New Data Shows El Mayor—Cucapah Earthquake Was Simple on Surface, Complicated at Depth

PASADENA, Calif.— Like scars that remain on the skin long after a wound has healed, earthquake fault lines can be traced on Earth's surface long after their initial rupture. Typically, this line of intersection between the area where the fault slips and the ground is more complicated at the surface than at depth. But a new study of the April 4, 2010, El Mayor–Cucapah earthquake in Mexico reveals a reversal of this trend. While the fault involved in the event appeared to be superficially straight, the fault zone is warped and complicated at depth.

The study—led by researchers at the California Institute of Technology (Caltech) and documenting findings from the magnitude 7.2 event, which was centered in the Baja California state of Mexico—is available online in the journal Nature Geoscience.

The El Mayor–Cucapah earthquake happened along a system of faults that run from Southern California into Mexico, cutting through the Cucapah mountain range and across the Colorado River delta. This system of faults forms a portion of the plate boundary between the Pacific Plate and the North American Plate. Two main segments of the fault tilt downward steeply from the surface at opposing angles: the northwestern half angles downward beneath the Mexicali Valley, whereas the southeastern half angles away from the valley.

In a standard model, transform plate boundary structures—where two plates slide past one another—tend to be vertically oriented, which allows for lateral side-by-side shear fault motion. In the case of this quake, however, lead author Shengji Wei, a postdoctoral scholar in geophysics, and colleagues showed that the 120-kilometer-long rupture involved angled, non-vertical faults and that the event was initiated on a connecting extension fault between the two segments.

"Although the surface trace is nearly linear, we found that the event, which started with a smaller quake, happened mainly on two faults with opposite dipping directions," says Wei.

In fact, the seismic rupture traveled through a relatively complicated set of preexisting faults that are dipping in various directions. "It was really surprising to see a straight fault trace that cuts through the Colorado delta and the rugged topography of the Sierra Cucapah as a result of this event," says Jean-Philippe Avouac, director of Caltech's Tectonics Observatory and principal investigator on the study.   

The team used interferometric synthetic aperture radar (InSAR) and optical images gathered from satellites, global positioning system (GPS) data, and seismological data to study the rupture process. By combining the GPS data and remote sensing techniques—which provide measurements of surface displacement—and seismological techniques to study the ground vibrations generated by the temblor, the researchers were able to produce an extremely well-resolved model of the earthquake.

The model describes the geometry of the faults that broke during the quake and the time evolution of the rupture. It shows that once the earthquake began with an extensional deep break that pulled the two segments apart, it spread bilaterally to the northwest and the southeast. As the rupture spread northwestward, it continued to break erratically through the faults below the Cucapah mountain range. Simultaneously, the rupture spread towards the southeast, breaking a fault that had been covered over by a blanket of sediments that forms the Colorado River delta.

"High-resolution satellite radar images allowed us to locate a previously unmapped fault—the Indiviso Fault—beneath the Colorado River Delta that had been buried by river sediments since its last earthquake," says NASA's Jet Propulsion Laboratory (JPL) geophysicist Eric Fielding, who was a coauthor of the study. "This fault moved up to 16 feet, or 5 meters, in the April 4, 2010, earthquake."

Wei says that since the new analysis indicates the responsible fault is more segmented deep down than its straight surface trace suggests, the evolution and extent of this earthquake's rupture could not have been accurately anticipated from the surface geology alone. Anticipating the characteristics of an earthquake that would likely happen on a young fault system (like the event in the study) is a challenge, since the geologic structures involved in the new fault system are not clear enough.

According to Avouac, the data can also be used to illustrate the process by which the plate boundary—which separates the Pacific Plate from North America— evolves and starts connecting the Gulf of California to the Elsinore fault in Southern California.

"We may have to wait for a couple of million years to clearly see the active fault zone in the topography, as we can now see further north in Central California, for example," Avouac says. "Earthquakes with magnitude 7.5 and lower are probably typical of this kind of younger fault zone, while fault zones with a longer geological history and simpler fault geometries are more prone to produce larger ruptures."

This is important information, since damage estimates from the earthquake, which mostly affected agribusinesses, topped $440 million in the Mexicali Valley of Baja California and $90 million in the Imperial Valley of California.

The paper, "Superficial simplicity of the 2010 El Mayor–Cucapah earthquake of Baja California in Mexico," appeared as an advanced online publication on July 31 in the journal Nature Geoscience. Sebastien Leprince, Anthony Sladen, Don Helmberger, Egill Hauksson, Risheng Chu, and Mark Simons, all from the Division of Geological and Planetary Sciences at Caltech; Kenneth Hudnut, geophysicist at the United States Geological Survey (USGS) in Pasadena; Thomas Herring, professor of geophysics at MIT; and Richard Briggs, research geologist at USGS in Golden, Colorado, also contributed to the study, which was funded by the National Science Foundation, USGS, the Gordon and Betty Moore Foundation, NASA and the Southern California Earthquake Center.

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Stolper Elected to Great Britain's Royal Society

PASADENA, Calif.—Edward M. Stolper, provost of the California Institute of Technology (Caltech) and William E. Leonard Professor of Geology, has been named a Foreign Member of Great Britain's Royal Society. He is one of eight scientists elected in 2011. Stolper's election brings to six the number of foreign members of the Royal Society currently on the Caltech faculty.

Membership in the Royal Society is bestowed each year on a small number of the world's scientists. The oldest scientific academy in existence, the Royal Society was established in 1660 under the patronage of King Charles II for the purpose of "improving natural knowledge," and helped usher in the age of modern science. Today, the Society seeks to promote science leaders who champion innovation for the benefit of humanity and the planet.

The Society cited Stolper for his "experimental and theoretical work on melting and igneous processes on the Earth, Mars, and asteroids." The citation noted Stolper's development of the so-called sandwich method for determining the phase equilibria that control melting in the mantles of Earth and other planets and his development of the first quantitative model of water speciation in glasses and silicate melts, which showed that H2O dissolves in magmas as both hydroxyl groups and as molecular water. The Society's announcement also recognized Stolper as the first to propose that a small but distinctive group of igneous meteorites (the "SNC" group, which comprises the shergottite, nakhlite, and chassignite meteorites) come from the planet Mars; the first to show that certain dense silicate minerals can float relative to coexisting silicate liquids at high pressures due to the very high compressibilities of magmas, a finding with implications for the differentiation of large silicate planets; and the first to demonstrate a linear relationship between the extent of melting in Earth's mantle and water content through studies of magmas that have erupted in the Mariana trough and in other subduction zone environments. 

In addition, Stolper was recently elected a foreign member of the Academia Europaea ("The Academy of Europe"), a pan-European academy of humanities, letters, and sciences founded in 1988 to promote learning, education, and research. Members are drawn from the physical sciences and technology, biological sciences and medicine, mathematics, the letters and humanities, social and cognitive sciences, economics, and the law.

A member of Caltech's faculty since 1979, Stolper was named the William E. Leonhard Professor of Geology in 1990. He served as chair of the Division of Geological and Planetary Sciences from 1994 to 2004. He was interim provost in 2004, and in 2007 he was named provost, the chief academic officer of the Institute.

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Genesis samples reveal new clues about sun's chemical makeup

Ever since a crash landing on Earth grounded NASA's Genesis mission in 2004, scientists have been gathering, cleaning, and analyzing solar wind particles collected by the spacecraft. Now, two new studies published in Science reveal that Earth's chemistry is less like the sun's than previously thought.

Because the sun, moon, planets, and meteorites in our solar system started from the same cloud of dust and gases, a long-held assumption has been that these objects share the same chemistry. However, data obtained from samples of material ejected from the outer portion of the sun, which Genesis collected over a two-year time period, show differences in isotopic content of both oxygen and nitrogen when compared to the Earth's atmosphere. Isotopes are variants of a particular element that differ and are identified by their number of neutrons.

One study found that the percentage of oxygen-16—the most prevalent kind of oxygen isotope in the solar system—was slightly higher in solar wind samples than it is in air on Earth and the other terrestrial planets. The second study examined nitrogen isotopes and found that although both the sun and Jupiter appear to have slightly more nitrogen-14 than Earth, they have 40 percent less N-15. These variations offer insight into how our solar system evolved.

"The sun houses more than 99 percent of the material currently in our solar system, so it's a good idea to get to know it better," said Don Burnett, professor of nuclear geochemistry, emeritus, at Caltech, and Genesis Principal Investigator. "While it was more challenging than expected, we have answered some important questions, and like all successful missions, generated plenty more."

Burnett says that the Genesis team will continue to mine the salvaged spacecraft for usable samples. To learn more about Genesis and keep up-to-date on new research findings from the mission, visit www.nasa.gov/mission_pages/genesis/main/index.html.

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Caltech-led Researchers Measure Body Temperatures of Dinosaurs for the First Time

Some Dinosaurs Were as Warm as Most Modern Mammals

PASADENA, Calif.—Were dinosaurs slow and lumbering, or quick and agile? It depends largely on whether they were cold or warm blooded. When dinosaurs were first discovered in the mid-19th century, paleontologists thought they were plodding beasts that had to rely on their environments to keep warm, like modern-day reptiles. But research during the last few decades suggests that they were faster creatures, nimble like the velociraptors or T. rex depicted in the movie Jurassic Park, requiring warmer, regulated body temperatures like in mammals.

Now, a team of researchers led by the California Institute of Technology (Caltech) has developed a new approach to take body temperatures of dinosaurs for the first time, providing new insights into whether dinosaurs were cold or warm blooded. By analyzing isotopic concentrations in teeth of sauropods, the long-tailed, long-necked dinosaurs that were the biggest land animals to have ever lived—think Apatosaurus (also known as Brontosaurus)—the team found that the dinosaurs were about as warm as most modern mammals.

"This is like being able to stick a thermometer in an animal that has been extinct for 150 million years," says Robert Eagle, a postdoctoral scholar at Caltech and lead author on the paper to be published online in the June 23 issue of Science Express. (Click here for video and additional images.)

"The consensus was that no one would ever measure dinosaur body temperatures, that it's impossible to do," says John Eiler, a coauthor and the Robert P. Sharp Professor of Geology and professor of geochemistry. And yet, using a technique pioneered in Eiler's lab, the team did just that.

The researchers analyzed 11 teeth, dug up in Tanzania, Wyoming, and Oklahoma, that belonged to Brachiosaurus brancai and Camarasaurus. They found that the Brachiosaurus had a temperature of about 38.2 degrees Celsius (100.8 degrees Fahrenheit) and the Camarasaurus had one of about 35.7 degrees Celsius (96.3 degrees Fahrenheit), warmer than modern and extinct crocodiles and alligators but cooler than birds. The measurements are accurate to within one or two degrees, Celsius.

"Nobody has used this approach to look at dinosaur body temperatures before, so our study provides a completely different angle on the longstanding debate about dinosaur physiology," Eagle says.

The fact that the temperatures were similar to those of most modern mammals might seem to imply that dinosaurs had a warm-blooded metabolism. But, the researchers say, the issue is more complex. Because large sauropod dinosaurs were so huge, they could retain their body heat much more efficiently than smaller mammals like humans. "If you're an animal that you can approximate as a sphere of meat the size of a room, you can’t be cold unless you’re dead," Eiler explains. So even if dinosaurs were "cold blooded" in the sense that they depended on their environments for heat, they would still have warm body temperatures.

A Jurassic sauropod.
Credit: Illustrated by Russell Hawley, Tate Geological Museum

"The body temperatures we've estimated now provide a key piece of data that any model of dinosaur physiology has to be able to explain," says Aradhna Tripati, a coauthor who's an assistant professor at UCLA and visiting researcher in geochemistry at Caltech. "As a result, the data can help scientists test physiological models to explain how these organisms lived."

The measured temperatures are lower than what's predicted by some models of body temperatures, suggesting there is something missing in scientists' understanding of dinosaur physiology. These models imply dinosaurs were so-called gigantotherms, that they maintained warm temperatures by their sheer size. To explain the lower temperatures, the researchers suggest that the dinosaurs could have had some physiological or behavioral adaptations that allowed them to avoid getting too hot. The dinosaurs could have had lower metabolic rates to reduce the amount of internal heat, particularly as large adults. They could also have had something like an air-sac system to dissipate heat. Alternatively, they could have dispelled heat through their long necks and tails.

Previously, researchers have only been able to use indirect ways to gauge dinosaur metabolism or body temperatures. For example, they infer dinosaur behavior and physiology by figuring out how fast they ran based on the spacing of dinosaur tracks, studying the ratio of predators to prey in the fossil record, or measuring the growth rates of bone. But these various lines of evidence were often in conflict. "For any position you take, you can easily find counterexamples," Eiler says. “How an organism budgets the energy supply that it gets from food and creates and stores the energy in its muscles—there are no fossil remains for that," he says. "So you just sort of have to make your best guess based on indirect arguments.”

But Eagle, Eiler, and their colleagues have developed a so-called clumped-isotope technique that shows that it is possible to take body temperatures of dinosaurs—and there's no guessing involved. “We’re getting at body temperature through a line of reasoning that I think is relatively bullet proof, provided you can find well-preserved samples," Eiler says. In this method, the researchers measure the concentrations of the rare isotopes carbon-13 and oxygen-18 in bioapatite, a mineral found in teeth and bone. How often these isotopes bond with each other—or "clump"—depends on temperature. The lower the temperature, the more carbon-13 and oxygen-18 tend to bond in bioapatite. So measuring the clumping of these isotopes is a direct way to determine the temperature of the environment in which the mineral formed—in this case, inside the dinosaur.

Camarasaurus tooth from the Jurassic Morrison Formation of North America that was analyzed in the study by Eagle et al.
Credit: Thomas Tutken (Bonn University)

"What we’re doing is special in that it’s thermodynamically based," Eiler explains. "Thermodynamics, like the laws of gravity, is independent of setting, time, and context." Because thermodynamics worked the same way 150 million years ago as it does today, measuring isotope clumping is a robust technique.

Identifying the most well-preserved samples of dinosaur teeth was one of the major challenges of the analysis, the researchers say, and they used several ways to find the best samples. For example, they compared the isotopic compositions of resistant parts of teeth—the enamel—with easily altered materials—dentin and fossil bones of related animals. Well-preserved enamel would preserve both physiologically possible temperatures and be isotopically distinct from dentin and bone.

The next step is to take temperatures of more dinosaur samples and extend the study to other species of extinct vertebrates, the researchers say. In particular, taking the temperature of unusually small and young dinosaurs would help test whether dinosaurs were indeed gigantotherms. Knowing the body temperatures of more dinosaurs and other extinct animals would also allow scientists to learn more about how the physiology of modern mammals and birds evolved.

The Science paper is titled, "Dinosaur body temperatures determined from isotopic (13C-18O) ordering in fossil biominerals." In addition to Eagle, Eiler, and Tripati, the other authors are Thomas Tütken from the University of Bonn, Germany; Caltech undergraduate Taylor Martin; Henry Fricke from Colorado College; Melissa Connely from the Tate Geological Museum in Casper, Wyoming; and Richard Cifelli from the University of Oklahoma. Eagle also has a research affiliation with UCLA.

This research was supported by the National Science Foundation and the German Research Foundation.

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Caltech-led Team Debunks Theory on End of "Snowball Earth" Ice Age

Finds that rocks used as key geologic evidence were formed deep within Earth millions of years after the ice age ended

PASADENA, Calif.—There's a theory about how the Marinoan ice age—also known as the "Snowball Earth" ice age because of its extreme low temperatures—came to an abrupt end some 600 million years ago. It has to do with large amounts of methane, a strong greenhouse gas, bubbling up through ocean sediments and from beneath the permafrost and heating the atmosphere.

The main physical evidence behind this theory has been samples of cap dolostone from south China, which were known to have a lot less of the carbon-13 isotope than is normally found in these types of carbonate rocks. (Dolostone is a type of sedimentary rock composed of the carbonate mineral, dolomite; it's called cap dolostone when it overlies a glacial deposit.) The idea was that these rocks formed when Earth-warming methane bubbled up from below and was oxidized—"eaten"—by microbes, with its carbon wastes being incorporated into the dolostone, thereby leaving a signal of what had happened to end the ice age. The idea made sense, because methane also tends to be low in carbon-13; if carbon-13-depeleted methane had been made into rock, that rock would indeed also be low in carbon-13. But the idea was controversial, too, since there had been no previous isotopic evidence in carbonate rock of methane-munching microbes that early in Earth's history.

And, as a team of scientists led by researchers from the California Institute of Technology (Caltech) report in this week's issue of the journal Nature, it was also wrong—at least as far as the geologic evidence they looked at goes.

This is a scanned image of a cut and polished slab of the cap dolostone from South China that contains highly carbon-13-depleted carbonate. The view shown is 3 inches wide.
Credit: Thomas Bristow

Their testing shows that the rocks on which much of that ice-age-ending theory was based were formed millions of years after the ice age ended, and were formed at temperatures so high there could have been no living creatures associated with them.

"Our findings show that what happened in these rocks happened at very high temperatures, and abiologically," says John Eiler, the Robert P. Sharp Professor of Geology and professor of geochemistry at Caltech, and one of the paper's authors. "There is no evidence here that microbes ate methane as food. The story you see in this rock is not a story about ice ages."

To tell the rocks' story, the team used a technique Eiler developed at Caltech that looks at the way in which rare isotopes (like the carbon-13 in the dolostone) group, or "clump," together in crystalline structures like bone or rock. This clumping, it turns out, is highly dependent upon the temperature of the immediate environment in which the crystals form. Hot temperatures mean less clumping; low temperatures mean more.

View from one of the cap dolostone collection sites in south China, looking along the cliffs of the Yangtze Gorges.
Credit: Thomas Bristow

"The rocks that we analyzed for this study have been worked on before," says Thomas Bristow, the paper's first author and a former postdoc at Caltech who is now at NASA Ames Research Center, "but the unique advance available and developed at Caltech is the technique of using carbonate clumped-isotopic thermometry to study the temperature of crystallization of the samples. It was primarily this technique that brought new insights regarding the geological history of the rocks."

What the team's thermometer made very clear, says Eiler, is that "the carbon source was not oxidized and turned into carbonate at Earth's surface. This was happening in a very hot hydrothermal environment, underground."

In addition, he says, "We know it happened at least millions of years after the ice age ended, and probably tens of millions. Which means that whatever the source of carbon was, it wasn't related to the end of the ice age."

Since this rock had been the only carbon-isotopic evidence of a Precambrian methane seep, these findings bring up a number of questions—questions not just about how the Marinoan ice age ended, but about Earth's budget of methane and the biogeochemistry of the ocean.

"The next stage of the research is to delve deeper into the question of why carbon-13-depleted carbonate rocks that formed at methane seeps seem to only be found during the later 400 million years of Earth history," says John Grotzinger, the Fletcher Jones Professor of Geology at Caltech and the principal investigator on the work described.

Unusual textures exposed in the cap dolostone from the field. Hand lens for scale is about 1 inch long.
Credit: Thomas Bristow

"It is an interesting fact of the geologic record that, despite a well-preserved record of carbonates beginning 3.5 billion years ago, the first 3 billion years of Earth history does not record evidence of methane oxidation. This is a curious absence. We think it might be linked to changes in ocean chemistry through time, but more work needs to be done to explore that."

In addition to Bristow, Eiler, and Grotzinger, the other authors on the Nature paper, "A hydrothermal origin for isotopically anomalous cap dolostone cements from south China," are Magali Bonifacie, a former Caltech postdoc now at the Institut de Physique du Globe de Paris, and Arkadiusz Derkowski from the Polish Academy of Sciences in Krakow.

The work was supported by an O. K. Earl Postdoctoral Fellowship, by the National Science Foundation's Division of Earth Sciences and its Geobiology and Environmental Geochemistry program, and by CNRS-INSU (French research agency).

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Lori Oliwenstein
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Caltech Researchers Release First Large Observational Study of 9.0 Tohoku-Oki Earthquake

Data yields surprising findings about energy distribution over the fault slip and stress accumulation in the Japan Trench

PASADENA, Calif.—When the magnitude 9.0 Tohoku-Oki earthquake and resulting tsunami struck off the northeast coast of Japan on March 11, they caused widespread destruction and death. Using observations from a dense regional geodetic network (allowing measurements of earth movement to be gathered from GPS satellite data), globally distributed broadband seismographic networks, and open-ocean tsunami data, researchers have begun to construct numerous models that describe how the earth moved that day.

Now, a study led by researchers at the California Institute of Technology (Caltech), published online in the May 19 issue of Science Express, explains the first large set of observational data from this rare megathrust event.

"This event is the best recorded great earthquake ever," says Mark Simons, professor of geophysics at Caltech’s Seismological Laboratory and lead author of the study. For scientists working to improve infrastructure and prevent loss of life through better application of seismological data, observations from the event will help inform future research priorities.

Simons says one of the most interesting findings of the data analysis was the spatial compactness of the event. The megathrust earthquake occurred at a subduction zone where the Pacific Plate dips below Japan. The length of fault that experienced significant slip during the Tohoku-Oki earthquake was about 250 kilometers, about half of what would be conventionally expected for an event of this magnitude. 

Furthermore, the area where the fault slipped the most—30 meters or more—happened within a 50- to 100-kilometer-long segment. "This is not something we have documented before," says Simons. "I'm sure it has happened in the past, but technology has advanced only in the past 10 to 15 years to the point where we can measure these slips much more accurately through GPS and other data."

For Jean Paul Ampuero, assistant professor of seismology at Caltech’s Seismological Laboratory who studies earthquake dynamics, the most significant finding was that high- and low-frequency seismic waves can come from different areas of a fault. "The high-frequency seismic waves in the Tohoku earthquake were generated much closer to the coast, away from the area of the slip where we saw low-frequency waves," he says.

Simons says there are two factors controlling this behavior; one is because the largest amount of stress (which is what generates the highest-frequency waves) was found at the edges of the slip, not near the center of where the fault began to break. He compares the finding to what happens when you rip a piece of paper in half. "The highest amounts of stress aren’t found where the paper has just ripped, but rather right where the paper has not yet been torn," he explains. "We had previously thought high-frequency energy was an indicator of fault slippage, but it didn’t correlate in our models of this event." Equally important is how the fault reacts to these stress concentrations; it appears that only the deeper segments of the fault respond to these stresses by producing high-frequency energy. 

Ampuero says the implications of these observations of the mechanical properties of tectonic faults need to be further explored and integrated in physical models of earthquakes, which will help scientists better quantify earthquake hazards.

"We learn from each significant earthquake, especially if the earthquake is large and recorded by many sensors," says Ampuero. "The Tohoku earthquake was recorded by upwards of 10 times more sensors at near-fault distances than any other earthquake. This will provide a sharper and more robust view of earthquake rupture processes and their effects."

For seismologist Hiroo Kanamori, Caltech’s Smits Professor of Geophysics, Emeritus, who was in Japan at the time of the earthquake and has been studying the region for many years, the most significant finding was that a large slip occurred near the Japan Trench. While smaller earthquakes have happened in the area, it was believed that the relatively soft material of the seafloor would not support a large amount of stress. "The amount of strain associated with this large displacement is nearly five to 10 times larger than we normally see in large megathrust earthquakes," he notes. "It has been generally thought that rocks near the Japan Trench could not accommodate such a large elastic strain."

The researchers are still unsure why such a large strain was able to accumulate in this area. One possibility is that either the subducting seafloor or the upper plate (or both) have some unusual structures—such as regions that were formerly underwater mountain ranges on the Pacific Plate—that have now been consumed by the subduction zone and cause the plates to get stuck and build up stress.

"Because of this local strengthening—whatever its cause—the Pacific Plate and the Okhotsk Plate had been pinned together for a long time, probably 500 to 1000 years, and finally failed in this magnitude 9.0 event," says Kanamori. "Hopefully, detailed geophysical studies of seafloor structures will eventually clarify the mechanism of local strengthening in this area."

Simons says researchers knew very little about the area where the earthquake occurred because of limited historical data.

"Instead of saying a large earthquake probably wouldn’t happen there, we should have said that we didn't know," he says. Similarly, he says the area just south of where the fault slipped is in a similar position; researchers don't yet know what it might do in the future.

"It is important to note that we are not predicting an earthquake here," emphasizes Simons. "However, we do not have data on the area, and therefore should focus attention there, given its proximity to Tokyo."

He says that the relatively new Japanese seafloor observation systems will prove very useful in scientists' attempts to learn more about the area.

"Our study is only the first foray into what is an enormous quantity of available data," says Simons. "There will be a lot more information coming out of this event, all of which will help us learn more in order to help inform infrastructure and safety procedures."

Other coauthors of the paper, "The 2011 Magnitude 9.0 Tohoku-Oki Earthquake: Mosaicking the Megathrust from Seconds to Centuries," are (from Caltech's Seismological Laboratory) Sarah E. Minson, staff seismologist; Anthony Sladen, visitor in geophysics; Francisco Ortega Culaciati, graduate student in geophysics; Junle Jiang, graduate student in geophysics; Lingsen Meng, graduate student in geophysics; Shengji Wei, postdoctoral scholar in geophysics; Risheng Chu, staff seismologist; and Donald V. Helmberger, Smits Family Professor of Geological and Planetary Sciences. In addition, Susan E. Owen, senior research scientist at the Jet Propulsion Laboratory (JPL); Eric Hetland, assistant professor of geological sciences at the University of Michigan; Angelyn W. Moore, scientist at JPL; and Frank H. Webb, principal scientist at JPL's Southern California Integrated GPS Network contributed to the study.

The work was funded by the Gordon and Betty Moore Foundation, National Science Foundation grants, the Southern California Earthquake Center, and NASA's internal Research and Technology Development program.

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Katie Neith
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Science in Progress: The Curious Case of the Shuram Excursion

About 540 million years ago, animal life on Earth suddenly boomed during an event known as the Cambrian explosion. In just tens of millions of years—a mere geological moment—life evolved rapidly, with all of the major groups of animals alive today making their first appearance. While researchers still aren't sure what could have triggered this burst of biodiversity, many suspect it had to do with a sudden rise in oxygen levels in the atmosphere, which would have allowed complex, multicellular organisms—and eventually higher organisms like ourselves—to flourish. Where this flood of oxygen came from is also a mystery, but whatever caused it, scientists know that it must have been a big event.

As one of the most important riddles in geology, this question has long interested Caltech geologists. In particular, professors John Grotzinger and Woody Fischer have been puzzling over a surprising discovery made in the early 1990s—a discovery that provided some clues as to what might have happened all those millions of years ago.

Digging in the oil-rich desert of Oman, geologists had looked at carbon isotopes in rocks. Measuring isotope ratios is a basic part of a geobiologist’s tool kit, allowing these scientists to piece together Earth's environmental history. The researchers found that sedimentary rocks from a time just before the Cambrian explosion—during the so-called Ediacaran period—were curiously short on a carbon isotope called carbon-13. In fact, the ratios of carbon-13 to carbon-12 that these researchers discovered were the lowest that have ever been seen.

The cause of this drop in carbon-13—named the Shuram excursion, after the rocks in which it was found—is a mystery that's still unsolved today. Now, Grotzinger, Fischer, and David Fike of Washington University in St. Louis have offered a new hypothesis in a review paper published in Nature on April 17. They still need to hunt for more data, but their idea is the latest attempt to reconcile seemingly contradictory evidence that has sparked a significant debate in the field.

When geologists first found this depletion in carbon-13, they thought it was just due to the usual set of processes that turn sediment into rock over time—rainwater can react with the rock, for instance, or high temperatures and pressures can induce chemical changes. Called diagenesis, this smorgasbord of reactions alters isotopic ratios. Diagenesis is typically a local process, depending on the environmental and geological conditions of a particular area—in this case, Oman.

In the last decade or so, however, geologists have found similar carbon-13 depletions widely distributed across other parts of the world, including southern China, southern Australia, and just a couple of hours away from Caltech, in Death Valley. In samples from each of these locations, geologists found a huge dip in the carbon isotope ratios at around the same depth—and therefore from around the same time in history. "We've never seen curves like this," Fischer says. "This is rare and unique in the rock record."

The fact that the Shuram excursion appears around the world seems to indicate that it's the chemical fingerprint of some global event that could be related to the rise of oxygen.

One of the most recent analyses came in February, when Caltech graduate student Charlie Verdel (now at the University of Michigan), geology professor Brian Wernicke, and Samuel Bowring at MIT published new data on the sediments in Death Valley. According to Grotzinger, this analysis provides some of the best evidence that the Shuram excursion was indeed a worldwide phenomenon. In another paper that's in press, graduate student Kristin Bergmann, along with Rebecca Zentmyer of Cerritos College and Fischer, have just published more observations of the Death Valley sediments that support this argument.

And yet, the data also support diagenesis. First, the depletion in carbon-13 only exists in inorganic carbon—carbonate minerals—and not in fossilized organic carbon deposited in the same sediment. If the Shuram excursion were caused by some global event, then it should also be recorded in the organic matter deposited in the same rocks.

The other piece of evidence that points to diagenesis is the oxygen-isotope ratios. In the rock samples that showed the Shuram excursion, researchers also found a relative lack of oxygen-18, which could be a sign that the sediment reacted with different kinds of fluids. The ratio of oxygen-18 to oxygen-16 dropped in much the same way as the ratio of carbon-13 to carbon-12 did, suggesting that whatever caused the carbon dip also caused an oxygen dip—a typical sign of diagenesis.

But at the same time, this data could be explained without invoking diagenesis. In 2006, Grotzinger and Fike, who was his graduate student at the time, along with Lisa Pratt at Indiana University and Roger Summons at MIT, published a paper in Nature suggesting that the data could be consistent with a global event related to the worldwide appearance of oxygen.

Field work along the coastline of Oman. Left to right: Lauren Edgar, John Grotzinger, Maggie Osburn, Woody Fischer.

In this scenario, Earth's ocean during the Ediacaran was chock full of organic carbon, which lacks carbon-13. A sudden increase in oxygen would have oxidized this oceanic carbon, producing carbon dioxide that then formed carbonate rock—inorganic carbon that's deficient in carbon-13, which is exactly what characterizes the Shuram excursion.

Another characteristic of the Shuram data is that the organic carbon found in the sediment did not show a similar decrease in carbon-13—but the researchers could explain this, too. There was so much organic carbon in the oceans that its presence would have masked any dip that might be seen in the sediment's organic carbon.

Despite this evidence, some researchers still argue for a purely diagenetic origin of the Shuram excursion, according to Grotzinger. But no matter which side they're on, all scientists in the field are wrestling with this perplexing data.

"We were sort of sitting around the room, thinking of how we could explain this," Grotzinger recalls. He and his colleagues wondered whether it might be possible for the Shuram excursion to have been caused by diagenesis, but—in a departure from the way people have always thought about it—via a unique global version of the process.

The researchers propose that a rise in atmospheric oxygen would have altered the planet's chemistry, helping sediment get better at oxidizing organic carbon by enriching that sediment with such compounds as iron oxide and sulfate-bearing minerals. The sediment could then oxidize any organic carbon that flowed through it as a hydrocarbon-rich fluid, like oil. Oxidizing organic carbon would produce carbon dioxide that would then form carbonate. Again, since the organic carbon in water or oil has low amounts of carbon-13, this would result in similarly low levels in carbonate (inorganic carbon), the prime feature of the Shuram data.

So far, the evidence has seemed to point to both diagenesis and a global event, and previous ideas favored one or the other. "What we're trying to do is open the idea that maybe it's a combination of both," Grotzinger says. Fischer adds, "I'm sure it'll be controversial." The researchers are now starting to gather more data to see if their new explanation is correct. Bergmann and another graduate student, Maggie Osburn, are now collecting more samples in Oman.

How all this links to the Cambrian explosion remains unclear. All researchers know for sure is that the Shuram excursion represents an unprecedented signal in Earth's geological record, and it happened right before the sudden surge in biodiversity. The evidence is circumstantial, but it’s certainly intriguing. "It requires a remarkable mechanism regardless of how you cut it—it's not a business-as-usual scenario," Fischer notes. "I'm totally open to how this shakes out. I can see it both ways now."

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Marcus Woo
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Caltech Researchers Use GPS Data to Model Effects of Tidal Loads on Earth's Surface

PASADENA, Calif.—For many people, Global Positioning System (GPS) satellite technology is little more than a high-tech version of a traditional paper map. Used in automobile navigation systems and smart phones, GPS helps folks find their way around a new neighborhood or locate a nearby restaurant. But GPS is doing much, much more for researchers at the California Institute of Technology (Caltech): it's helping them find their way to a more complete understanding of Earth's interior structure.

Up until now, the best way to explore Earth’s internal structures—to measure geological properties such as density and elasticity—has been through seismology and laboratory experiments. "At its most fundamental level, seismology is sensitive to specific combinations of these properties, which control the speed of seismic waves," says Mark Simons, professor of geophysics at Caltech's Seismological Laboratory, part of the Division of Geological and Planetary Sciences. "However, it is difficult using seismology alone to separate the effects that variations in density have from those associated with variations in elastic properties." 

Now Simons and Takeo Ito, visiting associate at the Seismological Laboratory and assistant professor of earth and planetary dynamics at Nagoya University in Japan, are using data from GPS satellite systems in an entirely new way: to measure the solid earth's response to the movements of ocean tides—which place a large stress on Earth's surface—and to estimate separately the effects of Earth’s density and the properties controlling response when a force is applied to it (known as elastic moduli).

Their work was published in this week’s issue of Science Express.

By using measurements of Earth’s movement taken from high-precision, continuously recording permanent GPS receivers installed across the western United States by the Plate Boundary Observatory (PBO), the researchers were able to observe tide-induced displacements—or movements of Earth's surface—of as little as one millimeter. PBO is a component of EarthScope, a program that seeks to understand the processes controlling earthquakes and volcanoes by exploring the structure and evolution of the North American continent.

The team focused on understanding the properties of the asthenosphere, a layer of weak and viscous upper mantle that lies below Earth's crust, and used those measurements to build one-dimensional models of Earth's response to the diurnal tides in the western United States.

"The asthenosphere plays an important role in plate tectonics, as it lies directly under the plates," explains Ito. "The results of our study give us a better understanding of the asthenosphere, which in turn can help us understand how the plates move."  

The models provided a look at the variations in density from Earth’s surface down to a depth of about 400 kilometers. The researchers found that the density of the asthenosphere under the western United States and the eastern Pacific Ocean is abnormally low relative to the global average.

"Variations in density can either be caused by variations in the chemical makeup of the material, the presence of melt, or due to the effects of thermal expansion, whereby a given material will decrease in density as its temperature increases," explains Simons. "In this study, we interpret the observed density anomaly to be due to the effects of elevated temperatures in the asthenosphere below the western United States and neighboring offshore areas. The required peak temperature anomaly would be about 300 degrees Celsius higher than the global average at those depths."

This type of data provides keys to understanding the chemical and mechanical dynamics of the planet, such as how heat flows through the mantle and how tectonic plates on Earth's surface are evolving.

"It is amazing that by measuring the twice-a-day centimeter-scale cyclic movement of Earth's surface with a GPS receiver, we can infer the variation of density 220 kilometers below the surface," says Simons.

Now that the researchers know it is possible to use GPS to derive measurements of internal Earth structures, they anticipate several new directions for this research.

"We hope to extend the observations to be global in scope, which may require temporary deployments of GPS in important areas that are typically tectonically bland—in other words, devoid of significant earthquakes and volcanoes—and thus do not have existing dense continuous GPS arrays already in place," says Simons. Next steps may also include going beyond the current one-dimensional depth-dependent models to build 3-D models, and combining the GPS approach with more conventional seismic approaches.

"The method we developed for gathering data from GPS devices has significant potential for improving 3-D images of Earth's internal structure," says Ito. 

The findings of the study, "Probing asthenospheric density, temperature and elastic moduli below the Western United States," are available online in Science Express and will be featured in a future issue of the journal Science.

This research was supported by the National Science Foundation; Grants-in-Aid for Scientific Research, part of Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT); JSPS Postdoctoral Fellowships for Research Abroad; and the Gordon and Betty Moore Foundation. 

Writer: 
Katie Neith
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New Caltech Research Suggests Strong Indian Crust Thrust Beneath the Tibetan Plateau

PASADENA, Calif.—For many years, most scientists studying Tibet have thought that a very hot and very weak lower and middle crust underlies its plateau, flowing like a fluid. Now, a team of researchers at the California Institute of Technology (Caltech) is questioning this long-held belief and proposing that an entirely different mechanism is at play.

"The idea that Tibet is more or less floating on a layer of partially molten crust is accepted in the research community. Our research proposes the opposite view: that there is actually a really strong lower crust that originates in India," says Jean-Philippe Avouac, professor of geology and director of Caltech’s Tectonics Observatory. 

These insights lead to a better understanding of the processes that have shaped the Himalaya Mountains and Tibet—the most tectonically active continental area in the world.

Alex Copley, a former postdoctoral scholar with Caltech’s Tectonics Observatory, along with Avouac and Brian Wernicke, the Chandler Family Professor of Geology, describe their work in a paper published in the April 7 issue of the journal Nature.

Tibet and the surrounding Himalaya Mountains are among the most dynamic regions on the planet. Avouac points out that underground plate collisions, which cause earthquakes and drive up the Himalaya and Tibet, are common geological processes that have happened repeatedly over the course of Earth's history, but are presently happening with a vigor and energy only found in that area.

Even though the elevation is uniform across the Tibetan Plateau, the type of stress seen within the plateau appears to change along a line that stretches east-west across the plateau—dividing the region into two distinct areas (southern and northern Tibet, for the purposes of this research.)

The researchers propose that a contrast in tectonic style—primarily east-west extension due to normal faulting in southern Tibet and a combination of north-south compression and east-west extension due to strike-slip faulting in northern Tibet—is the result of the Indian crust thrusting strongly underneath the southern portion of the Tibetan Plateau and locking into the upper crust. Strike-slip fault surfaces are usually vertical, and the rocks slide horizontally past each other due to pressure build-up, whereas normal faulting occurs where the crust is being pulled apart. They believe that the locked Indian crust alters the state of stress in the southern Tibetan crust, which can explain the contrast in the type of faulting seen between southern Tibet and northern Tibet.

To test their theory, the team performed a series of numerical experiments, assigning different material properties to the Indian crust. The simulations revealed evidence for a strong Indian lower crust that couples, or locks in, with the upper crust. This suggests that the "channel flow" model proposed by many geophysicists and geologists—in which a low-viscosity magma oozes through weak zones in the middle crust—­is not correct.

"We have been able to create a model that addresses two long-standing debates," says Copley, who is now a research fellow at the University of Cambridge. "We have constrained the mechanical strength of the Indian crust as it plunges beneath the Tibetan Plateau, and by doing so have explained the variations in the types of earthquakes within the plateau. This is interesting because it gives us new insights into what controls the behavior of large mountain ranges, and the earthquakes that occur within them."

According to Wernicke, the results have motivated the team to think of ways to test further the "weak crust" hypothesis, at least as it might apply to the active tectonic system. "One way we might be able to image an extensive interface at depth is through geodetic studies of southern Tibet, which are ongoing in our research group," he says.

The Gordon and Betty Moore Foundation funded the research, described in the Nature paper, "Evidence for mechanical coupling and strong Indian lower crust beneath southern Tibet." Pembroke College in the University of Cambridge provided additional funding for Copley.

Writer: 
Katie Neith
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Quake Expert Profiled in Los Angeles Magazine

Lucy Jones, a seismologist at the U.S. Geological Survey and visiting associate in geophysics at Caltech, gets personal in this month's Los Angeles magazine, recalling how she first became interested in earthquakes and what it's like to have two seismologists in the house—she's married to Egill Hauksson, senior research associate in Caltech's Seismological Laboratory. In the Q&A column titled "Shock Absorber," Jones also offers quake preparation advice to her fellow Californians: "However much water you've got, store some more. And get a couple of fire extinguishers and make sure you know how to use them." For the full story, click here

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Katie Neith
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