New Earthquake Model for the Marmara Region, Turkey

Europe's most populous city, Istanbul, is under significant seismic risk from the offshore transform faults in the Sea of Marmara. The 1999 Izmit M 7.6 earthquake further loaded the faults closest to Istanbul via Coulomb stress transfer (Parsons et al 2000). While the imparted stresses are permanent, the response of seismicity to the stress rapidly decays (according to rate/state friction of Dieterich 1994), and so after two decades without a successor mainshock, it is unclear whether the hazard is now higher or lower than it was shortly after the Izmit earthquake. Here we propose a new model that forecasts earthquake probability using a combination of GPS-derived strain rate, seismicity, and the Coulomb stresses. We consider that any model proposed for the Marmara Sea must be consistent with Istanbul's 500-year-long record of large, destructive earthquakes, as well as its well-recorded seismicity since 1999 (AFAD catalog). Our approach blends the Global Earthquake Activity Rate model (Bird et al 2015) with the decaying effects of stress transferred by the 1999 M 7.6 and 2019 M 5.6 earthquakes, and a fault slip model for characteristic M≥7 earthquakes. The model reproduces the historical record of 10 M≥7.0 shocks in the past 500 years, the current shear strain rate distribution (Kreemer et al 2014), and slip rates on major faults deduced from geology and bathymetry (Armijo et al 2002). In contrast, other recent models (Murru et al 2016, and Sesetyan et al 2019) give quake rates roughly four times higher than would be expected from the historical record. Given the decay of seismicity since the 1999 earthquake, we do not see how these high rates can be justified today. In the few places in the world where the historical record is rich enough to capture the long-term earthquake behavior—greater Istanbul, greater Tokyo and Kyoto, and central America—it is incumbent upon us to calibrate our models against this record. AGU T51C-04 - Earthquake hazards model for the Marmara Sea transform system consistent with contemporary and historical data Volkan Sevilgen, Temblor, Inc., Redwood City, CA, United States, Shinji Toda, Tohoku University, Sendai, Japan, and Ross S Stein, Temblor, Inc. and Scientist Emeritus, USGS, Redwood City, CA, United States