Surface-fault rupture and associated deformation

DOE-STD-1022-94

Probabilistic seismic hazard analysis also requires the specification of the recurrence or frequency of

occurrence of earthquakes of various magnitudes; each seismic source requires its own recurrence

relationship. For large areal source zones, historical seismicity data are usually used to estimate earthquake

recurrence rates. However, observed seismicity is usually insufficient to characterize adequately the

recurrence curve for a given source throughout the range of magnitudes up to the maximum. It is important

to correct the earthquake catalog for completeness of seismicity data to be used for probabilistic seismic

hazard analysis. The following geological data shall be used to estimate the repeat times for large events:

Geologic recurrence intervals. The geologic record captures the occurrence of earthquakes by recording

direct stratigraphic displacements within the fault zone; uplift, subsidence, or other tectonic deformation;

or secondary effects related to seismic shaking such as liquefaction and land sliding.

Fault slip rate. Fault slip rates, derived from the amount of slip that has occurred over a

geologically-defined interval, can be used to estimate average earthquake recurrence rates. Slip rates are

determined by assessing the amount of fault displacement of a geologic unit having a known age.

Temporal clustering. Earthquakes occurring on a seismic source may be clustered in time. The potential

effects of temporal clustering on estimated recurrence rate should be considered.

5.4.1.3 Surface-fault rupture and associated deformation

A site location that has a potential for surface-fault rupture and associated deformation from active faults

should be avoided. Where it is determined that surface deformation need not be taken into account, sufficient

data or detailed studies to clearly justify the determination should be presented. Requirements for setback

distance from active faults for hazardous waste treatment, storage, and disposal facilities can be found from

Environmental Protection Agency regulation (40 CFR 264).

The presence or absence of Quaternary faulting at the site needs to be evaluated to determine if there is a

potential hazard due to surface faulting. The potential for surface fault rupture should be characterized by

evaluating: (1) the location and geometry of faults relative to the site; (2) nature and amount of displacement

(sense of slip, cumulative slip, slip per event, and nature and extent of related folding and/or secondary

faulting); and (3) the likelihood of displacement during some future period of concern (recurrence interval,

slip rate, and elapsed time since the most recent displacement, see USNRC SRP 2.5.3).

For assessing the potential for fault displacement, the details of the spatial pattern of the fault zone (e.g., the

complexity of fault traces, branches, and en echelon patterns) may be important as they may define the

particular locations where fault displacement may be expected in the future. The amount of slip that might

be expected to occur can be evaluated directly based on paleoseismic investigations or it can be estimated

indirectly based on the magnitude of the earthquake that the fault can generate.