The results obtained from field studies of surface fault breaks on land, aftershock studies, and analysis of complex teleseismic body waves of the 17 August 1999 İzmit earthquake (Mw 7.4) indicate that the northern strand of the North Anatolian Fault Zone had been ruptured between 29.1° E and 31.1° E, with a maximum total fault rupture length of about 200 km. The 12 November 1999 Düzce earthquake (Mw 7.2) extended the surface fault break an additional 41 km to the east. Alignment of the aftershocks delineates the fault rupture on land and extends it to the west of the Hersek Delta, well into the Marmara Sea, to 29.1° E longitude. We have identified six fault rupture segments: two of them are located to the west of the Hersek Delta, and four are located to the east.

Rupture process analysis suggests two stages of rupture separated in space and time, with an estimated total scalar seismic moment of 2.42 × 1020 N m. The first stage initiated at the epicenter, near İzmit, and propagated asymmetrically—first eastward and then westward with a total scalar moment of 1.6 × 1020 N m, showing predominantly right-lateral strike-slip faulting followed by a minor subevent extending the rupture further east. The second stage took place at the western and eastern ends of the source area, rupturing two segments within the Marmara Sea, west of the Hersek Delta, and two segments between Akyazi and Gölyaka. The rupture process analysis indicates that six fault segments were ruptured during the mainshock: four of them on land and two within the Marmara Sea. The major moment release of 1.2 × 1020 N m is found on the 64-km-long Sapanca segments located between the easternmost tip of the Gulf of İzmit and Akyazi, with an estimated maximum slip of 5.3 m. The moment release on the Gölcük segment, with a length of 30 km, is about 3.7 × 1019 N m, and the estimated slip on this segment is 4.1 m. Rupture process analysis suggests two 20-km-long segments between Akyazi and Gölyaka striking N70°E. The scalar moment releases on these segments are 2.7 × 1019 and 1.9 × 1019 N m, with estimated offsets of 2.2 and 1.6 m, respectively. The last two subevents took place to the west of the Hersek Delta on the Yalova segment and within the Marmara Sea, with estimated offsets of 1.3 and 1.0 m, respectively.

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