Wide-band (320-0.001 Hz) and long period (0.01-0.0001 Hz) magnetotelluric (MT) data were acquired along two profiles crossing the western part of the North Anatolian fault zone (NAFZ), Turkey, which consists of two main fault branches. The first profile (Izmit profile) crosses the epicentral area of the 17 August 1999 Izmit earthquake. In fact, the NIT measurements along this profile started just a few weeks before the occurrence of the mainshock and four instruments happened to be in operation in the vicinity of the fault when the earthquake took place. The second profile (Adapazari profile) is located about 30 km east of the first profile. Two-dimensional modeling shows the following results. First, the hypocenters of mainshock and aftershocks are located on the highly resistive side near the edge of a conductive zone. Second, the long-period NIT data show a low resistivity zone extending down to 50 km between the two fault branches. Such a deep conductive zone is interpreted as representing partial melting resulted from the past complex tectonics in this region, and it is related to the non-seismic after-slip in the layer below the seismogenic zone characterized by the highly resistive layer. (c) 2004 Elsevier B.V. All rights reserved.