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Seismic reflection and gravity profiles collected across Yucca Mountain, Nevada, together with geologic data, provide evidence against proposed active detachment faults at shallow depth along the pre-Tertiary-Tertiary contact beneath this potential repository for high-level nuclear waste, The new geophysical data show that the inferred pre-Tertiary-Tertiary contact is offset by moderate to high-angle faults beneath Crater Flat and Yucca Mountain, and thus this shallow surface cannot represent an active detachment surface. Deeper, low-angle detachment surface(s) within Proterozoic-Paleozoic bedrock cannot be ruled out by our geophysical data, but are inconsistent with other geologic and geophysical observations in this vicinity, Beneath Crater Flat, the base of the seismogenic crust at 12 km depth is close to the top of the reflective (ductile) lower trust at 14 to 15 km depth, where brittle fault motions in the upper crust may be converted to pure shear in the ductile lower crust, Thus, our preferred interpretation of these geophysical data is that moderate- to high-angle faults extend to 12-15-km depth beneath Yucca Mountain and Crater Flat, with only modest changes in dip.

The reflection lines reveal that the Amargosa Desert rift zone is an asymmetric half-graben having a maximum depth of about 4 km and a width of about 25 km, The east-dipping Bare Mountain fault that bounds this graben to the west can be traced by seismic reflection data to a depth of at least 3.5 km and possibly as deep as 6 km, with a constant dip of 64 degrees +/- 5 degrees, Within Crater Flat, east-dipping high-angle normal faults offset the pre-Tertiary-Tertiary contact as well as a reflector within the Miocene tuff sequence, tilting both to the west, The diffuse eastern boundary of the Amargosa Desert rift zone is formed by a broad series of high-angle down-to-the-west normal faults extending eastward across Yucca Mountain. Along our profile the transition from east- to west-dipping faults occurs at or just west of the Solitario Canyon fault, which bounds the western side of Yucca Mountain. The interaction at depth of these east- and west-dipping faults, having up to hundreds of meters offset, is not imaged by the seismic reflection profile.

Understanding potential seismic hazards at Yucca Mountain requires knowledge of the subsurface geometry of the faults near Yucca Mountain, since earthquakes generally nucleate and release the greatest amount of their seismic energy at depth. The geophysical data indicate that many fault planes near the potential nuclear waste facility dip toward Yucca Mountain, including the Bare Mountain range-front fault and several west-dipping faults east of Yucca Mountain, Thus, earthquake ruptures along these faults would lie closer to Yucca Mountain than is often estimated from their surface locations and could therefore be more damaging.