Magnetotelluric investigations for the seismically active area in Northern Miyagi Prefecture, northeastern Japan

Abstract

The ELF-and the ULF-MT surveys were carried out in the northern part of Miyagi Prefecture, northeastern Japan. This area is one of the most seismically active areas in this region, where hypocenters of microearthquakes are distributed on a fault plane at depths from 2 to 16 km. The aim of the present study is to investigate the relationship between electrical resistivity structure and the hypocentral distribution of microearthquakes in the area. The calculated impedance tensor at each site has been obtained from the observed data and decomposed to remove galvanic distortion, provided that the regional strike is N32• E to obtain the 2-D apparent resistivity and phase responses. The resistivity structure obtained by the inversion process using smoothness constraint shows that the relatively electrically conductive layer at depths from 4 to 10 km corresponds to the zone where the microearthquakes occur. The fact that the conductive zone correlates with the hypocentral zone is probably attributed to fluids in the crust. Another more conductive block is found at depths from 1 to 3.5 km and the bottom boundary of this conductor appears to restrict the uppermost depth where the microearthquakes occur. This subsurface conductor is interpreted as a marine sediment deposited during the Tertiary period. In the lower crust, the relatively conductive blocks (lower than 5 ·m) exist below a depth of 15 km.

Microseismicity and stress in the vicinity of the Alpine Fault, central Southern Alps, New Zealand

“…occurs below or adjacent to these zones in high-resistivity zones. Similar distributions of earthquakes on the boundary between high and low-resistivity regions have been observed for microearthquakes [Ichiki et al, 1999], swarms [Tank et al, 2003] and large earthquakes and their aftershocks [Eberhart-Phillips et al, 1990;Kasaya et al, 2002]. Takeda et al [1999] observed the occurrence of earthquakes at shallow depths in areas of low water saturation and high crack density surrounded by areas of high saturation.…”

Electrical resistivity imaging of a seismic region in northern Baja California, Mexico

“…from 5 to 25 km, (Haberland et al, 2003;Sarma et al, 2004;Goto et al, 2005). Even though resolution decreases exponentially with depth, magnetotellurics is currently the most effective exploration tool to investigate the electrical properties of the crust at depths larger than 2-3 km (Ichiki et al, 1999;Ogawa et al, 2001;Ritter et al, 2003;Bedrosian et al, 2004;Tank et al, 2005).…”