PSHA with a mixed faults/area seismic sources model for the capital cities of four Iranian provinces.
Résumé
A probabilistic seismic hazard assessment (PSHA) has been produced for Iranian cities Kermanshah, Hamadan, Zanjan and Qazvin. Current PSHA method uses Poisson distribution of events in time with seismic hazard analysis based on instrumental and historical seismicity. But in the studied zones, these catalogues are not representative of real seismic activity because of incomplete or unreliable historic sources and short periods of observations covering instrumental seismicity. At the scale of a city, representation of seismic sources in source areas is inadequate. Dilution of activity in large zones can overestimate hazard far field and underestimate near field. Conversely, fault source model can be more accurate to better represent the concentration of activity on faults and attenuation of strong-motion with distance. A mixed approach combining fault and area sources is proposed as seismic source model. Major well-known active faults at short distance are represented by linear sources associated with narrow strips of seismic background zones. Seismic area sources are defined when state of knowledge of the fault system is too low and for long-distance sources. Slip rates, fault segmentation and seismogenic capacity are evaluated for the Main Recent Fault in Zagros and major faults in the south part of Alborz using the most recent literature and morphostructural interpretation of satellites images. With this method, very large earthquakes associated with active faults and little historical observations, can be taken into account in seismic hazard.