AGU journal highlights - 29 July 2005
I. Highlights, including authors and their institutions
II. Ordering information for science writers and general public
I. Highlights, including authors and their institutions
The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.
You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.
1. Reassessing the earthquake threat to San Francisco
New research suggests that authorities and scientists may need to reassess the threat posed by the Hayward fault, the crack in the Earth's crust responsible for San Francisco's highly destructive 1868 earthquake. The fault runs underneath the densely populated east side of San Francisco Bay and is considered the most likely future source of a major earthquake in the city. Williams et al. used a large shaking vehicle, known as a vibroseis truck, to send vibrations into the ground along a 1.6 kilometer [one mile] portion of the Hayward fault. By recording vibrations reflecting back from the fault, the authors developed a profile of its shape and location. Contrary to previous assumptions that the fault was vertical, they found that it angles about 70 degrees to the east at depths of 100-600 meters [300-2,000 feet]. At depths of 4-10 kilometers [2-6 miles], the fault is located 2 kilometers [one mile] east of its surface location, and may be responsible for previously unexplained seismic activity in that area, where, the authors say, the Hayward fault may directly connect deep underground with the Calaveras fault to the east. This connection may hold the potential for unexpected earthquakes.
Title: Seismic reflection evidence for a northeast-dipping Hayward fault near Fremont, California: Implications for seismic hazard
Robert A. Williams, William J. Stephenson, Jack K. Odum: U.S.
Geological Survey, Golden, Colorado, USA;
Robert W. Simpson, Robert C. Jachens, David A. Ponce: U.S.
Geological Survey, Menlo Park, California, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023113, 2005
2. Predict space weather by watching the Sun
Huge bubbles of electrified gas released from the Sun can drive plasma shockwaves that engulf the Earth's magnetosphere, the region in which Earth's magnetic field prevents most solar wind particles from hitting the planet. The shockwaves are driven by material released during solar explosions, known as coronal mass ejections (CMEs), and both reach Earth's magnetosphere within a few days of the solar event. The compression of the magnetosphere by the initial impact of the CME can induce currents in electrical power systems, sometimes leading to power blackouts. Fazakerley et al. compared spacecraft observations of the Sun during a CME on 20 January 2004 to measurements of the solar wind near Earth in the following days. They found that the source of the CME was a long-lasting solar flare, and showed that the magnetic structure of the CME inferred from study of the solar source region was compatible with the near-Earth observations. The study indicates that in at least some cases it may be possible to predict whether magnetic storms will occur after a CME strikes the Earth.
Title: Relating near-Earth observations of an interplanetary coronal mass ejection to the conditions at its site of origin in the solar corona
A. N. Fazakerley, L. K. Harra, J. L. Culhane, L. van Driel-Gesztelyi, S. A. Matthews, C. J. Owen: Mullard Space Science Laboratory, University College London, Dorking, UK;
E. Lucek, A. Balogh: Space and Atmospheric Physics Group, Imperial College of Science, Technology, and Medicine, London, UK;
C. Mazelle, H. Rème: Centre d'Etude Spatiale des Rayonnements, Toulouse, France.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022842, 2005
3. A rock-solid source for Martian methane
Researchers recently found the first definitive evidence of methane in the Martian atmosphere, but there is still much debate about its source. Methane is the first organic molecule found on Mars and has raised considerable interest, because of the possibility that it could have a biological origin. To date, however, no traces of life have been found. One source of methane on Earth is a chemical reaction that occurs at volcanic mid-ocean ridges when seawater comes into contact with rock in the planet's interior. Lyons et al. present a similar model to explain the production of Martian methane: interaction between a fluid containing carbon--most likely water--and rocks in the planet's basaltic crust. In this scenario, the heat from magma in Mars' interior melts subsurface ice; the resulting water acquires carbon dioxide released from the magma, which in the presence of basaltic crust is converted to methane as the fluid cools. From their calculations, the authors determined that a reaction of this sort between rock and fluids could have produced all of the methane found in the Martian atmosphere.
Title: Formation of methane on Mars by fluid-rock interaction in the crust
James R. Lyons: Institute of Geophysics and Planetary Physics, Center for Astrobiology, University of California, Los Angeles, California, USA;
Craig Manning: Department of Earth and Space Sciences, University of California, Los Angeles, California, USA;
Francis Nimmo: Department of Earth Sciences, University of California, Santa Cruz, California, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022161, 2005
4. Insight into the Earth's interior from the 2004 Sumatra quake
Measurements of vibrations throughout the Earth's interior after the massive 26 December 2004 Sumatra-Andaman earthquake may help researchers better measure the density of rock in Earth's mantle and core. The earthquake, which caused the devastating tsunami in the Indian Ocean, excited low-frequency vibrations in the planet's interior. Rosat et al. recorded the amplitude and frequency of the oscillations at 11 stations around the world for about 22 days after the earthquake, using superconducting gravimeters (SG), sensors that measured gravitational fluctuations resulting from the vibrations. The unusually strong vibrations created by the earthquake allowed them to obtain the clearest observations yet of certain low-frequency oscillations. The authors say that combining superconducting gravimeter data with other seismic data will help researchers better characterize the density of rock in Earth's mantle and core. These results, in conjunction with those gathered during a large earthquake in Peru in 2001, demonstrate that superconducting gravimeters are effective tools for studying low-frequency seismic vibrations, the authors say.
Title: High-resolution analysis of the gravest seismic normal modes after the 2004 Mw = 9 Sumatra earthquake using superconducting gravimeter data
S. Rosat, T. Sato, Y. Tamura: National Astronomical Observatory of Japan, Iwate, Japan;
Y. Imanishi: Ocean Research Institute, University of Tokyo, Tokyo, Japan;
J. Hinderer: Institut de Physique du Globe de Strasbourg (IPGS-EOST), Strasbourg, France;
H. McQueen: Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia;
M. Ohashi: Institute for Cosmic Ray Research, University of Tokyo, Tokyo, Japan.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023128, 2005
5. Phase change near Earth's core causes upwelling in the mantle
When subjected to high pressure and heat, many minerals transform into perovskites, dense, crystalline forms that resemble the mineral perovskite. The most abundant mineral in the lower portion of Earth's mantle, and possibly on the entire planet, is the perovskite form of the common mineral enstatite (MgSiO3). In the lowest part of the mantle closest to the molten core, the extreme pressure changes the structure of the mineral to what is known as a post-perovskite phase. Previous research has shown that the relationship between the pressure at which estantite transforms from one phase to another and the heat released during that reaction affects upwelling plumes of hot rock in the mantle and thus geological phenomena such as volcanic hot spots. Hirose and Fujita used a perovskite form of a similar but more stable mineral, CaIrO3, as a proxy to develop a curve to describe the temperature and heat at which enstatite transforms. They found that the transformation of enstatite probably releases a lot of heat, which promotes the upwelling of hot plumes of rock into the upper mantle.
Title: Clapeyron slope of the post-perovskite phase transition in CaIrO3
Kei Hirose, Yosuke Fujita: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023219, 2005
6. Volcanic rumblings from a South Atlantic seamount
Recordings of volcanic explosions from an underwater mountain range in the South Atlantic Ocean are helping researchers find the source of volcanic activity in the region. Haxel and Dziak used an array of underwater acoustic sensors moored in the North Atlantic Ocean off the coast of West Africa to record the sound waves of 365 volcanic explosions coming from the South Atlantic from November 2001 to March 2002. The explosions originated from an unnamed seamount that is part of the Walvis Ridge, a submarine mountain range that extends about 3400 kilometers [2,100 miles] southwest from the African continent into the South Atlantic. The authors say the Tristan da Cunha hotspot, located about 780 kilometers [480 miles] west of the seamount, is probably too far away to be the cause of the volcanic activity responsible for the explosion. Unless the hotspot is much larger than previously thought, a large fracture in the seafloor most likely plays a role in the volcanic activity near the seamount. This reopening of cracks long believed to have closed up is important for understanding long-term seafloor volcanism, the authors say.
Title: Evidence of explosive seafloor volcanic activity from the Walvis Ridge, South Atlantic Ocean
J. H. Haxel, R. P. Dziak: Cooperative Institute for Marine Resources Studies and NOAA Pacific Marine Environmental Laboratory, Oregon State University, Newport, Oregon, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023205, 2005
7. Space shuttle makes Antarctic clouds
Space shuttle exhaust can travel long distances on winds and form high-altitude clouds far from the launch site, suggests new research. The Columbia space shuttle blasted off 16 January 2003 from the Kennedy Space Center in Florida. Stevens et al. tracked the movement of exhaust released from the shuttle using satellite images and ground-based lidar, a radar-like system that uses lasers instead of radio waves. An exhaust plume, consisting of water vapor and trace amounts of iron, was emitted at an altitude of about 115 kilometers [70 miles] and then traveled on winds to Antarctica over the next few days. The authors also used satellite observations to determine the amount of ice-laden high-altitude clouds--known as polar mesospheric clouds--present over Antarctica in the days following the launch. During that period, these clouds increased dramatically as the exhaust plume reached Antarctica. The authors calculate that during the 2002–2003 austral summer, space shuttle exhaust accounted for 10-20 percent of the mass of these types of clouds over Antarctica.
[See also AGU press release 05-23 at http://www.agu.org/sci_soc/prrl/prrl0523.html] Title: Antarctic mesospheric clouds formed from space shuttle exhaust
Michael H. Stevens, E.O. Hulbert Center for space Research, Naval Research Laboratory, Washington, D.C., USA;
Robert R. Meier, School of Computational Sciences, George Mason University, Fairfax, Virginia, USA;
Xinzhao Chu, Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA;
Matthew T. DeLand, Science Systems and Applications Inc., Lanham, Maryland, USA;
John M. C. Plane, School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023054, 2005
8. Spacecraft-mounted device captures images of "elves"
During its first four months of operation, a new imaging device orbiting Earth on a Taiwanese spacecraft observed a large number of elves, high-altitude flashes of light often seen above large thunderstorms at altitudes around 94 kilometers [58 miles]. The flashes are caused by nitrogen molecules colliding with electrons that have been energized after an exceptionally strong bolt of lightning. The Imager for Sprites and Upper Atmospheric Lightning began operation in July 2004, and allows researchers to observe elves from space without the atmospheric interference that limits ground-based observations. Frey et al. report that about half of the elves observed were created by lightning that went through three distinct stages of brightness: bright, dimmer, and bright again. The authors say that these elves followed negatively charged lightning bolts, while sprites (another kind of flash that occur at altitudes of 30-90 kilometers [40-60 miles]) follow positively charged bolts. They plan to investigate whether the other half of the elves they observed were caused by dimmer, negatively charged lightning strikes, or, more probably, by positively charged lightning.
Title: Beta-type stepped leader of elve-producing lightning
H. U. Frey, S. B. Mende: Space Sciences Laboratory, University of California, Berkeley, California, USA;
S. A. Cummer: EE Department, Duke University, Durham, North Carolina, USA;
A. B. Chen, R.-R. Hsu, H.-T. Su: Physics, National Cheng Kung University, Tainan, Taiwan;
Y.-S. Chang: NSPO, Hsinchu, Taiwan;
T. Adachi, H. Fukunishi, Y. Takahashi: Geophysics Department, Tohoku University, Sendai, Japan.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023080, 2005
9. Ocean eddies feed California kelp forests
Rotating ocean currents in California's Santa Barbara Channel may deliver nutrients from the deep oceans to kelp forests off the coast of southern California. Bassin et al. used an array of three high-frequency radars to map current speed and direction in the Santa Barbara Channel from 1998–2002. They found rotating eddy currents 4–15 kilometers [2-9 miles] in diameter that lasted up to several days in the channel. While the cause of the eddies is not known, the authors speculate that friction between ocean currents and the sea floor may play a role in their formation. When two eddies formed in December 2001, the researchers also recorded water conditions such as temperature and nutrient and chlorophyll content. They found that salinity and nutrient levels increased when eddies were present, suggesting that the eddy currents transport nutrients, such as nitrate, from the deep open ocean waters to the giant kelp ecosystems off the southern California coast. The authors say these nutrients may be crucial for sustaining the kelp beds during summer and El Nino events, when nitrate concentrations fall dramatically.
Title: Sub-mesoscale coastal eddies observed by high frequency radar: A new mechanism for delivering nutrients to kelp forests in the Southern California Bight
Corinne J. Bassin, Libe Washburn: Institute for Computational Earth System Science and Department of Geography, University of California, Santa Barbara, California, USA;
Mark Brzezinski, Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California, USA;
Erika McPhee-Shaw: Moss Landing Marine Laboratories, Moss Landing, California, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023017, 2005
10. Using ship-based GPS to study climate and weather
Measurements of atmospheric water vapor taken with Global Positioning System (GPS) antennas aboard ships in the open ocean may help meteorologists better predict the weather. Satellite and weather balloon observations of the atmospheric temperature, humidity, and pressure over oceans often provide only a rough estimate of vapor distribution and can only be used in certain weather conditions. Rocken et al. explored the viability of taking these measurements at sea with the use of ship-based GPS during two week-long cruises in 2002 and 2003. They found that GPS water vapor readings were consistent with readings taken simultaneously from weather balloons. They also explored the ability of the GPS system to determine sea level height at the ship's location, finding that the GPS system was accurate to within about 10 centimeters [four inches], a highly precise measurement that may be useful for oceanographic research. They say that GPS measurements of water vapor and sea-surface information may be a valuable tool for future research on the oceans, climate, and weather.
Title: Atmospheric water vapor and geoid measurements in the open ocean with GPS
Christian Rocken, James Johnson, Teresa Van Hove, and Tetsuya Iwabuchi, COSMIC Program Office, University Corporation for Atmospheric Research, Boulder, Colorado, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022573, 2005
11. Deforestation alters rainfall in Africa and beyond
Deforestation in equatorial Africa may impact precipitation patterns throughout the world. Africa's rainforests are mostly located on the western side of the continent from the Congo River basin to the Ivory Coast. Werth and Avissar modeled the replacement of the rainforests with a mixture of shrubs and grassland to investigate the impact this might have on regional and global climate. Over the twelve years of the simulation, the removal of rainforests corresponded with a reduction in rainfall in the region, particularly during the driest season, in July. In addition, when African rainforests were removed, precipitation amounts changed in three equatorial regions around the globe. Rainfall decreased in the tropical Atlantic and the Gulf of Mexico, but increased in the Arabian Peninsula country of Yemen. The authors say that the extensive deforestation in African countries is not as uniform as the total removal in their simulation, but is actually a patchwork of forested and cleared areas. They say that new models that account for this variegated pattern of deforestation might clarify its implications for regional and local climate.
Title: The local and global effects of African deforestation
David Werth and Roni Avissar, Department of Civil and Environmental Engineering, Edmund T. Pratt, Jr. School of Engineering, Duke University, Durham, North Carolina, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022969, 2005
II. Ordering information for science writers and general public
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