The collision of the Arabian and African plates dominates the tectonic framework of the eastern Mediterranean (Jackson and McKenzie, 1984, 1988). In Turkey, this collision had induced the westward escape of the Anatolian block, which is accommodated by the right-lateral North Anatolian Fault on the north and the left-lateral EastAnatolian Fault on the south. Back-arc spreading behind the Hellenic and Cyprean arcs combined with the westward escape of the wedge-shaped Anatolian block results in a complex regional extension and transtension in western Turkey, where abundant geothermal activity is focused (Aydin et al., 2005; S¸engör et al., 1984; Westaway, 2003) (Fig. 15.1a). Hence the majority of the geothermal activity is observed in the tectonically active Aegean and Marmara regions of western Turkey which are located along or near major E-W striking normal faults and including major graben-bounding faults (Çag˘lar et al., 2005; Drahor and Berge, 2006; Faulds et al., 2006, 2009; Karamanderesi and Helvacı, 2003; Pfister et al., 1998; S¸ims¸ek, 1997; S¸ims¸ek and Demir, 1991; S¸ims¸ek and Güleç, 1994; Tezcan, 1995; S¸ims¸ek et al., 2000a; 2000b) (Fig. 15.1b). It is therefore critical to evaluate the type and position of the faults that are the most favorable for geothermal activity in such regions (Dewey and S¸engör, 1979; McKenzie, 1972; S¸engör, 1976).