Electric and magnetic field variations arising from the seismic dynamo effect for aftershocks of the M7.1 earthquake of 26 May 2003 off Miyagi Prefecture, NE Japan

Some examples of electric and magnetic field variations have recently been reported by Honkura and his colleagues in association with earthquakes, and these variations have been interpreted by them in terms of the seismic dynamo effect. In order to confirm that this effect is a universal phenomenon rather than a phenomenon appearing in a special local condition, we made magnetotelluric (MT) observations above the hypocentral area of the M7.1 earthquake which occurred off Miyagi Prefecture, northeastern Japan, on May 26, 2003. The MT site was selected at a location close to a seismic station belonging to the nation-wide seismic observation network called ‘Hi-net’, so that we can compare the MT signals with the seismic wave records. During the MT observation period after the mainshock, some moderate-size aftershocks of magnitudes between 2.8 and 4.1 occurred and MT signals appeared in association with all these aftershocks. In order to confirm that MT signals are not due to vibrations of MT equipment, we set up two sets ofMT equipment at the same location; in the case of electric field measurements, we used independent electrodes and arranged cables connecting electrodes on the ground for one set and in the air for the other set, and in the case of magnetic field measurements, we buried the induction coils under the ground for one set and hang them in the air for the other set. As for the electric field, the two sets showed exactly the same records. On the other hand, the magnetic field was different from one set to another, but we conclude that the induction coils buried in the ground are more likely to represent the magnetic field due to electric currents flowing in the ground as a result of the seismic dynamo effect.