We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.

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Seafloor faults having strong geomorphic expression and evidence for late Quaternary activity (i.e. < 150,000 years) are common geologic features associated with the Sigsbee Escarpment. Waterbottom maps derived from exploration 3D multichannel seismic data provided an early indication that several zones of seafloor faults are in the vicinity of the Mad Dog and Atlantis prospect areas. As part of the site investigation activities for field development BP initiated a study to characterize the potential hazard due to fault displacement. The fault displacement hazard study consists of five components:a site-wide structural geologic characterization of the style and origin of active faulting and fault-related deformation;development of a late Quaternary stratigraphic model to evaluate the history, recency, and rate of fault activity at the site;detailed characterization of faulting within limited study areas designated to capture fault behavior in areas of potential facilities development;a general description of the relationship between Quaternary active faulting and slope failure processes within the field area; anda probabilistic fault displacement hazard analysis (PFDHA) of the potential for fault rupture within the designated study areas that relates annual frequency of recurrence of faulting events to the size of the event. Changes in the style and origin of faulting and deformation of shallow (suprasalt) sediment across the individual field areas primarily is due to differences in the depth, geometry, and movement history of the underlying Sigsbee Salt Nappe. These relationships and the resulting geologic model for structural evolution of the suprasalt section has been used effectively to assess the site-wide geohazards not only for faulting, but also indirectly for slope failure and mass-gravity flows. Hazard from potential seafloor offset at fault crossings is judged to be moderate to low. Fault offsets of the shallowest horizons (less than 15 thousand years old) are typically less than ten meters to several tens of meters. Fault slip rates are on the order of tenths to several tens of metersper- thousand-years (m/kyr, also millimeters-per-year, or mm/yr), with most values in the range of 2-10 m/kyr Similarly, the probabilistic annual recurrence of 1-meter events is typically less than 10−3. These studies demonstrate that the presence of potentially active faults does not preclude safe development of seafloor facilities. To evaluate risk associated with potential seafloor faulting, integrated hazard studies can and should be conducted in the early stages of project development, with an underlying intent to understand the causative processes and quantitatively and explicitly evaluate the locations, magnitude and recurrence potential of displacement events.

Midway Valley, located at the eastern base of Yucca Mountain in southwestern Nevada, is the preferred location of the surface facilities for the potential high-level nuclear waste repository at Yucca Mountain. One goal in siting these surface facilities is to avoid faults that could produce relative displacements in excess of 5 cm in the foundations of the waste-handling buildings. This study reviews existing geologic and geophysical data that can be used to assess the potential for surface fault rupture within Midway Valley. Dominant tectonic features in Midway Valley are north-trending, westward-dipping normal faults along the margins of the valley: the Bow Ridge fault to the west and the Paintbrush Canyon fault to the east. Both faults displace Quaternary sediments. Published estimates of average Quaternary slip rates for these faults are very low (=10 3 mm/yr), but the age of most recent displacement and the amount of displacement per event are largely unknown. Surtace mapping and interpretive cross sections, based on limited drillhole and geophysical data, suggest that additional normal faults, including the postulated Midway Valley fault, may exist beneath the Quaternary/Tertiary fill within the valley. Existing data, however, are inadequate to determine the location, recency, and geometry of this faulting.To confidently assess the potential for significant Quaternary faulting in Midway Valley, additional data are needed that define the stratigraphy and structure of the strata beneath the valley, characterize the Quaternary soils and surfaces, and establish the age of faulting. The use of new and improved geophysical techniques, combined with a drilling program, _tfers the greatest potentinl for resolving subsurface structure in the valley. Mapping of surfici_ll get,logic units and logging of soil pits and trenches within these units must be completed, using accepted state-of-the-art practices supported by multiple quantitative numeric_tl and r clat ivc age-tinting techniques." e! "_' ';':" " i_':_ Hoover et al. (1981) and Hoover (1989) 17 3.3 Soil-Geomorphology Studies in Crater Flat (Peterson, 1988) (iv)LIST OF TABLES Bull and Ku, 1975 and Ku et al., 1979) 16 Table 3-2 -Correlation characteristics of surficial deposits (from Hoover et al., 1981 andHoover, 1989) 19 Table 3-3 -Age and characteristics of stratigraphic units of Hoover et al. (1981) and Hoover (1989) 21 Valley, the proposed location of the prospective surface facilities. The data obtained during this study will be used in conjunction with other site characterization activities to support:(1) the siting of the surface facilities, and (2) an assessment of the potential effects of surface faulting on the design of the surface facilities. SCOPE OF WORKThe scope of work for this task includes a review of available published and unpublished literature, maps, and data that are relevant to evaluation of the stratigraphy, structure, and tectonics of the Midway Valley area. Emphasis was placed on: (1) infbrmation pertaining to the nat...

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