Geotimes Logo NEWS NOTES  January 1999 

by Kristina Bartlett and Devra Wexler

 A layered inner core?
 Geo-soldiers
 Live from the ocean
 Researchers look ahead
 Dust trackers
 Improving geoscience curricula
 Loose cannons

A layered inner core?

Two seismologists recently published work suggesting that the inner core is composed of two distinct layers. They have found evidence that a sharp transition zone exists 100 to 300 kilometers below the top of the inner core. The behavior of the layer above this transition zone could provide clues about the core’s evolution and dynamics and its relationship to Earth’s magnetic field, they suggest.

Xiaodong Song of Lamont-Doherty Earth Observatory — one of the original researchers to propose that Earth’s core rotates faster than the planet — and Don Helmberger of the Seismological Laboratory at the California Institute of Technology, published their findings in the Oct. 30 issue of Science and presented their work during the December meeting of the American Geophysical Union in San Francisco. They measured the varying arrival times and phase changes of earthquake-generated seismic waves traveling from the South Sandwich Islands in the South Atlantic Ocean and from quakes just west of the islands, to seismic stations in Alaska, Canada, Korea, Russia, and Germany. The researchers believe their conclusions could be important in understanding the source of anisotropy in the inner core — the tendency for seismic waves to travel at different velocities in different directions when they pass through the inner core. The anisotropy arises from a crystal alignment of iron in the inner core.

Recent studies have shown that the anisotropy of the inner core in general decreases outward from its center. Song and Helmberger go further and suggest that an upper layer of the inner core is completely isotropic — the waves traveling through it have identical properties in all directions. Their analysis of the seismic waves points to a 50-kilometer transition zone between the inner core’s isotropic and anisotropic layers.

“This study provides direct evidence of a very sharp transition,” Song says. Just as light waves refract when they travel from air to water, so did the seismic waves refract as they traveled from one layer of the inner core to another, he adds. More importantly, waves traveling north-south through the inner core arrived at the seismic stations with more variations than did waves traveling east-west through the core. That’s because the inner core’s anisotropy is aligned in the north-south direction, Song explains. The change from an isotropic inner-core region to an anisotropic region would be sharper and easier to record for waves traveling north to south.

These observed phase and velocity differences could only arise from a discontinuity within the inner core, Song and Helmberger contend. More specifically, the different arrival times are evidence of a triplication — a branching of one wave into three different waves — of the earthquake waves as they traveled from the South Sandwich Islands and hit the proposed transition zone. Each wave would have branched into three different waves with three different arrivals at the seismic stations. “The simplest explanation to the observed phenomenon [arrival differences] is a triplication produced by a sudden increase in velocity along [north-south] paths due to a change in the inner core anisotropy,” they write in Science.

Most compelling, they observe, were the changes in waves traveling from earthquakes occurring just west of the South Sandwich Islands to the German Regional Seismic Network. These waves actually reflected from a point Song and Helmberger say was about 95 kilometers below the top of the inner core. This reflection supplies direct evidence for a discontinuity within the inner core, they write. “We actually saw reflected waves. If you have this kind of boundary you will see reflected waves — a very sharp change in the wave direction instead of small changes in wave direction,” says Song.

Wrestling with gravity?
Song and Helmberger haven’t pinpointed the exact location of the transition zone. In fact, they say, it probably varies with depth. At some points it could be 100 kilometers from the top of the inner core, at other points 300 kilometers. The transition zone probably varies laterally, they say.

Perhaps, Helmberger says, this isotropic layer of the inner core is perturbed by convection. The lower, anisotropic layer is not stirred by convection, so its crystals line up. A gravitational coupling between the mantle and core could churn the upper isotropic layer; the layer could also be affected by Earth’s magnetic field. Thus, Helmberger speculates, the varying thickness over time of this isotropic layer could coincide with changes in Earth’s magnetic field. “If that layer is changing over time, then the magnetic field would know about it and vice versa,” he says.

To observe these changes, Song and Helmberger say they’ll need more seismic arrays. “Better imaging of the inner core structure will help us distinguish different mechanisms of anisotropy as well as help us constrain the inner core rotation better,” says Song.

Researchers studying the inner Earth are still debating the dominant crystal structure of the inner core: is the iron arranged as a face-centered cubic, hexagonal close-packed, or recently discovered b structure? “Maybe in the inner core there are different phases of iron,” Song says. “We don’t know.” The main thing, he adds, is that the researchers observed changing wave behaviors for different depths within the inner core. Whether those changes point to phase changes or different crystal alignments within the inner core remains to be determined. “Why this is such a sharp change is really unclear yet,” Song says. “It’s all related to where the anisotropy comes from in the first place ... it may all be related to how the inner core is formed.”
 — K.B.
 


Geo-soldiers

Soldiers don’t just tote guns anymore — they’re also learning about geology. For training, offensive tactics, and peacekeeping, the earth sciences are becoming the latest strategic weapon in the military.

During a session of the Geological Society of America meeting in October (“Military Applications of Engineering Geology”), scientists from the United States and the United Kingdom described a variety of geoscience applications now being used by the military. Whether for identifying fracture zones to locate and blow up underground storage facilities or for evaluating terrain to build roads, geology has become a necessary part of military operations.

David Patrick and Suzanne Boyd of the University of Southern Mississippi described how the Army National Guard incorporates geology and other sciences into its training program at Camp Shelby Training Site (“Geologic Data in Military Training”) in order to comply with environmental regulations and cultural considerations.

Military training compounds are huge tracts of land that provide the space needed for recruits to practice maneuvers and fire weapons, explained Patrick. Camp Shelby is a 134,000-acre site that encompasses hardwood and pine forests as well as wetlands and is home to endangered species such as the gopher tortoise. It’s against the law to kill or endanger the tortoise, says Patrick, and forbidden for tanks to maneuver in the 15,000 acres of wetlands — some of which are easy to spot and some of which are not. “These are honest-to-goodness wetlands — but some have standing water at the surface and others don’t,” says Patrick.

Wetlands, streams, and slopes are all off-limits, so tanks must stay on hilltops. Trees are painted blue to show the boundaries and keep the tanks in place. But the tanks have to be able to travel and maneuver, so legal wetlands crossings, corridors of gravel, and sandbars have been brought up to grade. The Guard also needed a “plain-language classification” of the wetlands to make it easier to differentiate between the various areas. This was accomplished and a geologic map constructed as well,  showing the critical habitats and wetlands and the important geologic controls of the area: lithostratigraphy, weathering and soils, hydrology, and geomorphology.

On the offensive
Around the world, militaries have storage areas hidden underground for aircraft, ammunition, submarines — even troops. Thomas Eastler of the University of Maine and Robert Jacobi of the State University of New York at Buffalo described how to locate and “take out” enemy targets.

Since weapons can be safely stored underground, explained Eastler, “we need to find a way to neutralize them.” The trick, of course, is finding them first. Vegetation or sand often cover the bedrock that conceals a storage area. Eastler and Jacobi look for fracture systems (disturbed bedrock is the first clue) to locate weapons storage units and destroy them. “We must know the third dimension — we must be able to ‘rubble-ize’ the cover and target,” Eastler says. Understanding the characteristics of the fracture system is important for determining the trajectory of the missile that will be used in the strike.

Remote-sensing data can be used to recognize these “Fracture Intensification Domains” or FIDs, said Jacobi. By integrating lineament analyses of Landsat and hyperspectral images, air photos, and topographic maps with advanced analysis techniques of fractures observed in outcrops or boreholes, he and his colleagues are trying to model fractures in the “denied” (inaccessible or enemy) areas. The greater the density of the fractures, the more the integrity of the bedrock is impaired — implying a disruption to the bedrock and a possible military site.

War and peace
Scientists and military leaders in England are using geology to assist in peacekeeping efforts. Paul Nathanail, University of Nottingham, discussed different peacekeeping strategies and how they have changed over the years. There are three levels of peacekeeping involvement, he explained. The United Nations may have to step in “to keep the two sides apart, where two belligerents are not playing ball.” Or two antagonistic parties may allow a third party to serve as an intermediary. “Wider peacekeeping” tactics are called for in some highly volatile situations when a much stronger effort, and perhaps humanitarian relief, are required. Peacekeeping has expanded, Nathanail added. “There are more conflicts now in the 1990s than in the ’70s and ’80s. … And a multinational force is a reality.” At the same time, he added, “we have reduced armed forces [because of] the end of the cold war.” The new post-conflict priority has become de-mining operations.

Nathanail said that the British military uses geology extensively. Missions rely on proper protection, site selection, and mobility. Soldiers evaluate terrain to examine soils, studying the susceptibility to slope failure and the angles at which failure is likely to occur. They also study known fault zones, looking for silts and sands and making predictions of maximum impact of seismic activity, and examine flood-prone areas to gather historic data on severe floods.

When the United Nations and NATO peacekeeping troops were in Bosnia, the British soldiers needed to widen and maintain roads through mountains, an assignment “they weren’t used to doing,” says Nathanail. An aide memoire, focusing on geology, explained how to protect the slope beneath a road, to provide for drainage, and to avoid disposing spoil over the slope. In addition, they learned to take advantage of existing fractures to form slopes and to avoid excess blasting. Finally, the soldiers learned that persistent, clay-filled discontinuities could be the most dangerous. Without the geologic advice, says Nathanail, the army would have had much greater difficulty constructing the roads that were essential to their mission in Bosnia.
 — D.W.


Live from the ocean

To find out why cold water sinks annually in the North Atlantic, you have to be there when it happens. That’s what oceanographers tried to do last spring when they released two AUVs, or autonomous underwater vehicles, into the Labrador Sea. These automated submarines measured currents and temperatures 2,000 meters below the surface of the frigid waters. Technical problems prevented the AUVs from taking measurements throughout the entire winter, as originally planned. But the experiment was the first of what marine geologists and geophysicists hope will become a new research frontier: applying new technologies to measure ocean processes as they happen, where they happen.

“The rock record tends to smooth out what are actually a number of very discrete events,” says Marcia McNutt, president of the Monterey Bay Aquarium Research Institute. “We’re going to have to go about our research in a way we haven’t done before.” McNutt and other oceanographers reviewed the past and future of their science during a symposium, “Fifty Years of Ocean Discovery,” held last October at the National Academy of Sciences in Washington, D.C. The symposium celebrated 50 years of National Science Foundation (NSF) support for oceanographic research.

Submarine warfare during World War II demanded a better understanding of the oceans and led to the development of the science of oceanography. The Office of Naval Research funded oceanographic studies until NSF became the principal funder. Now oceanographers say they are feeling the pressure from society to answer complex questions and conduct comprehensive research with relatively small budgets. The oceans are a crucial part of the nonlinear systems that form the interactions among the lithosphere, biosphere, atmosphere, and hydrosphere. At the same time, oceanographers must study in an environment that can be as inaccessible as outer space.

“Developments in the understanding of the ocean have followed — and not preceded — technology,” says Walter Munk, a leading oceanographer since the 1940s and emeritus research professor of geophysics at the Scripps Institution of Oceanography. Scientists are developing new technologies to help them research the ocean at relatively low costs.

Into the ocean
Multidisciplinary teams of researchers are developing undersea observatories; creating computer models of ocean systems; assembling records of past climate changes from Ocean Drilling Program cores; deploying unmanned AUVs in deep-ocean regions; and looking at global ocean temperatures, currents, and biological densities through the all-seeing eyes of satellites like the National Aeronautics and Space Administration’s Sea-viewing Wide-Field-of-view Sensor (Sea WiFS). The first satellite view of the ocean came from the Coastal Zone Color Scanner in 1978 and “showed us that oceanography suffers from a terrible undersampling problem,” said Richard Barber of Duke University.

At the same time, sampling is taking a new turn. Oceanographers traditionally used Nansen Bottles to sample ocean water. The samples were studied in the lab, away from the ocean and its systems. Now scientists and their measuring devices are getting right into the water, collecting data constantly over time from a machine or buoy stationed in the ocean or from a satellite that can view thousands of sea miles at once. “We can only see everything by going into the ocean — in person,” Barber said.

Underwater laboratories
An example of an in situ ocean laboratory is the Hawaii-2 Observatory Seismic System, which sits on the ocean floor midway between the coasts of California and Hawaii. The seismometer is connected to a box that can also hold instruments that measure water temperatures, chemistry, and current velocities. Powering the box is an unused but still functioning undersea telephone cable that stretches 2,000 miles from Hawaii to California, enabling the seismometer to make constant measurements over time and transmit data back to labs at the University of Hawaii. Hawaii-2 is the first of many seismometers that
scientists hope will develop into the Ocean Seismic Network (Geotimes, August 1998).

Researchers at the University of Hawaii gathered three months worth of data from HUGO, the Hawaii Undersea Geo-Observatory. The ocean-floor observatory sends data from the summit of the Loihi underwater volcano (1,200 meters down) to the Big Island of Hawaii through a 47-kilometer fiber-optic cable. Led by Fred Duennebier, the Hawaii team presented HUGO’s initial findings during the December American Geophysical Union meeting in San Francisco. The system broke down in April because the cable could not withstand the rough terrain, Duennebier says. The team hopes to have a new cable in place this month.

Monitoring the Atlantic Ocean is the Long-Term Ecosystem Observatory (LEO-15), which includes scientists from Rutgers University and Woods Hole Oceanographic Institution. It is the world’s first long-term undersea observatory. A cable stretches from the shore to the observatory, which sits in 15 meters of water 9 kilometers from the New Jersey coast. As with Hawaii-2 and HUGO, the cable powers the observatory and carries data to the laboratories.
 — K.B.


Researchers look ahead

As we think about science in the next millennium, space exploration and medical advances often come to mind, but there is much to be discovered about our home planet, too. The National Science Foundation (NSF), working with the National Research Council (NRC), is formulating its future earth-science research priorities in a study, Basic Research Opportunities in the Earth Sciences. As part of the study, the NRC and NSF held forums at recent geoscience society meetings to obtain feedback from the research community. The first forum was held during the Geological Society of America annual meeting in Toronto in October 1998. A similar forum took place at the American Geophysical Union meeting in San Francisco last month. Additional workshops may be held later this year, as the committee seeks input from across the geosciences.

The study
The last major evaluation of NSF earth-science programs occurred in 1984 with the NRC report, Opportunities in the Earth Sciences. According to NSF Director of Earth Sciences Ian MacGregor, the report focused on “style not substance” by making broad recommendations, such as emphasizing the importance of facilities and a multidisciplinary focus. Since then, the NSF has worked to implement the recommendations, with minor adjustments, and has achieved most of its goals.

Seeking a new set of directives, NSF’s Division of Earth Sciences asked the NRC to conduct an assessment of the solid earth sciences in 1997. The committee, comprised of 14 members from a broad range of geoscience subdisciplinary backgrounds, will be chaired by Massachusetts Institute of Technology Seismology Professor Thomas Jordan. According to their staff director, Anthony deSouza, the committee will identify a small number of research priorities rather than a “laundry list” of topics that need attention. Although the committee will focus on basic research within NSF, it may also examine the links between research and societal needs as well as topics of interest to industry and the international community. The study is scheduled to be completed by late 2000, although deSouza anticipates that it may be completed earlier. This study coincides with several other NSF assessments. NSF is conducting an agency-wide study, Grand Challenges in Environmental Science, and the Geoscience Directorate is plotting its own future in a study, GeoVision 2000.

GSA session
The 10 geoscientists speaking at the GSA session, “Research Opportunities in the Earth Sciences: A Ten-Year Vision,” emphasized many different disciplines, but most agreed that future growth in the geosciences will occur in integrated areas — not along traditional disciplinary boundaries. In her talk, GSA President and Rutgers University Professor Gail Ashley said that research in the next decade must be both multidisciplinary and interdisciplinary, linking scientists from many disciplines together. She identified seven research areas that fall into these categories, including analysis of the surface-to-bedrock zone, dynamics of the cold region, biogeology, and periods of abrupt climate change. USGS geophysicist Mary Lou Zoback presented a paper by U.S. Geological Survey Associate Chief Geologist Steve Bohlen predicting that the next 100 years could be the “century of earth science.” Zoback mentioned three integrative activities that must be accomplished to achieve this goal: understand the surface of Earth as a coherent air, land, and water system; unify the geologic, biologic, and ecological sciences; and develop a synergy among federally funded research programs. University of Virginia Professor George Hornberger, who chairs the NRC Commission on Geosciences, Environment, and Resources, presented an overview of recent studies from the commission. He noted that common themes throughout many of the studies are the importance of interdisciplinary research, coordinated long-term datasets, and new measurements. Other talks examined paleobiology and the interface between biology and geology in isotope ecology as interdisciplinary areas that need further study.

Vic Baker, professor of planetary sciences at the University of Arizona, and other speakers emphasized how the rationale for scientific research has moved away from defense priorities since the end of the Cold War. Baker also spoke about the blurring of the line between the basic and applied sciences. David Simpson, president of the Incorporated Research Institutions for Seismology, pointed to the advances in seismology that have been made since the collapse of the Soviet Union and the resulting increased opportunities for data collection and international collaboration.
 — Kasey Shewey White, AGI Government Affairs Program
 


Dust trackers

Most people, other than those who suffer from allergies, might not realize that air currents, from light breezes to gale-force winds, carry solid matter. Dust particles are constantly picked up and carried elsewhere — sometimes halfway around the world — to settle onto Earth’s surface, until they are either picked up again or covered by other material. Where the dust goes depends on global circulation patterns. Where it stays seems to be largely a matter of climate. Where it comes from, however, is not well understood, but an international team of scientists is closing in on some answers.

During Earth’s glacial periods, the atmosphere contained a great deal more dust than it did during warmer eras. For some time, it has been known that the concentration of dust falling on high-latitude ice caps varied greatly, and that high concentrations reflected glacial periods, say Pierre Biscaye and Francis Grousset, scientists at Columbia University’s Lamont-Doherty Earth Observatory and France’s Université de Bordeaux, respectively. But they wanted to know why this was true. So, they and their French and Danish colleagues embarked on a study to determine the provenance, or place of origin, of dust contained in ice cores from Antarctica and Greenland.

“The causes of … extreme variations in the dust record … have been the subject of more speculation than analysis,” they wrote in the international newsletter PAGES (Past Global Changes) in August 1998. “The approach that we … have taken to the solution of this problem is that the first step must be to determine from what continental locations the dust has come.” In other words, knowing where the dust came from and where it ended up is the first clue to determining the ancient path of transport, and whether the variations were due to changes in the extent of arid source areas or to changes in wind intensity, or both. The long-term goal: a better understanding of paleoclimate and atmospheric paleocirculation.

In October 1998, Biscaye presented the group’s research at the Geological Society of America’s annual meeting. Entitled “Mineralogical and Isotopic Tracers of Aeolian Dust in Ice Caps and Marine Sediments,” his talk focused on the tracers they used to determine the origin of dust in two ice caps. Dust in the East Antarctic Dome C and Vostok ice cores was analyzed first, followed by dust extracted from the Greenland summit ice cores (GISP2 and GRIP). The samples were compared with wind-deposited or wind-blown samples from PSAs — Possible Source Areas.

The tracers for this comparison must be distinctive to the source area. The scientists used both clay mineralogy (reflecting the source-area climate) and radiogenic isotope composition (reflecting lithology and geologic age of the rocks from which the source area sediments were derived) to constrain the origin of the ice-core dusts. Plotted on one of several kinds of graphs, the samples clearly show whether or not they are from the same areas.

In their first attempt in 1992, the group took a sample of dust from the last glacial maximum (LGM) section (18,000 years B.P.) of the East Antarctic Dome C ice core. They compared this sample with samples from other conti-nental areas of the globe that were nearest, such as local Antarctic sand dunes, Australia, southern Argentina, and southern Africa. The strontium (Sr) and neodymium (Nd) isotopic ratios from all the samples were plotted on an eNd(o) vs. 87Sr/86Sr isotope composition diagram. (The eNd(o) notation is a convenient representation of the 143Nd/ 144Nd ratio.) The isotope composition fields of southern Africa and Australia overlap, but are distinctly different from those of Antarctica and South America (Argentina). The Dome C dust sample falls clearly in the last field, as does the dust from all other samples from East Antarctica, going back almost 200,000 years B.P. According to Biscaye, the dust from Dome C (and Vostok) therefore originated in southern Argentina and was transported to Antarctica through the atmosphere.

A similar experiment with the Greenland cores showed the researchers that dust contained in the GISP2 (Greenland Ice Sheet Project 2) ice core from Summit, Greenland, originated in eastern Asia. At the GSA meeting, Biscaye reported that the GISP2 core dust contained almost no smectite, and had a low kaolinite to chlorite ratio, indicating a high-latitude provenance. The isotopic compositions of strontium, neodymium, and lead all confirmed the origin as eastern Asia
— particularly China and Mongolia. The group is currently working on dust from western Antarctica and from annual layers of snow over the past decade in northern Greenland.

“The use of tracers, and particularly of multiple tracers, is a good way to determine provenance,” Biscaye concluded. And knowing the source area is the key to future research on how global atmospheric circulation has changed over time.
— D.W.


Improving geoscience curricula

For the past two summers, educators from across the country have gathered for a series of workshops on improving science curricula, working on projects to enhance undergraduate and K-12 education and examining the geosciences in particular. The project, called IDIG: Improving Delivery in Geoscience, is an educational initiative of the American Geological Institute (AGI).

A special symposium at the Geological Society of America (GSA) meeting in Toronto last October gave the IDIG participants a chance to reconvene and report on their individual projects, and to hear what the other teams were doing. Eleven teams, from the 1997 and 1998 summer workshops, provided reports and updates.

In most cases, teams are composed of groups of professors from a college or university, although a team from the Illinois State Geological Survey also participated this year (see Geotimes, August 1998). It’s also possible for professors — particularly if they are the only geoscience faculty member at their college taking part in IDIG — to join up with teams at other institutions, with nongeoscience professors at their own institutions, or with high-school teachers.

Team reports
Many different kinds of colleges and universities are involved in IDIG, from traditional four-year institutions to colleges for nontraditional students to two-year colleges. The College of Alameda, Chabot College, and Las Positas College, all two-year community colleges in northern California, are members of the College of Alameda Team. Because of another commitment — the members were run-ning a workshop on Geographic Information Systems (GIS) that week — the team could not travel to Toronto, but they did send an update report to the meeting.

Rita Haberlin of Alameda reported, “[The workshop] is an opportunity to showcase [the] new computer facility that we are using for web-based labs in geoscience. We are also gradually introducing students to GIS in our lab classes. ... I want to reassure [you] that we are continuing to redesign the way we teach and are benefiting from the opportunity to attend IDIG as a team. ... I went to Texas and participated in the First National GIS Teacher Training Institute. ... We have also shared our skills with other teachers in various staff-development workshops.”

The College of Charleston (S.C.) team is also working on improving lab classes in the geosciences. Charleston is a four-year, liberal arts, state-supported college with 10,000 students. More than 1,000 of them take geology classes each year in a department with 11 faculty members. The Charleston team is focusing on its introductory course, GEOL 101, says Leslie Reynolds Sautter, a geology professor at Charleston. There are so many sections taught by different professors that there is discontinuity in the lectures. Professors can choose their own books and lectures, so every lab has to start from scratch, “assuming zero knowledge on the students’ part, since the students didn’t have the same lectures.” Sautter described the faculty’s plan to link the lectures and labs by having a professor teach a lecture, and then have the same students in two labs. The pilot sessions will start in the spring.

Montclair State University, in Upper Montclair, N.J., also has 10,000 students, with about 1,000 students enrolled in geology courses. But a considerable percentage of the student body is made up of nontraditional or part-time students. The geology department has tried new approaches to improve delivery and enhance the program. Students have jobs and family commitments, which causes problems for scheduling labs and field trips. Montclair’s Jonathan Lincoln told the IDIG group that the geology faculty is exploring ways to “change the design of the courses to alleviate some of the disconnection that occurs as the nontraditional student moves from class to work to home.” They are also experimenting with new ways to do field work; the students in one section now do some of their lab work outside, collecting data for later use in lab. And families are included on many of the field trips. Lincoln also reported that a new web page includes exam results, homework assignments, news, and current events, allowing students to stay up-to-date on their work.

Lake Superior College in western Michigan, although not a member of IDIG, attended the symposium and asked the teams questions about designing a geoscience curriculum “from the ground up.” Advice from IDIG participants ranged from the need to analyze one’s existing program (departments should determine what their approach is, what they want their content to be, and what their modes of delivery are) to assessing the program. An important question that the Lake Superior staff needs to answer is whether to emphasize content knowledge (facts and figures of geology), skill knowledge (technical ability and problem-solving), or both, suggested IDIG participants.

IDIG is well on its way to accomplishing its goals of systemic change in classes and curricula. The teams are geared up for yet another year of hard work, and are looking forward to next summer’s workshops. For more information on IDIG and about participating in its workshops and programs, contact the American Geological Institute.
— D.W.


Loose cannons

For centuries, people have known that volcanic eruptions may trigger earthquakes. However, only recently has substantial evidence accumulated to show the reverse: Some earthquakes may cause volcanic eruptions. In the Oct. 29, 1998, issue of Nature, Dr. Alan T. Linde and Dr. I. Selwyn Sacks present research suggesting that seismic energy produced by an earthquake may cause a volcano to erupt, even at distances of 750 kilometers.

Linde and Sacks, both of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, became interested in the problem of earthquake-triggered volcanism, or triggered seismicity, after the 1992 Landers earthquake in Southern California. “Landers triggered an episode of deformation at the Long Valley caldera [and] we are rather confident that the deformation transient was what produced the triggered seismicity in the Long Valley area,” says Linde. To
support their hypothesis, the researchers searched through the historical records of earthquakes and volcanic events.

Linde and Sacks first analyzed the U.S. Geological Survey National Earthquake Information Center compendium “just looking for the really big guys” (earthquakes of Richter magnitude 7.0 and above). For each large earthquake, Linde and Sacks then searched the Smithsonian Glob-al Volcanism Program catalog for volcanic events within various distance and time separations. In order to exclude small-scale eruptions, Linde and Sacks only recorded volcanic eruptions that possessed a volcanic explosivity index greater than or equal to 2 (out of a possible 8). Upon plotting their results, Linde and Sacks discovered a significant correlation: 20 volcanic eruptions occurred within two days of nearby earthquakes. While most of the triggered eruptions were within a relatively close range, several were triggered from distances as great as 750 kilometers. In addition, the researchers found a mechanism for eruption pairs — volcanoes that are separated by a substantial distance, yet erupt in close succession. Linde and Sacks believe that the seismic energy produced by an erupting volcano may activate nearby volcanoes through triggered seismicity, even at distances of 200 kilometers.

Prompted by the Landers quake, several scientists have proposed theories to explain why seismic activity may trigger a volcanic eruption. For example, scientists have proposed that triggered seismicity could result from a static (permanent) increase in compressive strain at a volcano. However, Linde believes that this is an unlikely explanation for volcanoes greater than 200 to 300 kilometers away, where static strain field changes are “far too small to be a likely candidate.” Linde and Sacks propose a mechanism that focuses specifically on seis-mic waves. While earthquake-induced changes in static strain diminish relatively quickly over great distances, seismic waves maintain significant strengths for hundreds of kilometers. Linde and Sacks believe that the arrival of seismic energy at a caldera may disturb incorporated gases, causing them to rise in the magma chamber. If the chamber is a relatively sealed system, the rising gases will increase pressure to an unstable level, triggering a volcanic eruption.

Linde and Sacks are collecting new seismic data from modern volcanoes in an attempt to gain a better understanding of triggered seismicity and other eruption phenomena. In collaboration with fellow physicists and geologists, they are planting instruments on Vesuvius, Long Valley, Mauna Loa, and other volcanoes. Using such techniques as deep borehole tilt with strainmeters, the researchers are searching for a clearer understanding of the mechanics of the magma chamber. Although expensive, the experiments may lead scientists to a better understanding of the overall physics of eruptions.
 — Joshua A. Chamot
Department of Geology, University of Tennessee, Knoxville



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