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Earthquakes
Susan Hough


This story is expanded from the print version.


The year 2000 was notable for its lack of damaging earthquakes.But several damaging earthquakes struck in just the first two months of 2001. The year began, literally, with a magnitude-7.5 earthquake on Jan. 1 in the Philippines, an event that occurred away from populated regions. Scientists have not yet determined if and how this earthquake was related to another magnitude-7.5 earthquake that occurred over 200 kilometers to the southwest on March 5 this year. The 2002 event claimed at least 15 lives, damaged 87 buildings, and left widespread power outages and landslides. However, a magnitude-7.7 earthquake on Jan. 13 struck close to the shore of El Salvador, claiming at least 844 lives and 100,000 structures. Another scant two weeks later, the most deadly earthquake of the year, the magnitude-7.6 Bhuj earthquake in western India on Jan. 26, claimed more than 20,000 lives and more than 300,000 buildings.

The most deadly earthquake of 2001, the magnitude-7.6 Bhuj earthquake in western India on Jan. 26, claimed more than 20,000 lives and more than 300,000 buildings. Here, people in Ahmedabad in northwest India look at a building's remains. AP Photo/Martin Mejia


That was just January. On Feb. 13, El Salvador was hit again, this time with a magnitude-6.6 event that caused additional damage and fatalities. And on Feb. 28, the magnitude-6.8 Nisqually earthquake occurred in the Pacific Northwest near Seattle, the largest earthquake in the region in 35 years. The largest damaging earthquake of the year struck on June 1: a magnitude-8.4 event near the coast of Peru that killed more than 100 people and caused extensive damage.
All of these earthquakes had scientific as well as societal implications, perhaps none more so than the Bhuj earthquake. This event illustrated both the hazard posed by faults in low strain-rate environments and the potential for blind thrust faults to produce earthquakes with magnitudes greater than 7.5.

Also, some researchers almost immediately identified the Bhuj quake as a possible analog for the enigmatic New Madrid earthquakes of 1811 and 1812. Like the New Madrid mainshocks, the Bhuj earthquake caused damage as far away as 300 kilometers and was felt to distances exceeding 1,500 kilometers.

Unfortunately, no strong motion instruments were within 250 kilometers of the source, so the critical issue of near-source ground motions remains unresolved. However, teleseismic and other data have been used to determine a moment magnitude and other fault parameters, so the Bhuj earthquake will still provide an important calibration event that will help us understand large earthquakes that have happened in other intraplate regions around the world.

A number of other studies published in 2001 also highlighted earthquake hazard on the Indian subcontinent (Kumar et al., Science, vol. 294; Bilham and England, Nature, vol. 410; and Bilham et al., Science, vol. 293). Roger Bilham and Philip England. evaluated the slip deficit along the Himalayan front and concluded that several fault segments have enough stored slip to generate earthquakes with magnitudes reaching 8.0. Many of these segments are, moreover, close in proximity to densely populated regions. In the wake of these results and the Bhuj earthquake, the Indian government has strengthened its commitment to earthquake research and hazards mitigation (Jayaraman, Nature, vol. 409, 2001).

The El Salvador and Nisqually earthquakes occurred along subduction zones but had mechanisms indicating they ruptured faults within the slab. They were so-called slab-bending earthquakes. The Nisqually earthquake corroborated earlier observations that such events are associated with markedly few aftershocks, an observation that is now being explored with recently developed theories of stress triggering. The second El Salvador earthquake, however, illustrated the potential for slab-bending earthquakes to trigger substantial secondary events, as the second earthquake appears to have been triggered by the first. Although of smaller magnitude, the second event, because of its location, caused considerable damage and killed many people, illustrating the hazard posed by aftershocks or triggered earthquakes associated with the earthquakes along large subduction zones. The general hazard associated with remotely triggered earthquakes was further illustrated in other regions. Intriguing results showed that a triggered earthquake close to Lisbon, rather than a distant mainshock, might have caused the damage to that city from the great earthquake of 1755 (Fonseca and Vilanova, Transactions of the American Geophysical Union, vol. 82).

Apart from investigations tied to last year's earthquakes, research continued apace on developing theories of earthquake triggering. A study of the 1992 Landers and 1999 Hector Mine, Calif., earthquake sequences concluded that the dynamic stress changes associated with earthquake waves may be important for aftershocks as well as remotely triggered earthquakes (Gomberg et al., Nature, vol. 411). Other studies continued to investigate issues such as the role viscoelastic relaxation plays in static stress transfer.
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Another focus of earthquake seismology involved the physics of earthquakes, in particular the question of how radiated seismic energy scales as a function of magnitude. A number of studies have concluded that large earthquakes radiate energy more efficiently than do small earthquakes, an observation that, if true, would have important implications for the earthquake rupture process. However, a study by Ide et al. suggests that the observations may not be well-resolved (Transactions of the American Geophysical Union, vol. 82).

All of the research discussed above has important implications for seismic hazard, which has been explored in any number of individual studies. An additional focus of hazards research in 2001 was the evaluation of regional seismic hazard, including time-dependent hazard that takes into account factors such as stress transfer within and the nature of the earthquake cycle. For example, Bowman et al., also in volume 82 of the Transactions of the American Geophysical Union, presented a new physical model that accounts for the previous observation of accelerating regional seismic moment, or energy release, prior to large earthquakes. Although somewhat controversial, this model appears to explain a number of salient properties of earthquake and aftershock sequences. It also and offers hope that intermediate-term forecasting of earthquakes, if not prediction per se, might some day be possible.

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Hough is a research geophysicist with the U.S. Geological Survey in Pasadena, Calif. E-mail.

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