Tohru Mogi

Three-dimensional modeling of magnetotelluric data using finite element method

Abstract The magnetotelluric response of a resistivity model is usually obtained by solving a boundary value problem of the Helmholtz type equation which follows from the Maxwells equation. In this case, the electric field is not continuous at the boundary between different resistivity media. This condition gives us to errors in the numerical modelling using the finite element method or the finite difference method. To overcome this, I formulated the finite element equation using secondary field components resulting from the anomalies. The primary field is calculated analytically for a semi-infinite uniform or horizontally layered earth. I use the finite element method composed of 8 nodes isoparametric hexahedral elements to calculate the secondary field excited by electric charges appears at the anomalies having different resistivity from the host medium. Current sources at the resistivity anomaly depend on the resistivity contrast and the primary electric field, which appears on the right hand side of the finite element equations. I use boundary conditions which take account of the asymptotic behavior of the secondary field far away from the resistivity boundary. I applied this modelling method to simple three-dimensional prism models and compared the results with the responses of the hybrid FEM (Gupta et al., 1987) and the integral equation modeling (Ting and Hohmann, 1981).

Magnetotelluric survey of an active fault system in the northern part of kinki district, southwest japan

Abstract Combined geological and geophysical surveys are effective in finding hidden active faults and estimating the dimensions of their fracture zones in the northern part of the Kinki District, southwest Japan. The magnetotelluric method using electromagnetic energy in the ELF frequency band can define the dimensions of a fault and suggest whether it is active or not. The results of the survey show that individual active strike-slip faults may be linked through new faults found in unexposed areas. The distribution of major active faults suggest that they form conjugate sets defining boundaries to tectonic blocks. At least seven tectonic blocks are recognized with different patterns of occurrence of microearthquakes and distributions of active faults. Historically large earthquakes, magnitude >5 as determined by maximum velocity amplitude, occur at the boundaries of the blocks. These patterns are caused by the different stress fields applied to each block.

Theory of modulation interference laser radar (MILR) method and its computer simulation

Observation of volcanic gases including H/sub 2/O is very important for monitoring volcanic activities and volcano energy. The volcanic gases come up from a great depth and they indicate the state of the volcano directly. The laser radar technique is considered to be a useful one for monitoring purposes. The conventional pulse laser radar, however, is not sufficient because of the low signal to noise ratio in the infrared range. The authors propose a new method called Modulation Interference Laser Radar (MILR). Numerical simulation of the reflected wave intensity was performed by the MILR method in order to confirm the effectiveness of the method. As a result, the authors obtained a very sharp reflection at the expected position.<<ETX>>

Tectonics Deduced from Landsat Images in the Southern Part of Chubu District, Central Japan

A study of regional tectonics using the lineament map drawn from Landsat TM (Thematic Mapper) images was conducted for the southern part of Chubu district, Central Japan. Many long lineaments appeared along boundaries of tectonic blocks which are characterized by directional orientation of small lineaments. Tectonically significant NE-SW lineaments crossing Akaishi Mountains were specially obtained. It was found that the NE-SW lineaments coincide with geological or active faults in Akaishi Mountains and South Fossamagna areas, but they are not along with the sedimentary structure of the Paleogene and Neogene formation distributed in these areas. In the north side area of the NE-SW lineaments, main directions of small lineaments are N-S or E-W and seismic activity in the crust is low. In the south side area, on the contrary, the main directions of them are N-S or NE-SW and seismic activity is high. Moreover, the most north one of the NE-SW lineaments agrees with the boundary of the principal axis of the seismic stress. Many earthquakes, of magnitude more than 4, also occurred on the NE-SW lineaments in Akaishi Mountains. Therefore, the NE-SW lineaments are considered to reflect the active fracture zones and extend to the Median Tectonic Line.