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Earthquakes
Rachel E. Abercrombie

EarthScope, the largest infrastructure initiative in the geosciences, began in 2003. Researchers have installed the first GPS stations of the Plate Boundary Observatory and recovered the first data from the seismometers in the San Andreas Fault Observatory at Depth (SAFOD) pilot hole. Drilling of the SAFOD hole proper began in June.

The aim is to drill through the source region of a cluster of small, repeating earthquakes on the San Andreas fault at a depth of 3 to 4 kilometers. The main challenge is locating this region with sufficient precision. Typically, earthquake-location uncertainties of about 1 kilometer are considered good and uncertainties of 100 meters are superb, but neither adequately ensures that the drill hole will penetrate the target area. Recent intensive work on velocity structure and earthquake location by several groups, described in a special volume of Geophysical Research Letters, greatly increases the chance of drilling through the target and improving our understanding of this transitional part of the fault.

Researchers continue to focus on dynamics of the rupture process, including how earthquakes initiate, propagate and trigger one another. New information about how fault zones weaken and respond to shaking comes from John Vidale and Yong-Gang Li (Nature, v. 421, p.524). They observed a gradual increase in velocity in the fault zone of the Landers earthquake (magnitude 7.3, 1992) in the years after the earthquake, and then a decrease of velocity at the time of the Hector Mine earthquake nearby (magnitude 7.1, 1999), followed by continuing gradual increase. Vidale and Li attribute this gradual healing to the closing of cracks opened by shaking during the two earthquakes.

Compelling new evidence for supershear rupture during the 2001 Kunlun earthquake was published by Michel Bouchon and Martin Vallée (Science, v. 301, p. 824). Crack theory predicts that the rupture velocity during an earthquake should not exceed the shear wave velocity (ß). Most earthquakes have an average speed of ~0.8ß, but indications of rupture at super-shear speeds have been accumulating. The magnitude-7.8 Kunlun earthquake produced the longest surface rupture (about 400 kilometers) ever observed on land, providing good geometric resolution.

The multidisciplinary approach needed for progress in earthquake science was emphasized in the report, "Living on an Active Earth," published by the National Academies. The report summarizes earthquake science history, outlines current and future goals, and demonstrates the need for geologists and experts in rock and fracture mechanics to work with seismologists to understand earthquakes.

In 2003, researchers proposed two promising new analysis techniques. Kristine Larson and others (Science, v. 300, p. 1421) showed that permanent GPS stations recordings at high frequency (1 Hertz) can reliably observe seismic shaking and permanent displacements in the frequency range where long-period seismometers have limited resolution. Improving the recording of frequencies between seismic and long term is essential to further study of the slow slip events commonly detected on subduction zones and of other relatively slow and unusual sources such as the glacial earthquakes recently identified by Göran Ekström and others (Science, v. 302, p. 622). On a more practical note, Richard Allen and Hiroo Kanamori (Science, v. 300, p. 786) demonstrated a simple technique to obtain relatively reliable predictions of imminent large-amplitude shaking from smaller and earlier-arriving waves. Warnings up to 20 seconds could be possible, which would be very valuable to safety officials, industry and the public.

Major earthquakes

Last year's largest earthquake (magnitude 8.3) was a subduction interface earthquake off Hokkaido, Japan. It was well recorded and is already increasing our understanding of the subduction process in that region and elsewhere. In July, a magnitude-7.6 earthquake occurred just off the Carlsberg Ridge in the Indian Ocean. It began at the end of an active, left-lateral transform but ruptured into the Indian plate with right-lateral slip in the opposite sense to that of the transform fault! Such a large earthquake off the active transform is very unusual but work is showing that it is consistent with the complex, diffuse plate-boundary motions within the India-Australia plate.

Two events in December served as new reminders not only of the devastating power of earthquakes but also of the disconnect between magnitude and effect. On Dec. 22, a magnitude-6.5 earthquake in San Simeon, Calif. claimed two lives. Four days later, a magnitude-6.6 earthquake in Bam, Iran killed 40,000 people. Unreinforced masonry buildings, insufficient earthquake-preparedness strategies and higher population density spelled the difference between the two events. We cannot prevent earthquakes, but comparing these two event demonstrates that such a high level of suffering, injury and death need not be inevitable. The nonprofit organization Geohazards International promotes global safety by working with local people to reinforce existing buildings before earthquakes occur and by urging stricken communities to rebuild with inexpensive earthquake-resistant structures — a good model for moving forward.

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Abercrombie is associate professor in the Department of Earth Sciences at Boston University. E-mail: rea@bu.edu.

Links:
Earthscope
GeoHazards International
Last year's largest earthquake on the NEIC Web site
Living on an Active Earth
(National Academies Press)

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