GEOLOGIC AND ROCK MECHANICS LIMITS ON MAXIMUM POTENTIAL STRAIN IN THE 12.8-M.Y. HISTORY OF THE TOPOPAH SPRING TUFF AT YUCCA MOUNTAIN, NEVADA

Yucca Mountain (YM), Nevada, is the site of a proposed nuclear spent-fuel and high-level waste repository and structural effects from potential very low-probability, large-magnitude earthquakes must be evaluated. Limits can be determined on potential strain and peak ground velocity (PGV) at YM by comparing the geologic history of the rocks with predicted results of rock mechanics models. The proposed repository will be in the densely welded, crystallized, lithophysal and nonlithophysal Topopah Spring Tuff (Tpt, Miocene). The Particle Flow Code (PFC) for small-scale (<1 m) models with voids emulating lithophysal cavities and Universal Distinct Element Code (UDEC) for large-scale (10-40 m) models with equivalent material properties are consistent with laboratory results for lithophysal rock. Models can simulate various loading conditions, including tectonic and seismic stresses, and yield and fracturing of the rock mass thereby (1) establishing strain limits for the rock mass, and (2) identifying types of expected observable in situ damage if the rock had been strained beyond these limits in the geologic past. Faults and adjacent damage zones indicate local accommodation of strain; however, seismic waves affect the entire rock mass, so geologic conditions away from faults are important. In the Detailed Line Survey (DLS) along the 2.6-km-long “cross drift” tunnel, only 8% of the discontinuities are shears or faults with separations greater than 1 cm. On the basis of crystallization along 3835 fractures, including rims and vapor-phase mineralization in slabs of core from boreholes and re-categorization of DLS data in the cross drift, most formed during cooling and the rest have no diagnostic petrogenetic features so timing is indeterminate (but can be late-stage cooling). Some fractures intersect lithophysae, but PFC-predicted inter-lithophysal fracture patterns are not seen. Of 1410 lithophysae categorized by size and shape, none have UDEC-predicted stress-induced damage from rock-mass failure. In-progress results indicate that during the 12.8-million-year history of the Tpt the rocks did not experience (static or dynamic) strains greater than 0.1-0.2%, which are not large enough to induce significant damage to the rock mass. These strains can be used to determine limits on PGVs in the last 1.28x10⁷ years.