Masahiro Ichiki

Upper mantle conductivity structure of the back‐arc region beneath northeastern China

The upper mantle electrical conductivity structure of the pacific back-arc beneath northeastern China was investigated through long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) experiments. MT and GDS responses were obtained up to periods of 105 ∼ 106 seconds, and were inverted to one-dimensional (1-D) models with minimum and smooth structure constraints, respectively. The resulting conductivity model was compared with past models and the mantle transition zone beneath northeastern China is more conductive than those of other tectonic settings by almost one order of magnitude in the depth range of 400∼600 km. This feature may correspond to the presence of a stagnant slab beneath this region. In the mantle at a depth of less than 400 km, the conductivity profile has a similar feature to that in the thermal and extensional tectonic region in the southwestern United States.

Water content in the mantle transition zone beneath the north pacific derived from the electrical conductivity anomaly

Fukao et al. (2004) inverted semi-global electromagnetic network data for threedimensional electrical conductivity structure in the mantle transition zone beneath the north Pacific. In this paper we interpret the electrical conductivity structure in terms of the water distribution in the mantle transition zone, using partial derivatives determined by laboratory experiments on mantle materials. Fukao et al. (2004) explained both electrical conductivity and seismic P-wave velocity anomalies with thermal anomalies because of the overall coincidence of high electrical conductivity with low seismic velocity. However, a significant discrepancy is found beneath the Mariana islands where the seismic tomography would indicate little temperature anomaly, while electromagnetic tomography implies high temperatures. Despite limitations and differences in spatial resolution, this result indicates that this particular feature may not be explained by only a thermal effect. Taking into consideration that this region is well populated by subducted slabs, we further assume that this discrepancy is caused by water dehydrated from those slabs. Under this assumption, by combining the Nernst-Einstein relationship (e.g. Karato, 1990) and the recent result of laboratory measurements of hydrogen diffusivity in wadsleyite (Hae et al., 2006), the water content anomaly was estimated from the electrical conductivity anomalies. We find that the mantle transition zone beneath Mariana islands could contain about 0.3 weight % water.

Long-term seafloor geomagnetic station in the northwest Pacific: A possible candidate for a seafloor geomagnetic observatory

For two years, geomagnetic variations have been measured at the seafloor in the northwest Pacific. The seafloor data consist of the geomagnetic vector field measured by a three-component fluxgate magnetometer and the absolute scalar total force measured by an Overhauser (1953) magnetometer with attitude measurements for both orientation and tilt. Using the attitude data, the geomagnetic data at a site in the northwest Pacific (41o06′08″N, 159°57′47″E, -5580 m), hereafter referred to as NWP, were converted into the same reference frame as land and satellite measurements. Short-period variations of the converted vector data were examined by Hamano’s (2002) global time domain analysis method, which showed compatibility of the seafloor geomagnetic observatory data with the existing land observatory network. The smooth and gradual change of the Earth’s main field (i.e., the geomagnetic secular variation) was also found consistent with those predicted by the latest International Geomagnetic Reference Field (IGRF-10; IAGA, 2005) and by Ørsted Satellite (Olsen, 2002) for not only the scalar field but also the vector field. This means that observation of the geomagnetic vector secular variation is now feasible on the seafloor.

Three-dimensional magnetotelluric imaging of crustal fluids and seismicity around Naruko volcano, NE Japan

We analyzed the 3-D resistivity structure beneath Naruko volcano, northeastern Japan, with the aim of imaging 3-D distribution of fluids in the crust for its volcanic and seismogenic implications. The data were recorded at 77 sites in total: 30 sites are new and are arranged in an approximately 5 × 5 km grid whereas the remaining older sites constitute two separate east-west profiles. We ran a 3-D inversion using full components of impedance tensors in the period range between 0.13 and 400 s. The resulting model showed that a sub-vertical conductor exists a few kilometers below Naruko volcano. The conductor extends from the surface of the volcano and dips towards the south, away from the volcano towards the backbone range. High levels of seismicity are observed in the upper crust above and around the conductors. We suggest that the seismicity is fluid driven and that a fluid trap is created by the precipitation of quartz owing to a reduction in solubility at shallow depth. The Quaternary volcanic front is characterized by a sharp resistivity contrast and a high-resistivity zone and extends 10 to 15 km towards the east. A fore-arc conductor was observed at mid-crustal levels even farther towards the east. The sub-vertical conductors along the arc and the fore-arc conductor have resistivities of 1 to 10 Ωm. Assuming a Hashin-Shtrikman model with saline fluids of 0.1-Ωm resistivity, a porosity of 1.5% to 15% is required to explain the observed conductive anomalies.

Electrical conductivity measurements of brucite under crustal pressure and temperature conditions

Hydrous minerals are crucial because their occurrence is associated with seismic activity through the dehydration process that occurs in the earth’s crust and/or mantle. We have developed a technique to observe the dehydration reaction of brucite using electrical conductivity variation under sealed conditions. The electrical conductivity of brucite was measured as a function of temperature. The confining pressure for the measurements was 1 GPa, which represents that of the lower crust. Two types of remarkable electrical conductivity variation were observed. During the first heating, the conductivity of the sample showed a linear variation below 700 K, as was expected from the Arrhenius equation. Once the temperature was increased to near the dehydration boundary, the sample showed a high conductivity. Even though only a small amount of H2O was formed after dehydration, bulk conductivity of the sample varied greatly, presumably caused by a combination of the presence of coexisting solid and fluid phases and a mixed electronic and ionic conduction mechanism operating in the sample.

Resistivity structure of high-angle subduction zone in the southern Kyushu district, southwestern Japan

Magnetotelluric observations were carried out in the southern Kyushu district of southwestern Japan to investigate the characteristics of the electrical resistivity structure of a high-angle subduction zone. We constructed a 2-D resistivity model parallel to the subducting plate motion by using the inversion technique with the Akaike Bayesian Information Criterion (ABIC) smoothness constraint. The general features of the obtained resistivity structure are as follows: (1) a conductive block (below 1 Ω·m) is found beneath the volcanic zone and is widespread bilaterally below 40 km depth, (2) a resistive block (about 1000 Ω·m) distributes from 10 to 25 km depth in the forearc region and (3) a conductor (1 ∼ 30 Ω·m) is embedded beneath the resistive block, which may correspond to the negative Bouguer gravity anomaly observed in this region. We propose the following for the high-angle subduction zone: A serpentinized block is generated in the lower crust of the forearc region and a partial melting and hydrothermal fluid are well developed beneath the volcanic front.

Geomagnetic observatory operates at the seafloor in the northwest Pacific Ocean

The need to establish seafloor observatories has been important for decades. Multidisciplinary efforts aimed at establishing such observatories [e.g.,Beranzoli et al., 1998] are now culminating the ongoing Ocean Observatories Initiative [Copley, 2004]. This article reports on how the worlds first seafloor geomagnetic observatory NWP (North-Western Pacific) is producing data that are compatible with those obtained by satellite Orsted and the existing geomagnetic observatory network. In particular, the article reports on the remarkable agreement between the satellite-predicted and actual secular variation, which ensures the accuracy of the geomagnetic measurements both in space and at the seafloor.

Magnetotelluric investigations for the seismically active area in Northern Miyagi Prefecture, northeastern Japan

The ELF- and the ULF-MT surveys were carried out in the northern part of Miyagi Prefecture, northeastern Japan. This area is one of the most seismically active areas in this region, where hypocenters of microearthquakes are distributed on a fault plane at depths from 2 to 16 km. The aim of the present study is to investigate the relationship between electrical resistivity structure and the hypocentral distribution of microearthquakes in the area. The calculated impedance tensor at each site has been obtained from the observed data and decomposed to remove galvanic distortion, provided that the regional strike is N32?E to obtain the 2-D apparent resistivity and phase responses. The resistivity structure obtained by the inversion process using smoothness constraint shows that the relatively electrically conductive layer at depths from 4 to 10 km corresponds to the zone where the microearthquakes occur. The fact that the conductive zone correlates with the hypocentral zone is probably attributed to fluids in the crust. Another more conductive block is found at depths from 1 to 3.5 km and the bottom boundary of this conductor appears to restrict the uppermost depth where the microearthquakes occur. This subsurface conductor is interpreted as a marine sediment deposited during the Tertiary period. In the lower crust, the relatively conductive blocks (lower than 5Ω m) exist below a depth of 15 km.

Prediction of physical properties of water under extremely supercritical conditions: A molecular dynamics study

The physical properties of water under a wide range of pressure and temperature conditions are important in fundamental physics, chemistry, and geoscience. Molecular simulations are useful for predicting and understanding the physical properties of water at phases extremely different from ambient conditions. In this study, we developed a new five-site flexible induced point charge model to predict the density, static dielectric constant, and transport properties of water in the extremely supercritical phase at high temperatures and pressures of up to 2000 K and 2000 MPa. The model satisfactorily reproduced the density, radial distribution function, static dielectric constant, reorientation time, and self-diffusion coefficients of water above the critical points. We also developed a database of the static dielectric constant, which is useful for discussing the electrical conductivity of aqueous fluids in the earths crust and mantle.

Electrical image of subduction zone beneath northeastern Japan

We conducted long-period magnetotelluric observations in northeastern Japan from 2010 to 2013 to investigate the three-dimensional electrical resistivity distribution of the subduction zone. Incorporating prior information of the subducting slab into the inversion scheme, we obtained a three-dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the Ou Backbone Range. The conductive body indicates a saline fluid and/or melt pathway from the subducting slab surface to the lower crust. The lower crust conductor is less than 10 Ωm, and we estimate a saline fluid and/or melt fraction of at least 0.7 vol. %. Other resistivity profiles in the across-arc direction reveal that the conductive body segregates from the subducting slab surface at 80–100 km depth and takes an overturned form toward the backarc. The head of the conducting body reaches the lower crust just beneath Mt. Gassan, one of the prominent backarc volcanoes in the system.