|Measurements of the geoelectric potential and electric prospecting in Izu-Oshima Island, Japan|
Tanbo, T.; Toduka, Y.; Nagao, T.; Uyeda, S. (1999). Measurements of the geoelectric potential and electric prospecting in Izu-Oshima Island, Japan. Bull. Inst. Oceanic Res. & Develop., Tokai Univ. 20: 1-15
In: Bulletin of Institute of Oceanic Research and Development. Tokai University. Tokai University. Institute of Oceanic Research and Development: Shimizu. ISSN 0289-680X, more
|Authors|| || Top |
- Tanbo, T.
- Toduka, Y.
- Nagao, T.
- Uyeda, S.
Observations of earth current, or electrical potential, have been conducted since 1987 for the purpose of testing the VAN method of short-term earthquake prediction on Izu-Oshima Island, Japan. So far, there have been several instances of detecting unusual regional electric field variations of rectangular shape just before swarm earthquake activities off the east and south coasts of Izu Peninsula. These changes were found to be very strong locally at and around the Okata NTT Branch, suggesting the existence of strong electrical heterogeneity under the Okata area. This situation may satisfy the VAN's postulate that the Seismic Electric Signals (SES) are enhanced where underground electric conductivity is heterogeneous. In the recent years however, it has become increasingly more difficult to distinguish between potential precursory changes and artificial noise due to the increase of the latter on a regional scale. To understand the peculiaries of the Okata area and help distinguish between signals and noise, it was considered necessary to understand the electrical structure underlying the region. We investigated the electrical structure by three methods: 1) geopotential difference measurements at a number of sites in the surrounding area, 2) electric prospecting (pole-pole method and dipole-dipole method), and 3) VLF-MT survey.Although only a rough outline was obtained becuase of the limited number of measuring sites, the dipole-dipole method revealed a high resistivity area at about 200-300 m south of the Okata short-dipole network. In a more localized scale, the VLF-MT method showed that the center of a high resistive body with a diameter of about hundred meters was under the Okata NTT Branch, whereas the pole-pole method indicated that the resistivity directly under the Okata NTT Branch was low and the high resistive body appeared to be about 30 meters to the south. Although it is not appropriate to directly compare apparent resistivity values obtained by different methods, it can safely be said that the underground electrical structure in this area is extremely complex. The long-noted peculiarity of the Okata Station may be closely related to this complex structure.Probably reflecting the heterogeneous structure which may amplify geoelectric changes, the Okata Station has a high possibility of being a site sensitive to SES. However, the detection of SES from a multitude of equally amplified artificial noise may still be difficult. Most of the regional changes in the geopotential differences observed during this experiment were artificial noise. They were found to show remarkably anisotropic behavior at each measuring site; i.e., they were polarized in a distinctly preferred direction. Their preferred direction diferred from one site to another and was approximately the same as that for the magnetotelluric potential change at each site. The direction of the electric field generated by the dipole-dipole method also tended to coincide, with the preferred polarization direction of the regional geopotential change and magnetotelluric change, when the source/receiver distance was greater than a few hundred meters. These facts seem to indicate that, unless the source is very close, the polarization direction at any given site is site-specific, thereby complicating signal/noise discrimination. There still remains the possibility, however that the intensity distribution of changes related to real SES differs from those of magnetotelluric and other noise changes.