Two-dimensional resistivity modeling based on regional magnetotelluric survey in the northern Tohoku district, northeastern Japan.

Abstract

In 1984, a regional Magnetotelluric (MT) survey was carried out in the northern Tohoku district in the frequency range from 17.4 kHz to 1/256 Hz. MT stations wereThe shallow lateral inhomogeneity was evaluated from ELF-MT (8 to 20 Hz) and VLF-MT (17.4 kHz) survey at 35 sites, and the deep structure was investigated using ULF-MT (1/4 to 1/256 Hz) survey at 7 sites.Two-dimensional modelings with north-south strike are carried out. Analyses are done with special reference to the anisotropic behavior of phase of impedance. From the modeling with north-south strike, the following features are found. (1) For TM mode, where electric field is directed eastward, frequency characteristics and spatial dependence of impedance are primarily due to effect of oceans and shallow resistivity distribution. For 1/16 Hz, coastal effect alone cannot account for the observed impedance and conductor (300 ohm-m) is required beneath the eastern part of the profile whose top is located at 10 km depth in the crust. (2) On the other hand, for TE mode, where electric field is directed northward, the spatial and frequency dependence of impedance requires the existence of a conductor (10 ohm-m) in lower crust beneath the volcanic region whose depth ranges from 20 km to 30 km. This conductor is in harmony with low seismicity, low velocity, and shallow Curie isotherm depth.

Preliminary interpretation on detailed magnetovariational profilings in the northern Tohoku district.

“…On the other hand, in the western part of the surveyed area (sites 8, 18, 1, 2, 24, 30, and 31) the coastal effect is more significant. UTADA (1987) andOGAWA (1987) analyzed regional MT data. OGAWA et al (1987a) andOGAWA andUCHIDA (1987) studied detailed MT data for geothermal explorations.…”

“…UTADA (1987) andOGAWA (1987) analyzed regional MT data. OGAWA et al (1987a) andOGAWA andUCHIDA (1987) studied detailed MT data for geothermal explorations. These studies revealed the existence of a low resistivity body in the lower crust beneath the Quaternary volcanic zones in the Tohoku district.…”

“…Two-dimensional interpretation of data obtained in 1984 were already described by OGAWA (1987). Apparent resistivity data themselves are affected by the shallow resistivity distribution.…”

“…HONKURA (1974) and OGAWA et al (1986) used GDS, RESEARCH GROUP FOR CRUSTAL RESISTIVITY STRUCTURE, JAPAN (1983), SATO and SAITO (1987) and OGAWA (1987) used MT, and UTADA (1987) used both GDS and MT. Most of these previous studies are based on natural signals at periods longer than 300 sec.…”

Local Galvanic Distortions in the Central Part of North-Eastern Japan (Part 1)

“…Many geological and geophysical studies have been conducted in this district (see Fujinawa et al, 1997, hereafter cited as F). Ogawa (1987a) carried out two-dimensional resistivity modeling based on a regional broadband magnetotelluric survey following earlier studies by Honkura (1974), Ogawa etal. (1986) and Utada (1987) which used a more limited frequency band.…”