Featured items for the upcoming June 2007 issue:
New tool for monitoring nuclear tests:
The International Monitoring System (IMS) is a world-wide network of seismometers, hydrophones, infrasound arrays, and radionuclide detectors that operates in support of the Comprehensive Nuclear Test Ban Treaty. The goal of the IMS is to detect, locate, and classify seismic sources. There is an increasing need to discriminate among events that generate relatively small (M<4) seismic activity. Scientists at Los Alamos National Laboratory, the University of California, San Diego and Southern Methodist University have successfully tested a new discrimination technique on seismic recordings of delay-fired mine blasts and earthquakes. Delay-fired mine blasts, which consist of a series of individual shots arranged in a grid pattern and detonated in sequence, can introduce spectral modulations into recorded seismograms. Using array data, the authors applied the method to 76 recordings of regional events recorded by the Pinedale seismic array in Wyoming. Using the new technique, scientists achieved a success rate of 97%, or correctly identified 74 events, which included both earthquakes and mine blasts. The authors suggest that this new technique may make a strong contribution to the effort to monitor and identify explosions.
Corresponding author: Stephen J. Arrowsmith of Los Alamos National Laboratory.
California’s Hayward Fault System Examined:
For Northern California, the Hayward Fault System is considered to pose the greatest risk for producing a major quake in the next 30 years. It is necessary for seismologists to understand the structure of the East Bay and the mechanics of its motion in order to anticipate what will happen during an earthquake along the Hayward Fault. Scientists from USGS-Menlo Park created the most detailed 3-D model to date of the upper crust in the East Bay and geometry of the Hayward Fault. The model reveals the motion of small Hayward Fault earthquakes to be very similar to the over-all motion of the fault, with no complexities that could bound or restrict the rupture zones of large earthquakes. Seismic hazard assessments should therefore plan for earthquakes anywhere along the fault. Further, although the Hayward and Calaveras Faults are not connected at the surface, the model revealed a smooth connection between the Hayward and Calaveras Faults at depths greater than about 3 miles. Therefore, seismic hazard assessments should assume scenario earthquakes that span parts of both faults.
Authors: Jeanne L. Hardebeck, Andrew J. Michael, and Thomas M. Brocher of USGS-Menlo Park, California.
Long-term seismic behavior of an active fault: what can we learn from a 12,000-yr-long paleo-seismic record?
Daëron and colleagues from Institut de Physique du Globe de Paris (France) present results of the first paleoseismic study of the Yammoûneh fault, which is the main on-land segment of the Levant fault system (or "Dead Sea fault") in Lebanon, a region tectonically similar to the "Big Bend" in the San Andreas fault. This area offers a long historic record that spans more than 2000 years of activity. Researchers sought to answer several questions about the frequency and magnitude of historical quakes and to understand the mechanisms at work that govern the faults. They present evidence that the latest event was the great A.D. 1202 earthquake and resolve unanswered questions about the frequency of seismic activity. Large earthquakes on different fault segments appear to cluster temporally within a couple of centuries, followed by millennial spans of relative quiescence. Authors conclude that regional risk assessment needs to prepare for the possibility of a large (M>7) earthquake striking this densely populated region in the coming century.
Corresponding author: Mathieu Daëron, currently at Caltech in Pasadena, CA
The Bulletin of the Seismological Society of America (BSSA) is the premier journal of advanced research in earthquake seismology and related disciplines.
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Bulletin of the Seismological Society of America