New educational module on earthquakes now on-line

PASADENA-The ever-changing Earth and the forces that make it so are the theme of a new Web-based educational module from the Southern California Earthquake Center.

"Investigating Earthquakes Through Regional Seismicity" has been designed to provide students and others with the opportunity to learn about the nature of earthquakes. With two interactive sections already on-line and additional offering in the planning stages, the module will provide Web surfers with knowledge of matters such as faults, rates of occurrence, magnitudes, and geographic distribution.

The module is designed by John Marquis, Katrin Hafner, and Egill Hauksson of the Seismological Lab at Caltech, with funding provided by the Southern California Earthquake Center.

Hafner says that the material was originally designed to answer common questions about earthquakes, but that the project has now expanded to provide a significant education component for classrooms as well. "Because the module is Web-based, it is more than just a static educational product," says Hafner.

"Each section consists of a sequence of text 'pages'-with explanatory maps, diagrams, and other in-line images-hyperlinked to activities, in which students can develop an understanding of the concepts in a more interactive way."

Many of the activities are also linked to resources such as fault maps, which provide access to seismological data archived at the Southern California Earthquake Center's data center.

The content and format of the module have been reviewed by scientists and educators alike, and portions of the module have been field- tested in high school and community college settings.

The Web module can be accessed at the following address: http://www.scecdc.scec.org/Module/module.html

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Robert Tindol
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New study explains motions of the Emerson fault in the years following the Landers earthquake

PASADENA—For geophysicists, the 7.3–magnitude Landers earthquake of June 28, 1992 has yielded much in terms of understanding the basic mechanisms of seismic events. A new study appearing in this week's Science provides a new model to explain why the ground near the fault gradually shifted the first few years after the main shock. The work could be used in the future for the analysis of earthquake hazard.

In the Science article, Jishu Deng, a postdoctoral researcher at the California Institute of Technology, and his coauthors attribute the postseismic deformation to a viscous flow in the lower crust. Experts have known for some time that such slow motions around faults can occur, and in fact were quite aware of the effect near the Emerson fault on which the Landers earthquake was centered. But no one knew whether the ground was moving in small, quirky steps or slowly flowing like a viscous liquid.

Analyzing existing data from various satellites, Deng speculates that viscous flow must be the case, even though the "afterslip model" has for some time been the preferred explanation. Deng believes the "viscoelastic model" is preferable because the satellite data shows both a horizontal motion along the Emerson fault over about three or four years, as well as a vertical motion. While the viscoelastic model is not completely new, previous studies have been unable to distinguish between the viscoelastic and afterslip models. The Landers earthquake, however, provides the first opportunity to determine which mechanism is indeed at work.

Specifically, the area just west of the north–south fault has continued to move northward since the initial rupture. On the day of the earthquake, the fault slippage was measured to be about five to six meters along the fault line. But the GPS satellites show that the displacement has gradually expanded another 10 centimeters or so.

This continued slippage can be explained by the prevailing theory of postseismic slippage, but an additional result calls for a new theory: according to information gained from the Interferometric Synethetic Aperture Radar satellite (the ERS-1), the ground to the west of the fault has also sunk by about 28 millimeters, while ground east of the fault has risen slightly. And because the afterslip model cannot explain this motion, Deng shows that the effect must be the result of viscous flow.

"So we think the fault is not slipping," says Deng, who came to Caltech after earning his doctorate at Columbia University. "It must be in a flow." Deng further says the new information could be used in the future to assess the seismic hazard in specific locales. "Our new calculations will lead to a new generation of stress evolution models and help people understand how stress builds up and releases in seismic areas."

The other authors of the paper are Michael Gurnis and Hiroo Kanamori, both professors of geophysics at Caltech; and Egill Hauksson, senior research associate in geophysics at Caltech.

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Robert Tindol
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Galileo data shows Jupiter's lightning associated with low-pressure regions

MADISON, Wisconsin—Images of Jupiter's night side taken by the Galileo spacecraft reveal that the planet's lightning is controlled by the large-scale atmospheric circulation and is associated with low-pressure regions.

The new findings were reported October 13, 1998 by Andrew Ingersoll at the 30th annual meeting of the American Astronomical Society's Division for Planetary Sciences.

"Lightning is an indicator of convection and precipitation," says Ingersoll, a professor of planetary science at the California Institute of Technology and member of the Galileo Imaging Team. "These processes are the main sources of atmospheric energy, both on Earth and on Jupiter."

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In a terrestrial hurricane, Ingersoll explains, the low pressure at the center draws air in along the ocean surface, where it picks up moisture. Energy is relased when the moisture condenses and falls out as rain.

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On Jupiter, energy is transferred from the warm interior of the planet to the visible atmosphere in a similar process. The new findings show that lightning occurs in the low-pressure regions on Jupiter, too.

"On both planets, the air spins counterclockwise around a low in the northern hemisphere and clockwise around a low in the southern hemisphere," Ingersoll says. "The lows are called cyclones and the highs are called anticyclones."

On Jupiter the cyclones are amorphous, turbulent regions that are spread out in the east-west direction. In the Voyager movies they spawn rapidly expanding bright clouds that look like huge thunderstorms. The Galileo lightning data confirm that convection is occurring there.

"We even caught one of these bright clouds on the day side and saw it flashing away on the night side less than two hours later," says Ingersoll.

In contrast, the Jovian anticyclones tend to be long-lived, stable, and oval-shaped. The Great Red Spot is the best example (it is three times the size of Earth and has been around for at least 100 years), but it has many smaller cousins. No lightning was seen coming from the anticyclones.

"That probably means that the anticyclones are not drawing energy from below by convection," says Ingersoll. "They are not acting like Jovian hurricanes."

Instead, the anticyclones maintain themselves by merging with the smaller structures that get spun out of the cyclones. "That's what we see in the Voyager movies, and the Galileo lightning data bear it out. Whether the precipitation is rain or snow is uncertain," says Ingersoll.

"Models of terrestrial lightning suggest that to build up electrical charge, both liquid water and ice have to be present. Rain requires a relatively wet Jupiter, and that's a controversial subject.

"Water is hard to detect from the outside because it is hidden below the ammonia clouds. And the Galileo probe hit a dry spot where we didn't expect much water."

Fortunately the Galileo imaging system caught glimpses of a cloud so deep it has to be water, according to findings to be reported at the conference by Dr. Don Banfield of Cornell University and an imaging team affiliate. Banfield showed images of the water cloud near the convective centers in the cyclonic regions.

These results appear in the September issue of Icarus, the International Journal of Solar System Studies.

"We know the water is there, and we know where it's raining," says Ingersoll. "This is a big step toward understanding how Jupiter's weather gets its energy."

 

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Robert Tindol
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Crust of Tibetan Plateau is being squeezed by India and Asia, new study shows

PASADENA—Geophysicists have discovered why there are high plains and mountains in the Himalayas for trekkers to trek on. According to new data, the soft crust of the Tibetan Plateau is being squeezed like an accordion between the harder crusts of India and Asia.

According to Caltech professor of geophysics Donald Helmberger and his doctoral student Lupei Zhu, the results show for the first time that a portion of crust can be squeezed and thickened if plate tectonics is forcing a harder section of crust into another hard section. Before the current study, geophysicists were unsure whether the plateau was formed by actions in the mantle or more shallow movements of the crust.

In the August 21 issue of the journal Science, the researchers show that the northward tectonic motion of India is forcing the softer and younger crust of the Tibetan Plateau into the Qaidam Basin to the north. Like India, the crust of the Qaidam Basin is also old and hard.

Since the seismic data shows there is a dramatic change in the thickness of the crust at the edge of the Qaidam Basin, the researchers infer that the softer crust is being literally forced into a hard vertical wall beneath the surface.

Therefore, the crust of the Tibetan Plateau is being crammed up and thickened in the collision. In addition to providing uplift, the action is also grinding the materials laterally. The horizontal fault lines observed in the region also support this interpretation.

"This gives a different perception about how strong an old crust can be," says Helmberger. "There's a very sharp change in the thickness of the crust, from about 40 kilometers at the Qaidam Basin to about 60 kilometers at the Tibetan Plateau.

"Physically, this means the crust beneath the Qaidam Basin is like a solid wall," he adds. "This thing below the basin is cold and old and very tough."

Zhu and Helmberger's results come from raw data collected by the Institute of Geophysics at Beijing and the University of South Carolina during the joint PASSCAL project in the early 1990s. Zhu did some of the field work before entering Caltech in 1993 as a graduate student.

Zhu says he would like to return to Tibet for additional data at other sites, and both he and Helmberger think the work could herald a new understanding of how the crust figures into plate tectonics. 

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Robert Tindol
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Spring Colloquium 98: Mars Exploration—Past, Present, and Future

PASADENA—The San Gabriel Valley Section of the American Institute of Aeronautics and Astronautics will present "Spring Colloquium 98: Mars Exploration—Past, Present, and Future" on Tuesday, June 9, from 6–9:30 p.m. in von Karman Auditorium at the Jet Propulsion Laboratory, located at 4800 Oak Grove Drive in Pasadena. This year's program will provide a comprehensive overview of America's past, present, and potential future Mars exploration missions. The public is invited and encouraged to attend. Light refreshments will be served.

The program speakers, each an expert in a particular aspect of Mars exploration, together will provide a unique opportunity to learn about the nation's Mars exploration missions and how the missions work together to create a systematic approach to understanding and exploring the Red Planet.

Dr. Arden Albee will first present an overview of past and present Mars missions from a space science viewpoint. Next, Robert Manning will present an overview of NASA's ongoing Mars Exploration Program, a series of robotic missions that will greatly expand our knowledge of Mars, and also provide the knowledge needed to mount a future human mission to Mars. Finally, William Siegfried will present an overview of NASA's planning for a future human mission to Mars.

Please join us for a fascinating and informative evening dedicated to Mars Exploration.

Dr. Arden Albee is dean of graduate studies at the California Institute of Technology in Pasadena. Dr. Albee was a science investigator on a number of past Mars missions, and is currently the principal investigator for NASA's Mars Global Surveyor mission.

Mr. Robert Manning, of the Jet Propulsion Laboratory in Pasadena, is the chief engineer for NASA's Mars Exploration Program. Mr. Manning was previously the chief engineer for the Mars Pathfinder spacecraft.

Mr. William Siegfried, of the Boeing Company in Huntington Beach, California, has worked on a number of crewed space programs including Skylab, Space Transportation System, and the International Space Station. He has also served on several key space advisory committees, and is currently co-chair of the IAA Lunar-Mars Committee.

The San Gabriel Valley Section of the American Institute of Aeronautics and Astronautics sponsors the annual event as a forum for aerospace-related topics of interest to its membership and the public.

For more information, contact the AIAA Western Office at (800) 683-AIAA.

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Posthumous Paper by Gene Shoemaker Details Evidence of Comet Shower That Pummeled Earth 36 Million Years Ago

PASADENA—Geochemical evidence from a rock quarry in northern Italy indicates that a shower of comets hit Earth about 36 million years ago.

The findings not only account for the huge craters at Popagai in Siberia and at Chesapeake Bay in Maryland, but posit that they were but a tiny fraction of the comets active during a period of two or three million years during the late Eocene period. The work provides indirect evidence that a gravitational perturbation of the Oort comet cloud outside the orbit of Pluto was responsible for sending a wave of comets swarming toward the center of the solar system.

In a paper published today in the journal Science, a group from the California Institute of Technology, the U.S. Geological Survey Flagstaff office, and the Coldigioco Geological Observatory in Italy, report their evidence of a very large increase in the amount of extraterrestrial dust hitting Earth in the late Eocene period. The writers include the husband-and-wife team of Gene and Carolyn Shoemaker. Gene Shoemaker died in a car crash last year while the research was in progress.

According to lead author Ken Farley, a geochemist at Caltech, the contribution of Shoemaker was especially crucial in the breakthrough.

"Basically, Gene saw my earlier work and recognized it as a new way to test an important question: are large impact craters on Earth produced by collisions with comets or asteroids," Farley says.

"He suggested we study a quarry near Massignano, Italy, where seafloor deposits record debris related to the large impact events 36 million years ago. He said that if there had been a comet shower, the technique I've been working on might show it clearly in these sediments."

Carolyn Shoemaker said that she and her husband went to Italy last year to perform field work in support of the paper.

"Gene was pretty excited about the work Ken was doing," she said. "He was glad Ken was taking it on. It's exciting work, and it's a rather new type of work."

The matter involved detecting the helium isotope known as 3He, which is rare on Earth but common in extraterrestrial materials. 3He is very abundant in the sun, and some of it is ejected from the sun as solar wind throughout the solar system. The helium is easily picked up and carried along by extraterrestrial objects such as asteroids and comets and their associated dust particles.

Thus, arrival of extraterrestrial matter on Earth's surface can be detected by measuring its associated 3He. And even this material is unlikely to include large objects like asteroids and comets. Because these heavy, solid objects fall into the atmosphere with a high velocity, they melt or vaporize, giving their helium up to the atmosphere. This 3He never falls below very high altitudes, and soon reenters space.

But tiny particles entering the atmosphere are another story. These particles can pass through the atmosphere at low temperatures, and so retain helium. These particles accumulate on the seafloor, and seafloor sediments provide an archive of these particles going back hundreds of millions of years.

Elevated levels of 3He would suggest an unusually dusty inner solar system, possibly because of enhanced abundances of active comets. Such an elevated abundance of comets might arise when a passing star or other gravity anomaly kicks a huge number of comets from the Oort cloud into elliptical, sun-approaching orbits.

When Farley took Shoemaker's suggestion and traveled to the Italian quarry, he discovered that there was indeed an elevated flux of 3He-laced materials in a sedimentary layer some 50 feet beneath the surface. Because this region of Italy was submerged in water until about 10 million years ago, the comet impacts and microscopic debris had accumulated on the ocean bed, and this debris was preserved because dying organisms had cooperatively covered the debris over the eons.

The depth of the sedimentary layer suggested to the researchers that the 3He had been deposited about 36 million years ago. This corresponds to the dating of the craters at Popagai and Chesapeake Bay.

More precisely, the 3He measurements show enhanced solar system dustiness associated with the impacts 36 million years ago, but with the dustiness beginning 0.5 million years before the impacts and continuing for about 1.5 million years after. The conclusion is that there were a large number of Earth-crossing comets and much dust from their tails for a period of about 2.5 million years.

In addition to Gene and Carolyn Shoemaker and Ken Farley, the paper was cowritten by Alessandro Montanari, who holds joint appointments at the Coldigioco Geological Observatory in Apiro, Italy, and the School of Mines in Paris.

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Robert Tindol
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Geophysicists model the Cretaceous motions of Australia

PASADENA--The theory of plate tectonics says that Earth's crust has moved horizontally by thousands of miles over millions of years. For visual evidence, one need look no further than a map showing how nicely South America and Africa fit together.

But plate tectonics also literally has its ups and downs. In addition to the horizontal motions that long ago tore South America and Africa asunder, plate tectonics can also cause entire land masses to steadily rise and lower by thousands of feet. These vertical motions are driven by the internal planetary heat engine that makes the plates slide around horizontally.

One of the best examples of this vertical motion is Australia, and researchers have now completed a three-dimensional model of Earth's internal heat engine that provides the best-ever description of the process. The research is published in the current issue of Science.

According to Caltech geophysicist Michael Gurnis, the lead author of the paper, the research is aimed at improving the understanding of how heat convection in the mantle relates to the motions of the crustal plates. More specifically, the work answers nagging questions about oddities in the Australian plate.

"Normally, the ridges you see in the ocean floor are associated with upwellings of hot material from the mantle," Gurnis says. But in the ocean between Australia and Antarctica, researchers have long noted a downwelling at this mid-ocean ridge.

According to Gurnis, "We don't know why there's a downwelling, but it's at least 20 million years old and probably a few hundred million years old."

Also, Gurnis explains, there is longstanding evidence that Australia's seacoast significantly retreated during the Cretaceous period (about 60 to 125 million years ago). This was caused by the aforementioned vertical tectonic motions, which bowed the entire continent upward.

The result was a continent with terrain that at times was well above sea level—and thus dry—but at other times relatively low and thus inundated with water.

For example, the maximum flooding of Australia occurred about 120 million years ago and covered more than half the present land mass with water. But what has long perplexed geologists was that 70 million years ago, while nearly half the surface area of other continents (including the Americas, Europe, and Russia) was covered by shallow inland seas, Australia was high and dry!

The achievement of Gurnis and his colleagues is to model these risings and fallings of continents very precisely, so that a vertical level can be "predicted" for a certain time period.

Also, their dynamic models show that the entire effect was caused by a plate that subducted (or passed under) Australia, later stagnated in the mantle hundreds of miles below Earth's surface, and is now being drawn back up by the South East Indian Ridge.

"This is one of the best examples of this process, which is called 'continental epeirogeny,'" Gurnis says. "As such, it is an ideal place for tying vertical motion to how Earth's heat engine works."

The dynamic models published by Gurnis and his colleagues predict a downwelling between Australia and Antarctica in precisely the position it is observed. "The correspondence between sea-floor topography, chemistry, and crustal thickness is impressive," says Gurnis.

The work represents a breakthrough because of the ability to predict the timing and geography of geologic events separated widely in space and time. Gurnis suggests that this work opens up a huge new frontier in which the motions of plates can be predicted from computer models of the Earth's internal heat engine, in much the same way that scientists use sophisticated global circulation models to study climate change.

Says Gurnis, "Such models can be tested against the vast library of Earth's history locked in the geologic record within continents and on the sea floor."

The other authors of the paper are R. Dietmar Mueller of the University of Sydney in Australia and Louis Moresi of the Commonwealth Scientific and Industrial Research Organization in Perth, Australia.

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Robert Tindol
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Don Anderson Will Be Awarded the 1998 Crafoord Prize

PASADENA—The Royal Swedish Academy of Sciences is to award the 1998 Crafoord Prize in geosciences, with special emphasis upon "the dynamics of the deeper parts of the Earth," to Don L. Anderson of the California Institute of Technology and to Adam M. Dziewonski from Harvard University for their fundamental contributions to our knowledge of the structures and processes in Earth's interior. The 1998 Crafoord Prize is valued at $500,000 dollars, and will be presented to the prizewinners at a ceremony on September 16 in Sweden.

On hearing the news that he had been awarded the 1998 Crafoord Prize, Caltech's Eleanor and John R. McMillan Professor of Geophysics Don Anderson said, "I think it's very significant that deep-Earth geophysics is being honored by this award. It is rare for our field to be acknowledged in this way. I am really delighted that Adam Dziewonski, a close colleague of mine, is also being honored for his work. Most people, when they think of geophysics, think of earthquakes, but seismologists do other things, such as x-raying Earth using seismic tomography to see what is going on in the deep Earth."

Caltech president David Baltimore congratulated Professor Anderson and noted that "the Institute is very proud and pleased that Don will be receiving the Crafoord. It is exciting news. Don's work is truly deserving of this great prize. He is one of the world's most prominent scientists in the area."

According to the Royal Academy, Anderson and Dziewonski have together developed a generally accepted standard model of how Earth is organized and of the dynamics of the processes at its core and in its mantle that govern continental drift, volcanism, and earthquakes.

Anderson and his team have researched changes arising from the pressure deep down in Earth's mantle. Sudden changes in the rock types at depths of 400 kilometers and 660 kilometers are explained by conversions undergone by the rock types, so that they contain minerals entirely unknown at Earth's surface. At 400 kilometers, the mineral olivine, common in lava, changes to spinel, a high-pressure mineral. At 660 kilometers, the mineral perovskite is formed, a mineral otherwise only produced in the laboratory at very high pressures and temperatures. Anderson's research has shown that such changes in composition of the mantle may explain the occurrence of tensions in Earth's crust that can lead to earthquakes. Anderson and his research team have also used seismic data to study convection currents in the mantle, important for understanding continental drift and volcanism. Recently, Anderson has also used geochemical and chemical-isotope methods not only for mapping Earth's development, but also for understanding the development of the moon and the planets Mars and Venus.

Anderson was born in 1933 in Maryland and received his doctorate in geophysics from Caltech in 1962. He has been a leading figure in "deep Earth" research since the 1960s. He was director of the Seismological Laboratory at Caltech from 1967 to 1989. In 1989 he published his "Theory of the Earth," a remarkable synthesis of his broad and provocative research and a guide for geo-researchers from different fields for future exploration of the dynamics of the deep parts of Earth.

The Crafoord Prize is awarded at a ceremony held on September 16, Crafoord Day. On this occasion, the prizewinner gives a public lecture and the Royal Academy organizes an international scientific symposium on a subject from the chosen discipline of the year.

The Anna-Greta and Holger Crafoord's Fund was established in 1980 to promote basic research in mathematics, astronomy, the biosciences (particularly ecology), the geosciences, and polyarthritis. Both an international prize and research grants to Swedish scientists are awarded among the scientific fields mentioned above.

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Sue Pitts McHugh
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Geologists find more evidence for an active fault beneath downtown and east Los Angeles

LONG BEACH--Geologists report new evidence for a fault beneath Los Angeles that could cause damaging earthquakes in the future.

According to Michael Oskin, a graduate student at the California Institute of Technology (Caltech), the new study concerns an 11-mile-long, previously known geologic fold that runs through the hills north and east of Downtown Los Angeles. This fold provides indirect evidence for an underlying fault.

"Our evidence from the surface is that the fold is still growing," says Oskin. "This indicates that the fault that lies beneath it must also be active."

The fold, first associated with earthquakes at the time of the Whittier Narrows Earthquake, in 1987, is formally known as the Elysian Park Anticlinorium and runs northwest-southeast from Hollywood to Whittier Narrows. Three smaller "wrinkles" formed upon the southwest-facing flank of this fold have been investigated in detail by the Caltech scientists.

Their studies of sediment deposited by the Los Angeles River and its tributaries indicate that these small folds have been active during the past 60,000 years. During that time, the area has been contracting north-south at a rate of at least a half-millimeter per year.

"Our evidence that this structure is active does not increase the overall hazard in the metropolitan region," says coauthor Kerry Sieh, a professor of geology at Caltech. "Rather, it allows us to be more specific about how, where, and how fast deformation is occurring in the area.

The length of the surface features suggests that the underlying fault is about 11 miles in length and may extend 10 or so miles into the earth. Such a fault, if it ruptured all at once, could produce a 6.5- to 6.8-magnitude earthquake.

The rate of deformation suggests that such events might occur, on average, about once every one to three thousand years.

Also contributing data and resources to the study were the Southern California Earthquake Center, Metropolitan Transit Authority (MTA), Engineering Management Consultant, and Earth Technology Corporation.

Oskin and Sieh reported on their work at the Geological Society of America meeting in Long Beach April 7, 1998.

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Robert Tindol
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Yucca Mountain Is Possibly More Seismically Active Than Once Believed, Geologists Discover

PASADENA—Recent geodetic measurements using Global Positioning System (GPS) satellites show that the Yucca Mountain area in southern Nevada is straining roughly 10 to 100 times faster than expected on the basis of the geologic history of the area. And for the moment at least, geologists are at a loss to explain the anomaly.

In the March 28 issue of the journal Science, Brian Wernicke of the California Institute of Technology (Caltech) and his colleagues at the [Smithsonian Astrophysical Observatory] in Cambridge, Massachusetts, report on Global Positioning System surveys they conducted from 1991 to 1997. Those surveys show that the Yucca Mountain area is stretching apart at about one millimeter per year east-southeastward.

"The question is, why are the predicted geological rates of stretching so much lower than what we are measuring?" asks Wernicke. "That's something we need to think through and understand."

The answer is likely to be of interest to quite a few people, because Yucca Mountain has been proposed as a site for the permanent disposal of high-level radioactive waste. Experts believe that the waste-disposal site can accommodate a certain amount of seismic activity, but they nonetheless would like for any site to have a certain amount of stability over the next 10,000 to 100,000 years.

Yucca Mountain was already known to have both seismic and volcanic activity, Wernicke says. An example of the former is the 5.4-magnitude "Little Skull Mountain" earthquake that occurred in 1992. And an example of the latter is the 80,000-year-old volcano to the south of the mountain. The volcano is inactive, but still must be studied according to Department of Energy regulations.

The problem the new study poses is that the strain is building up in the crust at a rate about one-fourth that of the most rapidly straining areas of the earth's crust, such as near the San Andreas fault, Wernicke says. But there could be other factors at work.

"There are three possibilities that we outline in the paper as to why the satellite data doesn't agree with the average predicted by the geological record," he says. "Either the average is wrong, or we are wrong, or there's some kind of pulse of activity going on and we just happened to take our data during the pulse."

The latter scenario, Wernicke believes, could turn out to be the case. But if Yucca Mountain is really as seismically active as the current data indicate at face value, the likelihood of magmatic and tectonic events could be 10 times higher than once believed.

 

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Robert Tindol
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