Résumé
Over 20 trenches were excavated across the 1912 rupture of the Ganos strand of the North Anatolian fault near the Gulf of Saros, northwestern Turkey. A distinctive well-sorted fine sand channel, dated as younger than A.D. 1600, served as a marker unit. Isopach mapping shows that the sand is channelized north of the fault, flowed as a fan complex across a broad fault scarp to the south, and is now offset 8.5 m. This displacement occurred principally as two large events and a smaller, fissuring event. We attribute the two larger events to the large regional earthquakes of 1766 and 1912. The smaller event may be the result of minor bleed-over slip from the 1893 M6.9 earthquake in the Gulf of Saros to the west. We conclude from our observations that most of the slip likely results from the 1766 and 1912 earthquakes. If each was similar in size, then about 4 m of slip can be attributed to each event. We also excavated six trenches at two sites along the 1999 Izmit rupture to study past earthquake history along that segment of the North Anatolian fault. We found evidence for at least two and probably three surface ruptures during the past 400 years. One of the past events is almost certainly the large earthquake of 1719, but the presence of a probable third event during the past 400 years leaves open the possibility that either 1754 or 1894 also ruptured the onshore portion of the fault zone. Release of about 4 m of seismic slip both west and east of the Marmara Sea this past century (1912, 1999) support the contention that Istanbul is at high risk from a pending large earthquake.