Hiroaki Toh

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.

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

[1]xa0Four 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 Earths surface to the wedge and back-arc mantle in the form of serpentinite.

Magnetotelluric observations around the focal region of the 2007 Noto Hanto Earthquake (Mj 6.9), Central Japan

On 25 March 2007, a damaging earthquake (Mj 6.9) occurred near the west coast of the Noto Peninsula, Central Japan. A wideband magnetotelluric (MT) survey was carried out in the onshore area of the source region immediately after the mainshock, with the aim of imaging the heterogeneity of the crustal resistivity structure. The final observation network had consisted of 26 sites. As a preparatory step for imaging three-dimensional features of the resistivity around the focal region, we constructed two-dimensional resistivity models along five profiles using only the TM mode responses, in order to reduce three-dimensional effects. Four profiles are perpendicular to the fault strike, and a fifth profile is parallel to the strike through the mainshock epicenter. Significant characteristics of the resistivity models are: (1) beneath the mainshock hypocenter, there is a conductive body which spreads to the eastern edge of the active aftershock region; (2) a resistive zone is located in the gap of the aftershock distribution between the mainshock hypocenter and the largest eastern aftershock; (3) one of the largest aftershock occurred at the boundary of the resistive zone described above. These results suggest that the deep conductors represent fluid-filled zones and that the lateral heterogeneity could have controlled the slip distribution on the fault plane.

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]. n nThis 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.

A regional model of the geomagnetic field over the Pacific Ocean for epoch 2002

[1]xa0The Pacific region has been geomagnetically less surveyed in the sense that there are a very limited number of ground stations that continuously measure the Earths vector magnetic field. Recently, however, long-term vector geomagnetic observation has started at several stations, mostly on oceanic islands but including one seafloor station, which presently reinforces monitoring the geomagnetic field by the land-based observation network. The new geomagnetic data together with those collected by Intermagnet Magnetic Observatories (IMOs) being operated in the Pacific and its rim were combined to give a regional geomagnetic reference field over the western Pacific (RGRF-P) for epoch 2002 using spherical cap harmonics (SCHs). The resultant snapshot of the regional geomagnetic field was found to coincide well with existing global models, e.g., the 10th generation of International Geomagnetic Reference Field (IGRF-10). This implies that the regionally denser geomagnetic network still sees small contribution of non-dipole components over the Pacific.