Magnitudes of future large earthquakes near Istanbul quantified from 1500 years of historical earthquakes, present-day microseismicity and GPS slip rates

Abstract

In the past 1500 years, seven sequences of significant earthquakes have occurred on a 160-km-long segment of the North Anatolian Fault zone (NAFZ) centered on Istanbul. The most recent of these sequences will be complete with the future occurrence of one or more Mw ≥ 7.0 earthquakes near the city. From a summation of the seismic moment released by historical earthquakes between 500 and 2000 CE we calculate a mean slip deficit rate of 9.9 mm/year. Our moment summation procedure sums the seismic moment associated with known historical earthquakes in the past 1500 years based on rupture length and assumed seismogenic depth. We observed that in the 1999 Izmit earthquake, strongly coupled fault patches were associated with the highest co-seismic slip. In our calculated map of present-day slip-potential, we recognize several such patches from their low present-day rate of microseismicity, e.g. segments beneath the Kumburgaz and Eastern (Çınarcık) Marmara Basins. In contrast, intervening weakly coupled patches host high present-day background seismicity, e.g. the segment beneath the Western (Tekirdağ) Basin. This lends credibility to an interpretation of future earthquakes occurring in three seismogenic segments of the NAFZ corresponding to three deep basins in the Sea of Marmara. The present-day slip deficits reach up to 1.7 m beneath the Western (Tekirdağ Basin) segment, and 4.0 m and 5.4 m beneath the Central (Central High and Kumburgaz Basin) and Eastern (Çınarcık Basin) segments, respectively. These segments most recently ruptured in August 1766, May 1766 and October 1509 and currently have a potential to generate Mw 7.2, Mw 7.4 and Mw 7.5, earthquakes respectively. Although contiguous ruptures have not occurred historically, ruptures of contiguous segments could occur as a Mw 7.5 earthquake in the west, or a Mw 7.6 earthquake in the east or as a single through-going Mw 7.7 rupture.