Detecting Strain in the Yucca Mountain Area, Nevada

Abstract

From repeated surveys of the relative motions between geodetic stations Claim, Black, Mile, 67TJS, and Wahomie (Fig. 1) from 1991 through 1997, Wernicke et al. (1) deduce a N65°W strain accumulation rate of 50 ± 9 nanostrains per year (2) across the proposed high-level radioactive waste disposal repository at Yucca Mountain, Nevada. That strain rate is sufficient to indicate a higher-than-expected earthquake hazard at the repository. An earlier (1983–1993) U.S. Geological Survey (USGS) measurement (Table 1) of the strain accumulation in the network (black triangles) in Fig. 1 found a N65°W strain rate of 8 ± 20 nanostrains per year, a rate not significantly different from zero (3). Although the two measurements are marginally consistent at the 95% confidence level [that is, the difference 42 ± 22 nanostrains per year is less than 2 standard deviations, (SD)], the earthquake hazard implications are somewhat different. The USGS strain determination is the better of the two measurements. Wernicke et al. did not include the effects of monument instability in their error budget and, as a consequence, have significantly underestimated the uncertainties in their measurements. Moreover, in their interpretation of the data, they did not give proper weight to the coseismic and postseismic effects of the Little Skull Mountain earthquake (June 29, 1992, M = 5.4).