Timothy E. Dawson

Paleoseismology of the Gazikoy-Saros segment of the North Anatolia fault, northwestern Turkey: Comparison of the historical and paleoseismic records, implications of regional seismic hazard, and models of earthquake recurrence

We excavated five trenches across the North Anatolia fault zone (NAFZ)along the Ganos fault (Gazikoy-Saros segment), which last produced surfacerupture in 1912, near Kavakkoy where the fault enters the Gulf of Saros. The trenches exposed faulted sediments in a flood-plain environment withabundant detrital charcoal and scattered land-snail shells. Twenty-tworadiocarbon dates place constraints on the ages of the exposed sediments,which range from less than a few hundred years to about 6000 years inage. In two closely spaced trenches, we identified five discrete earthquakeevent horizons in the upper 2.5 m of stratigraphy based on abruptupward termination of shear zones, folding, fissuring, and abruptstratigraphic thickening, four of which may corresponded to historicallyrecorded large regional earthquakes. The earliest of the identified eventsoccurs below an unconformity and dates to about 4 ka B.P. The morerecent four events all occurred within the past 1000–1200 years and maycorrespond to large earthquakes in A.D. 824, ca 1354, 1509, 1766 and1912 (Ambraseys and Finkel, 1987, 1991, 1995). In another trench,we identified at least two events that have occurred during the past 500years and probably correspond to the large events of 1766 and 1912. These observations support an average return period of about 250–300years for the Gazikoy-Saros segment of the NAFZ. They also suggest thatthis segment, which is bound both to the east and west by large releasingstepovers, behaves in a quasi-periodic fashion, at least for the past severalsurface ruptures.Most of the 23 mm/yr of dextral shear between Anatolia and Europeobserved by GPS occurs on the North Anatolian fault. We use18 mm/yr and the ∼ 250–300 year recurrence rate, as determined fromour trenching and the historical record, to suggest that each of theearthquakes observed in our trenches produced several meters of slip,consistent with their inferred sizes from the extent of historical damage. Considering that Istanbul has not suffered a large nearby event in theMarmara Sea since 1766, we suggest that about 4 m of strain hasaccumulated across faults in the Marmara during these past centuries. Thisis similar to the average slip in many of the large earthquakes on the NorthAnatolian fault this century. If released seismically, this could result in anearthquake in the M 7.2–M 7.6 range, similar to the August and November,1999 earthquakes east of the Marmara Sea.