Electromagnetic Survey of Fluid Distribution and Migration

摘要

Mega-thrust earthquakes with magnitude greater than 8 occur repeatedly at the Nankai Trough seismogenic zone. Existence of fluid on the seismogenic fault has a key role on such great earthquakes. In this study, we report a field experiment and a numerical simulation of electromagnetic observations near the Nankai Trough, which can give us useful information on the fluid distribution and migration. Marine magnetotelluric soundings around the Nankai Trough and the Kumano basin were carried out in 2002-2003. On the basis of the observed data, the electrical conductivity model below the sea floor was estimated. According to this model, the Philippine Sea plate has a conductive oceanic crust before subduction. As the plate goes down the Kumano basin, the conductivity becomes low at the depth of 10 km below the sea floor, which approximately coincides with the up-dip limit of the Nankai mega-earthquake zone. Such a decrease of conductivity can be explained by discharge of fluid from the subducting Philippine Sea plate, terminating at the depth of 10 km. Less fluid condition of the subducting plate may be related to the locked plate boundary at the mega-earthquake zone. Although electrical conductivity can be used as an index of fluid distribution, self-potential measurement is sensitive to 'movement' of fluid. On the basis of our numerical simulation, self-potential gradient of 0.3 mV/m can be expected when the fluid velocity is almost 10~(-9) m/s within the frontal thrust zone, at the toe of the accretionary prism. Also, high vertical gradient of 1 - 3 mV/m can be expected along boreholes. These gradient values can be observed by using conventional observation techniques. The electrical conductivity structure is essential to model a distribution of fluid pressure and velocity by using self-potential distributions. Thus, combination of marine magnetotelluric and self-potential surveys can be a powerful tool to get information on fluid distribution and migration around the seismogenic zone.