Earthquake Hazard Research at Yucca Mountain

Winter 1989

Yucca Mountain, a flat-topped volcanic ridge located about 90 miles northwest of Las Vegas, is at present the only site under consideration for disposal of the high-level nuclear wastes produced by the United States' commercial nuclear power plants. The federal Nuclear Waste Policy Act of 1982 outlined the steps to be followed for the selection of a site for deep underground storage of the highly radioactive materials. The act called for preliminary assessment by the U.S. Department of Energy (DOE) of nine sites, with three finalists selected for detailed study, or "site characterization. " Yucca Mountain (volcanic tuff), Hanford, Washington (basalt), and Deaf Smith, Texas (bedded salt) were selected as the three finalists. In 1987, an amendment to the Nuclear Waste Policy Act designated Yucca Mountain as the sole site for detailed study. Recent completion of the 6,000-page, 9-volume "Site Characterization Plan" by DOE marks the beginning of the site characterization phase expected to last at least until 1995. Following site characterization, licensing by the U.S. Nuclear Regulatory Commission and facility construction will require several additional years. Even if approved and licensed, the site will not begin receiving waste until well into the twenty-first century.

Federal guidelines for site selection call for locating a site that can safely isolate the wastes for at least 10,000 years. Never before has an engineered project called for prediction of natural processes over such a long period, so this poses an unprecedented challenge for earth scientists. Adequate predictions of geological processes, groundwater hydrology, and future climate are needed in order to assess whether the site will be suitable.

The Nevada Bureau of Mines and Geology has been involved in research on earthquake hazards at Yucca Mountain since 1986. Headed by John Bell, Engineering Geologist at NBMG, this is one of several studies being conducted on geological issues pertinent to Yucca Mountain by the Center for Neotectonic Studies at the University of Nevada, Reno (UNR). Others at NBMG involved in the Yucca Mountain studies include Craig dePolo and Alan Ramelli (Research Associates), Hal Bonham (Research Geologist), and Keryl Fleming (Computer Specialist). The UNR studies are contracted to the Nevada Nuclear Waste Project Office in Carson City, a state agency created to oversee research at Yucca Mountain and independently assess the suitability of the site.

The earthquake potential at Yucca Mountain is one of the key issues for the nuclear waste project. Earthquakes and volcanic activity could potentially disrupt the waste packages and speed up release of radioactive materials. Along with the rest of Nevada, Yucca Mountain lies within the Basin and Range province, a region of considerable Quaternary and historical earthquake activity. In the Yucca Mountain area, there are many signs of active tectonism, even though the immediate area has had few recorded earthquakes. Several active faults surround and may transect the repository site, and nearby Quaternary cinder cones indicate the recent occurrence of basaltic volcanism.

Earthquake-hazard research conducted by NBMG has covered several interrelated topics on Quaternary geology and active faulting at Yucca Mountain. Adequate estimates of earthquake hazards require such aspects as thorough mapping of active fault traces, accurate estimates of ages of faulted deposits and surfaces, accurate determinations of the amounts of fault slip, and appropriate comparisons to similar areas. In early stages of the project, aerial photographs were taken under low-sun-angle conditions over Yucca Mountain and surrounding areas. These photographs allowed identification of several previously unrecognized active faults in the area and better delineation of fault extensions and connections. Combined with existing mapping, this work showed extensive Quaternary faulting in the area. The recently active faults are highly interconnected and may rupture together during earthquakes that cause surface deformation over a broad area. In east Crater Flat, immediately west of the repository site, most of the Quaternary alluvium has well developed soils with up to several feet of carbonate and silica accumulation. Analysis of patterns and distributions of fractures in the strongly cemented alluvium show that fractures are most abundant near, and closely parallel, Quaternary faults. This close association of faults and fractures supports the concept that earthquakes cause deformation over a broad area.

Accurate determinations of the ages of alluvial deposits and geomorphic surfaces are essential for precise earthquake hazard estimates, Studies of soils, alluvial fan geomorphology, and Quaternary dating methods have therefore been a large part of NBMG's efforts. Because many of the Quaternary dating techniques being used at Yucca Mountain and elsewhere are still experimental, various researchers were contracted for their expertise: Fred Peterson (Department of Range, Wildlife and Forestry; College of Agriculture; UNR) for studies of soils and alluvial-fan geomorphology; Ron Dorn (Arizona State University) for dating of desert varnish on alluvial fan surfaces; Richard Ku (University of Southern California) for uranium-series dating of pedogenic carbonate deposits; and Steve Forman (University of Colorado) for thermoluminescence dating of eolian silts.

There have been several historical surface-rupturing earthquakes in the Basin and Range province. Comparisons of the Yucca Mountain site to these events can add considerable assurance that earthquake hazard estimates are appropriate. Part of NBMG's effort has been devoted to analysis of faulting patterns of historical Basin and Range earthquakes. Because the 1932 Cedar Mountain earthquake area in west-central Nevada has many similarities with the faulting at Yucca Mountain, more extensive work, including exploratory trenching, has been conducted in this area.

Barring the discovery of a "fatal flaw " (a problem of great enough magnitude to disqualify the site), or a drastic change in federal legislative policy, Yucca Mountain will likely remain the top candidate for construction of a nuclear waste storage facility. As long as it does, this project will have significant political and economic effects on the State of Nevada. The detailed studies to be conducted during site characterization will require several years for completion. Through its research efforts, NBMG will continue to provide data pertinent to earthquake hazards at the site.

---Alan R. Ramelli, Research Geologist