HR: 13:40h
AN: S13E-01    [Abstracts]
TI: Including foreshocks and aftershocks in time-independent probabilistic seismic hazard analyses
AU: * Boyd, O S
EM: olboyd@usgs.gov
AB: Traditional or time-independent probabilistic seismic hazard analysis (PSHA) treats each source as being temporally and spatially independent; hence foreshocks and aftershocks, which are both spatially and temporally dependent on the mainshock, are removed from earthquake catalogs. Yet, intuitively, these earthquakes should be considered part of the seismic hazard, capable of producing damaging ground motions. In this study, we consider the mainshock and its dependents as a time-independent cluster, having a recurrence time of the mainshock, such that each earthquake in the cluster can contribute to seismic ground motions and hazard. We produce PSHA maps and compare ground motions resulting from the clustered model to a traditional analysis. The concept of clustering in PSHA was outlined by Silva and Toro for the New Madrid series or cluster of earthquakes, in which they suggest that every 500 years, the probability of exceeding a specified ground motion at a given location could be achieved by any of the three segments in the cluster: the southern segment, the central segment, or the northern segment. They considered this mathematically as a union. Therefore, the probability that the New Madrid cluster, pc, will exceed a specified ground motion is expressed by the equation p_c=1-(1- p₁)(1-p₂)(1-p₃) where p₁, p₂, and p₃ are the individual ground motion exceedance probabilities of the earthquakes making up the cluster. Generalizing this for n foreshocks and aftershocks, this equation can be written p_c=1-\prod{1-p_n}. There are several interesting properties of this equation. If any one of the exceedance probabilities is equal to one, the exceedance probability for the cluster is one. As the individual probabilities decrease, as happens for very large and infrequent ground motions or great distances from the fault, the individual exceedance probabilities tend to add. Take, for example, hazard curves for a New Madrid earthquake cluster. At return periods on the order of the cluster recurrence interval, 500 years, only a single earthquake appears to contribute to the hazard. But at longer return periods, say 2500 years, all earthquakes in the cluster appear to contribute fully to the hazard. Because of this behavior, including foreshocks and aftershocks will never increase the hazard by more than the ratio of a catalog containing all shocks to one having the dependent earthquakes removed. Typically, seismic hazard based on earthquake catalogs considers a range of magnitude between 5 and 7. Over this range, we find that for the Central and Eastern US, the rate of all earthquakes is about a factor of 2.5 greater than the rate of independent earthquakes, where the declustered earthquake catalog is obtained with a Gardner and Knopoff declustering algorithm. In the Western US, this ratio is closer to 2. These ratios imply a potentially significant increase in time-independent estimates of seismic hazard at long return periods relative to the recurrence interval of the mainshocks.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2007 Fall Meeting