A Deep Transient EM Experiment in the Northern Part of Miyagi Prefecture, Northeastern Japan

Abstract

In November 1993, a time domain electro-magnetic (TDEM) experiment was carried out in the northern part of Miyagi Prefecture, northeastern Japan. The observation area is of high seismic activity, where a magnitude 6.5 earthquake occurred in 1962 and its aftershocks can be still observed. The purpose of this observation is to investigate how this seismic activity is related to the electric resistivity structure. We aimed to investigate the resistivity structure down to a depth of 10 km or so, where most recent earthquakes took place. For the field experiment, an approximately 2 km long grounded wire was used as an artificial source. Receivers were located at a distance of several kilometers from the transmitter, where three components of the magnetic field were measured using a high sensitivity fluxgate magnetometer. High quality transient data were obtained from measurements made at 10 sites for 4 days, with total data length of about 27 hours. After proper techniques of the data analysis were applied, the resistivity structure was estimated by comparing not only the vertical but also the horizontal magnetic components with the synthetic master curve for one dimensional (1-D) resistivity model. Preliminary result shows that a highly conducting crustal layer exists below the north-eastern part of the survey area where micro seismicity is rather shallower. On the other hand, we could not find a crustal conductor within the range of field penetration depth in the southwestern part, where seismicity is deeper.