The first magnetotelluric study in the Marmara Sea, Turkey, was undertaken to resolve the structure of the crust and upper mantle in the region, and to determine the location of the westward extension of the North Anatolian Fault (NAF) in the C ¸ınarcık area. Long-period ocean bottom magnetotelluric data were acquired at six sites along two profiles crossing the C ¸ınarcık Basin, where a significant increase in microseismic activity was observed following the devastating 1999 İzmit and Düzce earthquakes. 2-D resistivity models indicate the existence of a conductor at a depth of ∼10 km in the middle of both profiles along with a deeper extension into the upper mantle, implying the presence of fluid in the crust and partial melting in the upper mantle. The northern and southern boundaries of this conductor are interpreted to represent the northern and southern branches of the NAF in the Marmara Sea, respectively. These conductors have been previously identified farther to the east along the NAF, suggesting that the electrical characteristics of this fault are continuous from onland areas into the Marmara Sea. Microseismic activity in the C ¸ınarcık area is located above the conductor documented here, and indicates a possible seismogenic role of crustal fluids present in the conductive zone. In comparison, resistive zones along the NAF may act as asperities that could eventually result in a large earthquake.
The magnetotelluric (MT) method has been applied to show resistivity variations in the vicinity of the epicenter of the 1999 Düzce earthquake (M w = 7.2) where a large rupture velocity difference was seen between the western and eastern parts of the epicenter. MT data for frequencies ranging between 320 and 0.0005 Hz were acquired along two parallel profiles to the west and the east of Düzce earthquake's epicenter. Both profiles crossed the Düzce basin and the Düzce fault in the north and the North Anatolian Fault (NAF) in the south. MT data analysis shows almost two-dimensionality in the region, at the depth starting from 2-3 km beneath the surface. According to two-dimensional MT modeling, a higher resistive layer extends beneath the eastern side of Düzce earthquake epicenter, compared with the western side. This resistivity structure correlates well with the aftershock distribution of the Düzce earthquake.
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