ALBERT

Description: Electromagnetic response is examined for a local three-dimensional resistivity anomaly embedded in the earth's crust of uniform resistivity, in order to interpret time-dependence of horizontal amplitudes as well as transfer functions for short-period geomagnetic variations at an observation site in the Izu region, Japan. The results of numerical estimation indicate that the detected enhancement of about ten percent in horizontal amplitude can be well explained by a local decrease, amounting to one order of magnitude, in crustal resistivity, on the condition that the observation site is located over the area of resistivity decrease. In this situation no marked change is expected for transfer functions; this is consistent with the observation. The results of numerical estimation also suggest that electric as well as magnetic field measurements are useful for detecting crustal resistivity changes; for instance, changes are expected in the amplitude of induced electric field, as would be reflected in apparent resistivity changes in magnetotellurics, and also in polarization angle of the electric field. It should be noted, however, that no detectable precursory changes are expected unless observation sites are located close to the area of resistivity change presumably corresponding to an earthquake source area and its vicinity. © 1986, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.