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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS016 Oral Presentation 2345
Present status of seafloor electromagnetic stations in the northwest
Pacific
Dr. Hiroaki Toh
Earth Sciences University of Toyama IAGA
Yozo Hamano
WP1 and WP2 are known as the locations of borehole seismometers in the northwest Pacific. Close to
each site, we are maintaining SeaFloor ElectroMagnetic Stations (SFEMSs) that monitor the geomagnetic
field as well as the short-period geoelectric field. The EM sites are respectively called WPB (West
Philippine Basin) and NWP (North West Pacific). SFEMS measures the absolute scalar geomagnetic total
force by an Overhauser proton precession magnetometer, geomagnetic three components by a fluxgate
vector magnetometer, horizontal two components of the geoelectric field and tilts in addition to
temperature and the instruments azimuth at the seafloor by a fibre optical gyro. The 3.5-year long EM
time series observed at NWP have been analyzed to yield the following two results: (1) Absolute
measurements of not only the scalar geomagnetic total force but also the vector geomagnetic field
components have been successfully conducted even by unmanned and pop-up type seafloor
instruments. (2) The electrical structure beneath the 129Ma-old seafloor has been revealed as deep as
850 km using both magnetotelluric and geomagnetic deep sounding signals. It was not until we were
able to acquire precise attitude data at the seafloor such as tilts and orientation that the former result
was achieved. The scalar and vector secular variations at the seafloor were further confirmed by
comparison with a couple of global geomagnetic field models based on recent magnetic satellite data
(Oersted and CHAMP). It was a good surprise to see an excellent agreement between the observed and
predicted geomagnetic scalar and vector secular variations, even though the seafloor and satellite
measurements were completely independent. In this paper, we will report the present status of SFEMSs
operated in the northwest Pacific, which covers the details of SFEMS, ability and limitation of the
present instruments, the quality of the EM data derived so far, and possible future plans. Necessity of
measuring precise tilts to recover the vector geomagnetic secular variation will also be emphasized.
Keywords: seafloor em station sfems, the northwest pacific basin, geomagnetic secular variation