Implementing the Effect of the Rupture Directivity on PSHA for the City of Istanbul, Turkey

Abstract

In the present study, we improve the probabilistic seismic-hazard assessment (PSHA), taking into account fault rupture-related parameters that sensibly affect the azimuthal variability of the ground motion. The study area is the Marmara region (Turkey), characterized by one of the highest levels of seismic risk in Europe and the Mediterranean region. The seismic hazard in the city of Istanbul is mainly associated with two active fault segments having well-defined geometry, focal mechanism, and rate of activity. Deterministic dynamic models are also available in this area (Aochi and Ulrich, 2015) that aimed at evaluating the seismic potential of the Marmara region. These models provide the statistical distribution for the hypocenter position, which is particularly relevant for rupture directivity. The aim of this work is to incorporate all the available information about the seismic potential of the Marmara region in a PSHA framework. We use an analytical model for directivity (Spudich and Chiou, 2008; Spudich et al., 2013) to integrate rupture-related parameters inside the PSHA standard procedure. Because the directivity effect is conditional on the hypocenter position, which is not a priori known, we assume at first ad hoc Gaussian distributions centered in the western, eastern, or middle part of the two fault segments. Our results show that the correction for directivity introduces a significant contribution (up to 25% of relative increase at 2 s) to the hazard maps computed with the standard PSHA practice (given in terms of pseudospectral accelerations having 10% probability of exceedance in 50 years). The hazard maps sensibly change when we use the distribution for the hypocenter position informed by the statistical treatment of dynamic simulations. Thus, integrating new variables in the PSHA in combination with properly informed probability density functions is not only feasible, but also recommended for a comprehensive PSHA.