AGU journal highlights -- Dec. 19, 2006
1. The 17 July 2006 Java earthquake and tsunami
The 17 July 2006 Java earthquake (Magnitude 7.2) involved thrust faulting near the Java trench and triggered a tsunami with run-ups between five and eight meters [20 and 30 feet] that killed more than 637 people along the southern coast of Java. Ammon et al. analyzed surface and body waves and found that the rupture was unusually long (about three minutes), propagated slowly, and occurred near the up-dip edge of the subduction zone thrust fault, attributes shared with past earthquakes that generated larger than expected tsunami for events of their surface-wave magnitude. Most of the earthquake's large aftershocks involved normal faulting. Further, the rupture included five or six pulses of energy release as it propagated along 200 kilometers [120 miles] of the trench. Because earthquakes that generate tsunami must be identified quickly by tsunami warning operators, the authors expect that similar retrospective studies of such earthquakes will help authorities better respond to future hazards.
Title: The 17 July 2006 Java Tsunami Earthquake
Charles J. Ammon: Department of Geosciences, Pennsylvania State University, State College, Pennsylvania, U.S.A.;
Hiroo Kanamori: Seismological Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;
Thorne Lay: Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, U.S.A.;
Aaron A. Velasco: Department of Geological Sciences, University of Texas, El Paso, Texas, U.S.A.
Geophysical Research Letters (GRL) paper 10.1029/2006GL028005, 2006
2. A new technique for early estimates of earthquake source and strength
Earthquake early warning systems have the potential to benefit many active seismic areas of the world, if alert signals from dense seismic networks can be sent to urban settlements in advance of the arrival of destructive seismic waves. Further, if reliable real-time estimates of earthquake location and magnitude are obtained in an evolving, continually updated form, they can be used to rapidly simulate shake maps, a process critical to assessing emergency disaster response needs. To facilitate earthquake early warning calculations, Zollo et al. studied near-source strong motion records, which provide unsaturated recordings of moderate to large earthquakes. They found that low-pass filtered peak amplitudes of initial primary and secondary wave seismic signals recorded in the vicinity of the earthquake correlates with the earthquake magnitude and may be used for real-time estimations of event size before the earthquake rupture itself is completed. The authors say that the probability that a fracture grows to a large size should scale with the energy initially available.
[See also AGU Press Release 06-42: http://www.agu.org/sci_soc/prrl/prrl0642.html]
Title: Earthquake magnitude estimation from peak amplitudes of very early seismic signals on strong motion records
Aldo Zollo and Maria Lancieri: Department of Physics, University of Naples Federico II, Naples, Italy;
Stefan Nielsen: Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy.
Geophysical Research Letters (GRL) paper 10.1029/2006GL027795, 2006
3. Subsidence and southward displacement of southeast Louisiana through normal faults
Tectonic processes such as faulting and load-induced flexure of the lithosphere [Earth's crust and uppermost mantle] have played a substantial role in lowering the land surface in the Gulf of Mexico basin over geologic time. Dokka et al. studied GPS data collected between 1995 and 2006 and found that Louisiana, including New Orleans and the larger Mississippi River Delta, are both subsiding vertically and moving southward with respect to North America. They hypothesize that this subsidence occurs in part because the area is situated on the hanging wall of a normal fault system separating North America from deltaic sediments. These sediments and underlying bedrock are moving southward due to gravity instabilities created by sediments of the Mississippi River delta loading Earth's crust and mantle, and by rising sea levels during continental glacial retreat. Because New Orleans and other communities of southeastern Louisiana hit hard by hurricanes Katrina and Rita lie atop this active fault system, the authors state that future motion of this land should be considered during the reconstruction of the region.
Title: Tectonic control of subsidence and southward displacement of southeast Louisiana with respect to stable North America
Roy K. Dokka: Center for Geoinformatics, and Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana, U.S.A;
Giovanni Sella: National Geodetic Survey, National Oceanic and Atmospheric Administration, Silver Spring, Maryland, U.S.A.;
Timothy H. Dixon: Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida, U.S.A.
Geophysical Research Letters (GRL) paper 10.1029/2006GL027285, 2006
4. Volcanic explosivity could be monitored by measuring ground deformation near vents
Volcanic explosivity is excited by gas bubbles in magma conduits; if sufficient amounts of gas are lost during magma ascent, the probability of an explosive eruption is reduced. Thus, measuring the magnitude degassing from a magma body will help determine volcanic hazards. Direct measurements of degassing is difficult, and past research has linked bubble growth in deep magma chambers with ground deformation, so Takeshi Nishimura used a numerical model of the degassing process to examine magma ascent before an eruption with and without degassing, taking into account bubble growth in melt and elastic stress from the volcanic conduit. His results show that when magma containing gas bubbles reaches the surface without degassing, ground deformation accelerates just before the eruption due to magma expanding as pressure is released. However, the rate of change in ground deformation is almost constant when magma degasses as it ascends. Thanks to this mechanism, Nishimura suggests that geodetic measurements, usually used to estimate eruption time and dyke intrusion, could also provide information needed to predict the degree of volcanic explositivty.
Title: Ground deformation due to magma ascent with and without degassing
Takeshi Nishimura: Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan.
Geophysical Research Letters (GRL) paper 10.1029/2006GL028101, 2006
5. Source and consequences of a large interplanetary shock near 79 Astronomical Units from the Sun
In 2006, as Voyager 2 moved past 79 Astronomical Units [79 times the distance of Earth from the Sun], it experienced a high-intensity interplanetary shock when it encountered charged solar particles that propagate from the Sun during periods of solar unrest. This was followed by a merged interaction region of enhanced magnetic field lines and plasma density. Richardson et al. studied this event and found it to be comparable to other shocks observed by Voyager 2 in 2001 and 1991. Observing that near 79 Astronomical Units, Voyager 2 was close to the termination shock region, the boundary where the speed of the solar wind drops abruptly as it meets interstellar wind, the authors sought to define how the interplanetary shock region interacted with termination shock. They determined that the interplanetary shock and the merged interaction region probably pushed the termination shock outward. The authors also modeled possible sources of the interplanetary shock using data from the Ulysses solar probe and conclude that a series of flares and coronal mass ejections observed by Ulysses in 2005 was a possible source of the interplanetary shock experienced by Voyager 2 in 2006.
Title: Source and consequences of a large shock near 79 AU
J. D. Richardson: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.; and State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China;
Y. Liu: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.;
C. Wang: State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China;
D. J. McComas: Southwest Research Institute, San Antonio, Texas, U.S.A.;
E. C. Stone and A. C. Cummings: Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, California, U.S.A.;
L. F. Burlaga: Laboratory for Geospace Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.;
M. H. Acuna: Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.;
N. F. Ness: Institute for Astrophysics and Computational Sciences, Catholic University of America, Washington D.C., U.S.A.
Geophysical Research Letters (GRL) paper 10.1029/2006GL027983, 2006
6. Observations of dust storms help constrain radiation budgets
Saharan dust storms pose health hazards in Africa, are thought to suppress hurricane growth, and transport large quantities of nutrients to the Atlantic Ocean. Though Saharan dust storms have been observed from space, their impact on the Earth's radiation balance through absorbing solar energy is poorly known, because of limited surface observations in areas affected by such storms. Since surface data are needed for validating satellite observations, Slingo et al. were the first to study simultaneous observations from space and from a comprehensive new mobile facility in Niamey, Niger, of a major dust storm in March 2006. Their results indicate major perturbations in the radiation balance both at the top of the atmosphere and at the surface. Using the observations on the ground in radiation models, they also showed that models based on satellite data underestimate the observed absorption of solar radiation in the dusty atmosphere over Niamey. They expect that better spatial coverage for ground truth will help improve more regional models of the radiative effects of dust storms.
Title: Observations of the impact of a major Saharan dust storm on the atmospheric radiation balance
A. Slingo, R. P. Allan, and G. J. Robinson: Environmental Systems Science Centre, University of Reading, Reading, United Kingdom;
T. P. Ackerman, E. I. Kassianov, S. A. McFarlane, and J. C. Barnard: Pacific Northwest National Laboratory, Richland, Washington, U.S.A.;
M. A. Miller: Atmospheric Sciences Division, Brookhaven National Laboratory, Upton, New York, U.S.A.;
J. E. Harries and J. E. Russell: Blackett Laboratory, Imperial College, London, United Kingdom;
S. Dewitte: Royal Meteorological Institute of Belgium, Brussels, Belgium.
Geophysical Research Letters (GRL) paper 10.1029/2006GL027869, 2006
7. Changes in ocean dynamics may affect top predators
The California sea lion is the most abundant top predator in the California Current system, an ocean current characterized by warm water conditions in winter and cold upwelled water in spring and summer. Distributions, abundances, and behaviors of top marine predators are related to oceanographic features and the abundance of prey, so Weise et al. tagged 25 male sea lions off the coast of California in the fall of 2003 and 2004. They found that in early 2004, the sea lions overwhelmingly stayed near the shore. However, in early 2005, two males altered their foraging efforts by spending more time at sea and venturing up to 450 kilometers [280 miles] offshore. The onset of spring conditions in 2005 were delayed in the California Current system, and the authors suggest that the two sea lions altered their behavior in order to find food. Further, changes were found in these sea lions' diets, similar to those seen during other episodes of shifting ocean dynamics. The authors note that their study is the first to concentrate on male sea lion behavior with respect to oceanic conditions.
Title: Movement and diving behavior of male California sea lion (Zalophus californianus) during anomalous oceanographic conditions of 2005 compared to those of 2004.
Michael J. Weise and Daniel P. Costa: Center for Ocean Health, Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California, U.S.A.;
Raphael M. Kudela: Ocean Sciences Department, University of California Santa Cruz, Santa Cruz, California, U.S.A.
Geophysical Research Letters (GRL) paper 10.1029/2006GL027113, 2006
8. Modeling how underwater ridges affect oceanic turbulence
When ocean currents and tides encounter undersea topography, disturbances called internal waves can be generated that add to the complexity of oceanic mixing. Such waves can influence how nutrients are distributed and how energy is transported through out the ocean. Xing et al. developed a model to study sills, which are shallowly submerged ridges that separate basins of water, and their contribution to internal wave generation. Through varying parameters on a vertical cross section of turbulent water above the sill, the authors show that when the width of a sill is reduced, its effect on internal wave generation is also reduced, although internal mixing on the lee side of the sill increases. Small-scale topography on the sill slope produces the same effects. Further, the authors find that when surface layers less susceptible to vertical perturbations are added to this model's water column, internal waves created by the sill will be less energetic.
Title: Influence of stratification and topography upon internal wave spectra in the region of sills
Jiuxing Xing and Alan M. Davies: Proudman Oceanographic Laboratory, Liverpool, United Kingdom.
Geophysical Research Letters (GRL) paper 10.1029/2006GL028092, 2006
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 (GRL).
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