Report of Hydrographic And Oceanographic Researches

No.44 (March, 2008)
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Contents

Original Papers

  1. abstract pdf Undersea crustal movement off the Tokai District, central Japan, detected by GPS/Acoustic seafloor geodetic observation
    Yoshihiro MATSUMOTO, Mariko SATO, Masayuki FUJITA, Tadashi ISHIKAWA, Hiroaki SAITO, Masashi MOCHIZUKI, TetsuichiroYABUKI and Akira ASADA , p.1
  2. abstract pdf 3D-magnetic structure model of Io To
    Kaoru KOYAMA, Noboru SASAHARA, Koichi KUMAGAWA, Ken-ei ONODERA and Ryuji KUBOTA , p.9
  3. abstract pdf Crustal structure and geophysical parameters of seamounts in the western Pacific as derived from topography and potential field anomalies
    Yoshio UEDA, You IWABUCHI and Shigeru KASUGA , p.17
  4. abstract pdf Evaluation of Marine Geoid model around Japan
    Noboru SASAHARA, Hiroyuki KUDO and Masayuki FUJITA , p.43
  5. abstract pdf Temporal Trends of the Concentrations of Pollutants in the Surface Sediments from the Major Bays of Japan
    Junko SHIMIZU, Ken-ich NOGUCHI, Yukihiro MIURA and Takeshi TOMOHISA , p.57
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Abstracts

Undersea crustal movement off the Tokai District, central Japan, detected by GPS/Acoustic seafloor geodetic observation

Yoshihiro MATSUMOTO, Mariko SATO, Masayuki FUJITA, Tadashi ISHIKAWA, Hiroaki SAITO, Masashi MOCHIZUKI, TetsuichiroYABUKI and Akira ASADA

We have been carrying out GPS/Acoustic seafloor geodetic observations at a reference point situated off the Tokai District, central Japan, where the Tokai Earthquake is expected to occur in the near future. A linear fit to the time series of horizontal coordinates at the reference point obtained from five campaign observations for the period 2002-2007 gives an intraplate crustal movement velocity, which is in realistic range and implies strong interplate coupling around this region.

3D-magnetic structure model of Io To

Kaoru KOYAMA, Noboru SASAHARA, Koichi KUMAGAWA, Ken-ei ONODERA and Ryuji KUBOTA

We report on the 3D-magnetic structure model of Io To based on the airborne geomagnetic surveys in January 2007 and December 1998. We used a 3D-magnetic tomography inversion method where the volcanic edifice was sliced into three horizontal layers (above 250m, 250-750m, 750-2500m in depth) and horizontally divided into prisms with a mesh size of 1km x 1km. We assumed that each prism possessed a homogeneous magnetization, and analyzed the intensity of the magnetization of each prism using a conjugate gradient method for calculation. The result showed a characteristic feature that the magnetization intensities in the central part of layers 2 and 3 are lower than a circumference part. The result may suggest participation of magma.

Crustal structure and geophysical parameters of seamounts in the western Pacific as derived from topography and potential field anomalies

Yoshio UEDA, You IWABUCHI and Shigeru KASUGA

Hydrographic and Oceanographic Department of Japan has conducted geophysical surveys by multi-beam soundings, KSS-30type ship gravity meter, and sea-surface proton magnetometer in the western Pacific as a part of continental shelf surveys since 1992. The surveyed area includes Jurassic Magnetic Quiet zone and a large number of seamounts were confirmed. Using the topographic depth soundings and potential field data (free-air gravity and magnetic anomalies), Bouguer gravity anomalies on the assumed density values of 2300kgm-3 and 2700kgm-3 were compiled. The effective elastic thickness was estimated for the several seamounts. These values ranging from 5km to 30km, with the largeset occurrences in 10km are consistent with the thermal rejuvenation due to mantle plumes occurred in Middle to Late Cretaceous time. Geophysical parameters (volume, density, magnetization) of 85 seamounts were calculated. A volume histogram indicates bi-modal pattern, one peek is around 1000-2000 km3, and the other 5000km3. This bi-modal pattern may be related with the different scale of magma plumes forming these seamounts. Density values of the seamounts were estimated from the correlation between calculated and observed free-air gravity anomalies. These density values show a mono-modal peek around a mean density of 2694±253kgm-3. Magnetizations of the seamounts are also estimated by the least square inversion method using a topographic model and a gravity basement model derived from Bouguer gravity of the assumed density of 2300kgm-3. The precision index parameters (GFR) on the topographic model give usually larger values than those on the gravity basement model. This may suggest the depressed structure observed in the gravity basement is not real but artificial resulting from filtering process. Histogram of magnetization intensities of the seamounts shows considerable wide range distribution ,contrary to the density pattern. This may be ascribed to differences in magnetic field intensity at the origin or in metamorphoses of magnetic minerals composing seamounts. The Virtual geomagnetic poles (VGPs) of these seamounts generally coincide with the VGP curve estimated from DSDP and ODP results (Sager, 2006), however, Hanzawa and Katayama seamount (4, 6) and seamount D4(48) show considerable displacement from the VGP. The origin of this misfit may arise from tilting effects of the seamounts , non-dipole components of magnetic field, or the older seamount's formation ages in comparison with radiometric age of 80Ma. Stage Euler poles derived from hotpot tracks (Koppers et al., 2001) failed to the reproduction of the APWP estimated by paleomagnetic method. Stage Euler poles reproducing the paleomagnetic APWP and South Pacific Isotopic and Thermal Mantle Anomaly (SOPITA) origin of West Pacific seamounts, are calculated and listed in this article for the consideration of the plate motions.

Evaluation of Marine Geoid model around Japan

Noboru SASAHARA, Hiroyuki KUDO and Masayuki FUJITA

In 2005, Hydrographic and Oceanographic Department(H.O.D.), Japan Coast Gurad, determined a new precise Marine Geoid model around Japan (Sasahara et al., 2006b). The new model covers the area 15N-50N 120E-160E with the spacing of 1'x1'. By applying the remove-restore technique the geoid undulations were calculated, and the Stokes's integral was computed with the modified Stokes's kernel (Featherstone, 2003) and 1D Fast Fourier Transform (Haagmans et al., 1993). We determined the model by using the gravity data derived from altimeter. In this work, we corrected the long wavelength component of this gravity data. The new version of model is called MGM２００８by using the corrected data.

We evaluated the accuracy of Marine Geoid model. The method of evaluation is the comparison of the SSDH (Sea Surface Dynamic Height). SSDH can be calculated by using the SSH(Sea Surface Height) derived from altimeter and the geoid height. SSDH can be calculated from the CTD(Conductivity-Temperature-Depth) data, too. We compared both SSDH, and evaluated the standard deviation of the difference between SSDHgeo(SSH-Geoid) and SSDHctd(CTD). The result showed the small standard deviation.

Moreover, we calculated the geostrophic current with SSDH. The result that the high velocity part of geostrophic current mostly agreed with the axis of Kuroshio current with the Quick Bulletin of Ocean Conditions.

Temporal Trends of the Concentrations of Pollutants in the Surface Sediments from the Major Bays of Japan

Junko SHIMIZU, Ken-ich NOGUCHI, Yukihiro MIURA and Takeshi TOMOHISA

The temporal trends of the concentrations of pollutants such as oil, PCBs, and heavy metals in the major bay areas of Japan were examined by using the data measured by JCG's marine pollution survey.

It was found that the concentrations of the pollutants tended to decrease at most of the survey areas, except in the case of a few areas and certain substances. However this tendency of decrease reduced after the 1990s as compared to that before the 1980s.

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