Numerical testing of certain features of probabilistic aftershock hazard assessment

Probabilistic aftershock hazard assessment (PAHA, Wiemer, 2000), provided for California in the frame of the STEP project, is based on a methodology having features, two of which are addressed in detail: 1) independence of parameter c in the Omori's law on a lower magnitude cut-off, and, 2) application attenuation relations in the expression for the probability of PGA exceedance. Concerning the first point, in STEP, c is assumed constant with respect to magnitude. However, in paper by Shcherbakov et al. (2004) the authors conclude that c scales with a lower magnitude cut-off. We show, using Japanese attenuation relations and four different earthquake models, that this modification change the hazard curves for very early time interval (<1 day) after the mainshock substantially. For later times (>1 day), the effect is minimal. As regards the second point, we try to substitute attenuation relations and their uncertainties by strong ground motion simulations for a set of scenarios. The main advantage of such an approach is that the simulations account for details of the aftershock source effects (faulting style, slip distribution, position of the nucleation point, etc.). Mean PGAs and their variances are retrieved from the simulations and they are used for the PAHA analysis at a station under study. The method is tested for the Izmit A25 aftershock (Mw=5.8) that occurred 26 days after the main shock. The resulting PAHA maps are compared with those obtained by the use of attenuation relations. We conclude that the two types of the PAHA maps do not differ significantly provided equal occurrence probability is assigned to each nucleation point location. However, possible constraint on this location (e.g., occurrence within the red Coulomb stress change areas) would change the maps considerably.