Probabilistic seismic hazard maps of Alaska

Probabilistic seismic hazard maps have been prepared for Alaska portraying ground motion values (peak ground acceleration and spectral amplitude at periods of 0.2,0.3 and 1.0 seconds) at probabilities of exceedance of 2% and 10% in 50 years. Preparation of these maps followed the same general strategy as that followed for the U.S.G.S. seismic hazard maps of the contiguous United States, combining hazard derived from spatially-smoothed historic seismicity with hazard from fault-specific sources. Preparation of the Alaska maps presented particular challenges in characterizing the hazard from the aaskaAleutian megathrust. In the maps of the contiguous United States the rate of seismicity for recognized active faults was determined from slip rates estimated from geologic data. This approach is not appropriate for the megathrust because it has been demonstrated that a sigmhcant fraction of the subduction occurs aseismically. The characteristic earthquake hypothesis, based on recurrence rates determined from geologic data, is appealing for the portion of the megathrust that ruptured in the 1964 Alaskan earthquake, but is shown to be inappropriate for the western portion of the megathrust by the recent large earthquakes in the region which did not follow the characteristic model. Consequently the hazard from the western portion was estimated based on a truncated Gutenberg and Richter model derived from historic seismicity, and the hazard for the 1964 zone was estimated from a combination of a Gutenberg and Richter model derived from historic seismicity and the characteristic earthquake hypothesis with recurrence rates estimated from geologic data. Owmg to geologic complexity and limited data, hazard models of the easternmost portion of the megathrust in the vicinity of Yakataga are the least satisfactorily constrained. Hazard is estimated for the recognized crustal faults of the Denali, Fairweather-Queen Charlotte and Castle Mountain fault systems based on available geologic slip rates. Hazard from other sources is estimated from spatially smoothed historic seismicity. Disaggregations of the hazard for Anchorage, Fairbanks and Juneau reveal the dominant sources of the hazard at each location.

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