Wide-band magnetotelluric (MT) data across the Izmir-Ankara Suture Zone (IASZ), the Bornova Flysch Zone and the Menderes Massif of western Turkey images shallow and deep crustal structures. Inversions of the MT data reveals two subzones of the crust with varying thickness; the viscoelastic lower crust appears as a conductive zone (2000 ohm m) at a depth from 5 km to 25 km under the IASZ. A hypocentral cross-section of earthquakes along the profile shows more dense seismic activity in and around the twin-core structure. Due to this correspondence, we interpret that decoupling of the twin-core structure could be responsible for the denser seismic activity around the IASZ. The depth of the lithosphere/asthenosphere boundary is rather shallow in the geoelectrical model, about 45 km at the SE and NW end of the profile, and 55 km in the middle. ¿ 2001 American Geophysical Union