Earth Planets Space, Vol. 56 (No. 2), pp. 109-114, 2004
Yoshimori Honkura1, Hideyuki Satoh2, and Naoto Ujihara1
1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan
2AIST, Tsukuba, Japan
(Received October 31, 2003; Revised January 25, 2004; Accepted January 29, 2004)
An earthquake of M7.1 occurred off Miyagi Prefecture, NE Japan, on May 26, 2003 at the depth of about 70 km. Just two months later, on July 26, 2003, another shallow earthquake of M6.4 occurred in northern Miyagi Prefecture. In order to investigate whether small signals precursory to the arrival of seismic wave appeared for these earthquakes, we examined the MT records which have continuously been acquired at two stations, Mizusawa and Esashi of the Geographical Survey Institute, located about 50 km northwest of the epicenter of the M7.1 earthquake and about 100 km north of the M6.4 earthquake. Unfortunately, seismometers were not available at these stations, and hence direct comparison between the MT and the seismic records were not possible. We therefore refer to the seismic record obtained at a nearby station belonging to the nation-wide seismic network called 'Hi-net' and also we used the arrival times estimated by Japan Meteorological Agency (JMA) for these MT stations. In conclusion, no clear signals preceding the arrival of seismic wave could be detected, although the possibility is suggested that a slight signal may have appeared in one magnetic component at the Mizusawa station before the arrival of seismic wave for the M7.1 earthquake.
Key words: Seismic dynamo effect, magnetotellurics, electric and magnetic field variation, seismic wave.