This computer program makes probabilistic seismic hazard calculations at sites affected by earthquakes occurring on faults which are defined by the user as a series of line segments. The length of rupture of the fault as a function of earthquake magnitude is accounted for, and ground motion estimates at the site are made using the magnitude of the earthquake and the closest distance from the site to the rupture zone. Uncertainty in the earthquake magnitude, in the rupture given magnitude, in the location of the rupture zone on the fault, in the maximum possible magnitude of earthquakes, and in the ground motion at the site given the earthquake, its size, rupture length, and location, are accounted for explicitly. FRISK (Fault RISK) was written to take advantage of repeated calculations, so that seismic hazard analyses for several ground motion parameters (for instance, peak ground acceleration, velocity, and displacement), and for several sites, are most efficiently made with one execution of the program rather than with repeated executions.
The program uses a step-truncated exponential distribution for earthquake magnitude, a lognormal distribution for rupture length given magnitude, a uniform distribution for rupture location on faults, and a lognormal distribution of site amplitude given magnitude of the earthquake and distance from the rupture zone to the site. The program has been structured so that other functions may easily be substituted if this is appropriate for a particular problem; for example a wide range of deterministic or probabilistic geophysical models for estimating ground motion may be incorporated, and the program will yield probabilistic estimates of seismic hazard.
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USGS Numbered Series
FRISK: computer program for seismic risk analysis using faults as earthquake sources