Seismic source identification data

DOE-STD-1022-94

(Johnston, 1996). Historic or prehistoric EUS earthquakes associated with specific geologic structures

have been identified in only a few instances (e.g., Meers fault in Oklahoma). Thus, in the EUS,

earthquake sources are usually defined for ground motion analyses as source zones (areas or volumes of

the earth's crust) having uniform earthquake potential (uniform earthquake recurrence and uniform

maximum earthquake magnitude throughout the area). Typically, source zones are defined primarily on

the basis of the spatial distribution of observed seismicity complemented by information on the regional

geologic structure and tectonics. Such source zones also exist in the Western U. S. (WUS) in areas of

lower seismicity and alluvial valleys where active faults have not been identified.

4. Active faults. As defined in NRC R G. 1.165 (1997a), an active fault is part of capable tectonic source

which is essentially characterized by the presence of surface or near surface deformation of geologic

deposits of a recurring nature within the last approximately 500,000 years or at least once in the last

approximately 50,000 years or/and an association with one or more large earthquakes or sustained

earthquake activity which are usually accompanied by significant surface deformation. All Quaternary

faults within about 25 to 50 km of a site should be assessed to determine if they are significant

contributors to the seismic hazard of the site. Detailed site characterization is necessary for active faults

within a radius of 8 km (5 miles) of a site as input to the probabilistic seismic hazard analysis and the

vibratory ground motion estimation. In the WUS, the focus of seismic source identification is on

identifying active faults or fault-related features (such as fault-related anticlinal folds) that are observed

at the ground surface. This focus is appropriate because large earthquakes (historic and prehistoric)

typically have occurred on mapped active faults in the WUS. It is also appropriate to define seismic

source zones in the WUS to incorporate that portion of the seismicity (typically

small-to-moderate-magnitude earthquakes) that does not appear to be associated with identified discrete

faults. The geological, seismological and geophysical investigations to identify the locations and geometry

of faults that may be significant seismic sources shall, to the extent practical, address the following

factors:

Rate of Fault Movement. In evaluating the rate of fault movement, the following factors must be

considered: historical and geologic evidence regarding the displacement history (especially the Quaternary

displacements) of the fault, historical and instrumental seismicity data, structural relations that may

suggest kinematic linkages to a known active fault, and the regional tectonic setting.

Sense of Slip (style of faulting). For cases in which a fault has experienced slip in more than one direction

during its history, emphasis should be on assessing its most recent slip sense. The horizontal and vertical

components of displacement and at least a general assessment of the fault dip are required to properly

classify the sense of slip on a fault.

Fault Dip and Down dip Width. To model fault sources in three-dimensions, an assessment must be made

of the dip of the fault throughout the seismogenic crust. The down dip width of a fault cam be assessed

indirectly based on the estimated maximum depth of the seismogenic crust and the dip of the fault source.

Example approaches to determine the angle of dip are: (1) Use geometry of foreshock/aftershock

earthquake foci to define fault plane orientation; (2) Seismic reflection profiles, where available; and (3)

Details of outcrop patterns along range-front.

Buried or Blind Faults. Blind potential seismic sources can be identified by a combination of subsurface

interpretations (e.g., balanced cross sections, seismic reflection) coupled with evidence for

geologically-young deformation (e.g., folding of Quaternary deposits and surfaces). As an example, the

location and dimensions of an interpreted blind thrust ramp are important to the assessment of the

maximum magnitude that the ramp may be capable of generating, and the rate of slip will be important

to estimating recurrence.