Two‐dimensional electrical section beneath the eastern margin of Japan Sea

Abstract

Four ocean bottom electromagnetometers provided seafloor geoelectromagnetic data for more than nine months in order to reveal mantle dynamics beneath the eastern margin of Japan Sea, where several large and hazardous earthquakes have repeatedly occurred so far. High‐quality magnetotelluric (MT) data from the seafloor array, together with those of four additional land sites in northeast Japan, were analyzed and carefully corrected for topographic/bathymetric effects to extract two‐dimensional (2D) MT responses at each site. Here we show how the 2D electrical section derived by inverting the corrected 2D MT responses depicts fluid injection deep into the wedge and back‐arc mantle by the cold and fast subducting Pacific plate. A high conductivity anomaly found beneath the Sado Ridge at depths ranging from 150 to 200 km may be direct field evidence that supports water transportation from the Earth's surface to the wedge and back‐arc mantle in the form of serpentinite.

Subduction Zone Tomography

“…Clear S-wave reflectors are also detected in the upper and lower crust beneath the active volcanoes in NE Japan, which reflect sheets of cracks containing melts or magmatic fluids (e.g., Matsumoto and Hasegawa 1996; Horiuchi et al 1997). Magnetotelluric soundings have revealed high-conductivity anomalies in the crust and upper mantle wedge under NE Japan (e.g., Ogawa et al 2001;Toh et al 2006;Mishina 2009), consistent with seismological results. The spatial correlation between active volcanoes, low-V zones in the crust and mantle wedge, and low-frequency microearthquakes is also found in SW Japan (e.g., Zhao et al 2004Zhao et al , 2011aLiu et al 2013b).…”

Electromagnetic View of the Seismogenic Zones Beneath Island Arcs

“…Injection of water into the deep mantle can produce electrical conductivity anomalies beneath back-arc regions. In order to image such kind of anomalies, we constructed a seafloor MT array in the Japan Sea (Toh et al, 2006). The seafloor array consisted of six ocean bottom electromagnetometers (OBEMs) that are capable of measuring both vector geomagnetic and horizontal geoelectric fields in addition to horizontal tilt variations.…”

“…The inversion converged at an rms of 3.55 using both TE and TM mode responses. The derived electrical 2-D section (Toh et al, 2006), which is an EW slice of the non-volcanic part of northeast Japan, reveals a resistive shallow mantle and a conductive anomaly beneath the back-arc region at depths 150-200 km (Fig. 2).…”

“…It can be attributed to the fact that the slice covers a non-volcanic part, viz., a region between the hot fingers, of the well-developed island arc. An electrical section (Toh et al, 2006), which covers the non-volcanic part of northeast Japan, reveals a resistive shallow mantle and a conductive anomaly beneath the back-arc region at depths 150-200 km. The electrical conductivity anomaly can be interpreted as a direct manifestation of slab dehydration associated with collapse of the high-temperature type serpentine such as antigorite.…”

Efficient Inversion of Multi-frequency and Multi-source Electromagnetic Data: Final report

“…It was made freely available to the MT research community in 2006 and has since become one of the standard tools for 3-D inversion and interpretation (e.g., Tuncer et al, 2006;Heise et al, 2008; among others). The inversion algorithm used closely follows the two-dimensional (2-D) data space Occam's inversion of which has also been widely used for 2-D interpretation (e.g., Pous et al, 2002;Oskooi and and Perdersen, 2005;Toh et al, 2006; among others). Here we describe extensions to this code, which we illustrate with tests on synthetic and real data.…”