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On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

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