On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

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[1] Volatile releases, in terms of CO 2 efflux and noble gas of mantle origin, are investigated in the Marmara region of the seismically active North Anatolian Fault (NAF) zone. A total of 180 CO 2 efflux measurements (1 to 309 g m À2 d À1 ) were carried out along four transects across the NAF. Spatial variability of surface CO 2 effluxes was higher at faulted sites, relative to comparable background areas. The d 13 C values of soil CO 2 are À24.0 to À15.6% without correlation with CO 2 efflux values, which suggest that CO 2 efflux anomaly is caused by fault-related biogenic gas flow. Furthermore, 36 gas and water samples from spring sites were investigated for 3 He/ 4 He ratios which resulted in a range from 0.26 R/R A to 4.22 R/ R A . Large mantle helium contributions were observed from several sources along the NAF. The high CO 2 efflux sector on the Gaziköy-Saros fault along the NAF coincides with the area of high mantle helium flux. This shows that fault zones play an important role both in transferring mantle helium to the surface and in causing fault-related biogenic gas flow.