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Loma Prieta Fault Not So Weak?
December 20, 2007A new study adds to evidence that the fault responsible for the 1989 Loma Prieta earthquake is not as unusually weak as had been thought.
In general, a "weak" fault is one that ruptures relatively easily, resulting in smaller but more frequent earthquakes, while a "strong" fault can accumulate more strain before breaking in larger shocks, said Robert Twiss, professor emeritus of geology at UC Davis and co-author of the study with Jeffrey Unruh of William Lettis and Associates of Walnut Creek, Calif.
The Loma Prieta fault has been described as unusually weak based on studies of the aftershocks from the 1989 earthquake. By looking at the direction and movement of the aftershocks, researchers could calculate the strains in the fault. Previous studies looked at the aftershocks in total.
Twiss and Unruh took a new approach, breaking the aftershocks up into 17 separate clusters. They found that the fault is complex, with different kinds of deformation in different places. When they looked at the aftershocks in greater detail, they found more stresses in the structure.
"You're losing most of the essential mechanical information about the fault by piling these details together," Twiss said.
The researchers also found that the stresses and slip directions of the main earthquake were reproduced in the aftershocks, implying that the stresses continue to be present.
Twiss said that there is "no way" to predict the strength or timing of future earthquakes on the same fault using these results. But the findings would be relevant for other researchers seeking to build models of earthquake faults in the Bay Area, he said.
The paper was published in the September/October 2007 issue of the Geological Society of America Bulletin.
University of California - Davis
Twiss and Unruh took a new approach, breaking the aftershocks up into 17 separate clusters. They found that the fault is complex, with different kinds of deformation in different places. When they looked at the aftershocks in greater detail, they found more stresses in the structure.
"You're losing most of the essential mechanical information about the fault by piling these details together," Twiss said.
The researchers also found that the stresses and slip directions of the main earthquake were reproduced in the aftershocks, implying that the stresses continue to be present.
Twiss said that there is "no way" to predict the strength or timing of future earthquakes on the same fault using these results. But the findings would be relevant for other researchers seeking to build models of earthquake faults in the Bay Area, he said.
The paper was published in the September/October 2007 issue of the Geological Society of America Bulletin.
University of California - Davis
The impact of 1989 Loma Prieta Earthquake -- 20 years later
The Loma Prieta earthquake transformed the earthquake sciences and engineering and remains a major focus of study, some twenty years later. The 17 October 1989 magnitude 6.9 Loma Prieta earthquake severely shook the San Francisco and Monterey Bay regions and initiated major changes in earthquake science and engineering, disaster response and public policy well beyond California.
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Like geological ninjas, earthquakes can strike without warning. But there may be a way to detect the footfalls of large earthquakes before they strike, alerting their potential victims a week or more in advance. A Stanford professor thinks a method to provide just such warnings may have been buried in the scientific literature for over 40 years.
Discovery sheds new light on cause of earthquakes
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Prelude to an Earthquake?
A geophysicist from the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has identified possible seismic precursors to two recent California earthquakes, including the 1989 Loma Prieta earthquake that wreaked havoc throughout the Bay Area.
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