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Non-technical summary: This study calculates the probability of large (M≥7) earthquakes beneath the Sea of Marmara, the shores of which are home to 16 million people. The paper was written because of emerging information that required a reassessment of the hazard. Calculation of what will happen in the future depends on what happened in the past. Thus probability calculations require knowledge of the frequency of past large earthquakes. Written descriptions of damage from the historical record (going back 2000 years) are available for the region. However, major earthquake faults in northwest Turkey are submerged beneath the Sea of Marmara, which has made it difficult to know which fault caused historic earthquakes. A recent multi-beam bathymetric survey and fault map combined with a new determination of how energy attenuates through the crust enabled Parsons (2004) to assign historic earthquakes to faults and determine frequencies. Additionally, the 1999 M=7.4 Izmit earthquake is calculated to have raised stress in the Sea of Marmara region, and the availability of new, detailed earthquake slip-models has revised stress-change calculations on Sea of Marmara faults. Combination of these new parameters led to a revised M≥7 earthquake probability. For example, the revised 30-year probability at Istanbul was revised to 41±14% from the previous estimate of 62±15%. Uncertainties associated with each stage of the assessment were carried through all the calculations. The second aim of the paper was to assess the sensitivity of the answers to parameter uncertainties, and to determine whether incorporation of stress transfer was justified. It was determined that in the case of the Izmit earthquake, where the magnitude of the stress change was large, stress transfer should be used.

Figure 1. Segments of the North Anatolian fault beneath the Sea of Marmara [Armijo et al., 2002] and locations of M 6.6 earthquakes (A.D. 1–1999) as located by Ambraseys [2002]. Click on image for the whole pdf article or on the link above.