Nobuhiro Isezaki

A new shipboard three-component magnetometer

The three components of the geomagnetic field were measured at sea by STCM (Shipboard Three Component Magnetometer) with a relative accuracy of 50 ± 25 nT. The STCM was controlled by a microcomputer, and the three geomagnetic components were calculated by reducing the magnetic field due to the ship’s remanent and induced magnetic moments, the only assumption being that the magnetic field is linearly related to the magnetic moments. Although STCM has no particular magnetic compensation device, it can be set at any place on board of any kind of ship. In the KH82-5 cruise, the three component measurements were conducted along latitude 42u2009°N north of the Mendocino fracture zone where there are well‐defined magnetic anomaly lineations trending north and south. STCM measured the north component anomalies with very small amplitudes compared to the east and downward component anomalies, which shows that lineations trend north and south. The three measured component anomalies are almost identical to the anomalies ...

Sea-floor magnetization in the eastern part of the Japan Basin and its tectonic implications

Abstract The tectonic history of the Japan Basin is revealed by the distribution of magnetization and the topography of basement in the eastern part of the Japan Basin: 40°30–43°00′N, 136°30–139°00′W. The vector and total intensity of the geomagnetic field were measured on the R/V “Hakuho-maru” expedition in 1986 and the R/V “Tansei-maru” expedition in 1987 with a track spacing of about 4 nautical miles. Geomagnetic anomalies were obtained from these data and from other cruises using IGRF/DGRF. Furthermore, data on sediment thickness were compiled with all the available seismic profiles from the area. A depth to basement map was prepared using a regression equation of velocity in the sediments and one-way reflection time. Inversions were performed using both the gridded geomagnetic anomaly field and the depth to basement data to calculate a gridded distribution of magnetization. The key results of the distribution of magnetization are summarized in the following three categories: 1. (1) Magnetic lineations exist only in the northern part of the area studied where the basement is deepest and fairly flat. The lineations have two trends: N40°E in the north and N60°E in the south. 2. (2) No magnetic lineations were observed in the morphologically high relief area in the southernmost part of the area studied. (3) Some round-shaped distributions of magnetization exist. The three categories correspond to oceanic, continental and volcanic areas, respectively. They are probably characteristic features of rifting along the continental margin.

Geophysics of the Pacific Basin

Gravity anomalies show the effects of both the topography of the solid earth and its compensation. Free-air gravity anomalies, therefore, approximately represent isostatic anomalies, and the small range of free-air anomalies of about 800 mgal (1 mgal = 10-5 m sec-2), or 0.08% of the earth’s gravity field, shows that the earth is nearly in isostatic equilibrium (Bowin et al., 1982). Large-amplitude free-air anomalies, or deviations from isostasy, are maintained by tectonic activities (Vening Meinesz, 1932). Isostasy is fairly well achieved in wavelengths longer than about 500 km in the Pacific basin, and the corresponding free-air anomalies are as small as ±20 mgal (McKenzie et al., 1980). The value of 20 mgal is 0.002% of the earth’s gravity field, and the small value indicates that gravity measurements need high precision. It was difficult for the prealtimeter gravity field to discuss such gravity anomalies, because the spatial resolution of the gravity data obtained from satellite orbit perturbations was not sufficient, and because surface observations were sparse and not free from measurement errors of about ±10 mgal.