INFLUENCE OF SEISMIC SOURCE GEOMETRY IN PSHA PREDICTIONS IN STABLE CONTINENTAL REGIONS (SCR)

摘要

Seismic activity is higher along inter-plate boundaries, decreasing in intra-plate regions. Although a few locations in Stable Continental Regions (SCRs) have been subjected to earthquakes with magnitudes above M_w = 8, the largest events do not exceed M_w ≈ 7 and their prediction presents great difficulties on account of the scarce evidence on seismic activity in intra-plate regions. In view of the difficulty to identify seismic sources in those areas, the assumption of diffuse seismicity is often accepted in Seismic Risk Analysis. The model justifies the assumption of a Poisson process to describe the occurrence in time of seismic events. In the paper the authors examine available seismic data for a 1200km square region in the South American SCR, showing conclusively that the distribution of amplitudes is not exponential, but may be satisfactorily approximated by a Weibull (minimum) distribution, giving rise to a relation that differs from the G-R Law. The approach permits the identification of seismic events that should not be included in the background seismicity data base, but belong to a set of large earthquakes, similar to the so-called characteristic earthquake, for which instrumental or historical records are very limited. On the basis of a preliminary probability distribution of large earthquake for the SCR considered herein, the authors examine in this paper the influence on the probability of occurrence of PGA amplitudes at a site of a zone free of active faults around it.