction coils in the period range of 16 to 256 s, which was shorter than the periods used in the preceding studies because of the need to clarify the structure of deep crust but was narrow as compared with present-day wideband magnetotelluric measurements (typically 10−2 to 104 s) Those periods turned out to be sensitive to the distributions of deep crustal and upper mantle conductive anomalies, as shown below. These data were qualitatively interpreted in terms of conductors along the volcanic front that were reducing the lengths of the northward arrows (Ogawa 1987a)

Methods

Results and discussion

Conclusion