PALEOSEISMIC CHARACTERIZATION OF QUATERNARY FAULTS AT YUCCA MOUNTAIN, NEVADA

Paleoseismic investigations were conducted by a team of scientists to characterize the Quaternary activity at or near the planned national high-level radioactive waste repository at Yucca Mountain (YM). An initial screening was conducted of 94 faults within 100 km of YM with known or suspected Quaternary activity. Near the site, ten local faults within 10 km of YM, and two major faults within 25 km of YM, were selected for detailed studies. These consisted of: (a) field mapping along surface traces to document evidence for or against Quaternary surface displacements, and (b) excavation of about 50 trenches across faults suspected of Quaternary movement. Stratigraphy and soils exposed in trench walls were mapped in detail to identify the presence/absence, number, size, and stratigraphic context of displacements in bedrock and/or surficial deposits. Geochronologic control on displacement ages were provided by (a) thermoluminescence (TL) dates on fine-grained deposits, (b) U-series dates on secondary pedogenic carbonate, and (c) geochemical correlation of basaltic ash layers to nearby dated eruptive centers. Evidence for 1-7 co-seismic surface ruptures was found on all but one of the 12 faults studied in detail. Surface displacements occurred during the middle to late Quaternary and ranged in size from 5 cm to 350 cm per event. The timing constraints provide estimates of recurrence intervals from 10 ka to >100 ka with slip rates of 0.001 to 0.05 mm/yr. Fault combinations and distributed rupture scenarios were evaluated, as well as a possible temporal linkage between fault displacements and the 80 ka Lathrop Wells volcanic eruption. The Quaternary fault characteristics of the rupture length, per-event displacement, slip rate, and recurrence interval, along with fault geometry, were used to characterize the probabilistic and deterministic seismic hazards at Yucca Mountain.