Earthquakes & Faults
Three issues that the State of Nevada argues make the Yucca Mountain area unsuitable for a nuclear waste repository are active earthquake activity, volcanism and faulting. While both earthquakes and volcanos affect Yucca Mountain, from a scientific standpoint they are a relatively insignificant threat to the site. Faulting may pose more problems, not as much from shearing of the repository block as from presenting a path for water flow through the repository.
In fact, tectonic and volcanic activity created Yucca Mountain. Unfortunately, the emotional volatility of the geologic catastrophy theories proposed by the state became imprinted in the minds of Nevadans in the ongoing propaganda wars. Massive earth upheavals and flowing magma are visions quite able of stirring mass hysteria but have little to do with the safety of the nuclear repository. Nevertheless, there are real questions still unresolved regarding geologic dynamism at the site which still deserve study. Some background is in order.
Yucca Mountain was formed millions of years ago by a series of volcanic eruptions that deposited ash and material that compressed together to create layers of rock called tuff (see Figure 14). The explosive type of volcano that formed Yucca Mountain is extinct. There are, however, seven small and dormant volcanos scientists are studying in the Yucca Mountain area to determine whether one might erupt in the 10,000 years the repository is designed to last, and whether they could affect the site.
There are three main types of volcanos worldwide: composite, shield and cinder cone. Composite volcanos have explosive eruptions, such as Mount St. Helens in Washington State. Shield volcanos have less explosive eruptions and slow moving lava, such as the Hawaiian volcanos. Cinder cones generally are the smallest volcanos. The seven dormant volcanos near Yucca Mountain are cinder cones.
The seven cinder cone volcanos located near Yucca Mountain are among the most common types of volcanos on earth. Two cones located 12 and 27 miles away may have been active within the last 10,000 years. The other five, located 8 to 27 miles away, had their eruptions from 3000,000 years to 1.2 million years ago. All seven cones consist of less than .06 cubic miles of material, a relatively small amount.
The presence of volcanos near Yucca Mountain leads to questions: of how likely are volcanic intrusions s near the site and what effect would such an eruption have. After intensive studies of the soil and rock to determine the age and type of volcanos in the Yucca Mountain area, the conclusion most scientists have come to is that the chances of a volcano affecting the repository are remote. An extensive review of these issues conducted by the National Research Council in its efforts to understand local groundwater issues led to the following statement:
In assessing what processes are likely to cause a perturbation of the water table, the panel considered the long and complex Tertiary volcanic history of the region. A possible recurrence of the earlier highly explosive silicic volcanism that produced the ash flow tuffs, which are the predominant bedrock of the Yucca Mountain area, was dismissed because the subduction zone that caused it is now extinct in the Great Basin region. Concurrent and subsequent basaltic basaltic volcanism, related to the change in extensional tectonic that created the Basin and Range structure, has experienced a progressive decline in volume, as expressed in the low-volume volcanic eruptions of Crater Flat which bounds Yucca Mountain on the west, and the latest and lowest in volume, Lathrop Wells cone, a short distance to the south.
Thus the geologic record of waning basaltic volcanism indicates that the only likely style of intrusion into the Yucca Mountain area during the lifetime of the repository is a low-volume basaltic dyke. . . . Thus, dike intrusion appears to be inadequate to cause the rise of the water table level of more than 10-20 meters.
The calculated probability of occurrence of a dike intrusion that would affect the proposed repository is a very small number, on the order of 10-8 per year.
["Groundwater At Yucca Mountain: How High Can It Rise?", Report of the Panel on Coupled Hydrologic/Tectonic/Hydrothermal Systems at Yucca Mountain, National Academy Press, April 1992, p143]
The activity line of area volcanos seems to have been moving northward over the last few hundred thousand years and the size of eruptions has been diminishing with time. Non-explosive basaltic eruptions seem to be occurring near Yucca Mountain only about every 200,000 years. Moreover, the fact that Yucca Mountain is composed of many layers of volcanic tuff that haven't been disrupted would seem to indicate that the chance of the site being disrupted is virtually nill.
The very real possibility of earthquakes occurring near Yucca Mountain turns out to be a less pressing issue than is often portrayed by opponents of Yucca Mountain repository. The two main threats earthquakes pose are through effects on the water table and disruption of tunnels and support structures. However, historically earthquakes seem to cause only minor fluctuations in water tables (less than 10 meters) and ground tremors do not affect underground structures the same as above surface facilities.
When an earthquake occurs, the layers of rock take on a movement that is in some ways related to the movement of jello. If one visualizes one cherry buried in the jello and one on the top, it is clear that the cherry on the free surface is much more likely to be affected by movement. Similarly, as one nears the surface of the earth, earthquake movement is accentuated because the surface has greater freedom to move. This is because the stiffness of the rock increases with depth and the degrees of freedom with which material can move is limited below ground compared to surface structures. Consequently, mining tunnels buried in a mountain act like cherries buried in the jello and are relatively unaffected by earthquake movement. Indeed, tunnels at Skull Mountain near Yucca Mountain were little affected by a series of earthquakes in 1992.
Over the last hundred years many engineered ways to protect buildings have been developed that allow us to build skyscrapers capable of withstanding 7.0 Richter scale readings, a rather violent earthquake. Designing the repository to withstand such stresses actually requires less finesse. Unlike a skyscraper, the repository will be embedded in rock and therefore not free to move and sway the way surface buildings do.
A surface receiving facility for the nuclear waste won't have the advantage of being situated a thousand feet below the surface in rock, however, the surface waste handling facility will be built just sub surface to maximize earthquake resistance. With five foot thick walls used as part of the radiation shielding of the receiving building, its vulnerability to earthquakes will be inconsequential.
Of course, the possibility that Yucca Mountain is not very vulnerable to earthquakes has been ignored by the State of Nevada in its attempts to raise fear levels. Judy Treichel, the state's nuclear information expert was heard to exclaim, "Earthquakes, push earthquakes!" at the June 1992 Sawyer Commission meeting, shortly after a number of earthquakes had hit both California and Nevada, obviously aware of the sensationalism of those words.
Carl Johnson, the State geologist for NWPO, correctly admitted to this author at that same Commission meeting that buildings could be designed for forseeable earthquake threats and admitted that surface structures were much more vulnerable than subsurface structures like the repository itself. The defense Johnson made for the State's contention that earthquakes disqualified Yucca Mountain as a site was that there wasn't enough data from buildings monitored during earthquakes to make an informed decision about the vulnerabilities of the site. It appears that Johnson was avoiding the empirical evidence and fact that many of our cities have been subjected to earthquakes and building designs are regularly validated by this real world testing.
Another issue sometimes raised is the possibility of complex interactions between volcanos and earthquakes, but these effects if present seem secondary.
More interesting than volcanos or earthquakes in terms of their possible effects at Yucca Mountain is the existence of faults within the repository block. Intensive mapping of these faults, notably the Ghost Dance and Sundance fault systems, are presently being carried out.
The existence of deep faults crossing the repository does not in itself disqualify the site but it does raise a number of difficult questions. The obvious concern, though not necessarily the most problematic, is the question of future movement along these faults causing the fracturing of storage canisters. The more important concern is that the faults present an access channel for groundwater, concentrating the flux through the repository to vulnerable hot-spots. Resolving this question is not straightforward because the acretion of various calcites within water channels can lead to the cementing of water channels like faults. This makes it conceivable that the faults may in many circumstances be less permeable to water than the normal tuff matrix. According to Science News:
In its preliminary designs for Yucca Mountain, the Energy Department has indicated it will avoid placing waste directly within the Ghost Dance fault zone, which is believed to reach the depth of the repository 300 m below the surface. If the Sundance fault zone also extends to this depth, DOE will have to decide whether to work around these faults as well.
At present, geologists do not know whether the Sundance or Ghost Dance faults have generated earthquakes within the last several million years. But even if these structures are not active, they may still threaten the storage facility. Because fractured rocks fill these faults, they could provide a path for groundwater to reach the repository, potentially speeding up the rate at which radionuclides leak into the environment. Faults could have the opposite effect, however, if they contain natural mineral cement that inhibits water flow. [Science News, 5/14/94]