In order to investigate the resistivity structure beneath the northern and the southern branches of the western part of the North Anatolian Fault Zone (NAFZ), which has been known as a seismic gap area, we started wideband magnetotelluric (MT) measurements on 27 July 1999, along a north-south profile crossing the northern branch near İzmit and the southern branch near Iznik. When the İzmit (Kocaeli) earthquake (Mw 7.4) occurred on 17 August 1999, MT fields were being measured at five sites, four of which happened to be located near the surface rupture zone associated with the mainshock, and large variations in the MT fields were observed in association with seismic waves. We propose motional electromagnetic induction in the electrically conducting crust, which vibrates under the Earth's magnetic field, as one of the possible generation mechanisms for such MT field variations. For more detailed studies of such a mechanism, we installed a wideband MT instrument and a three-component seismometer at two of the four sites on 16 September 1999 and made simultaneous measurements during several aftershocks. The estimate of the order of the magnitude of motional electromagnetic response, derived from the MT fields and ground motion observed during a large aftershock (M 4.5), supports our claim that the seismo-dynamo effect is a plausible mechanism for variations in the MT fields associated with the seismic waves.

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