A 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.
Source: University of California - Davis
Related stories:
May 2008 earthquake in China could be followed by another significant rupture
Researchers analyzing the May 2008 Wenchuan earthquake in China's Sichuan province have found that geological stress has significantly increased on three major fault systems in the region. The magnitude 7.9 quake on May 12 has brought several nearby faults closer to failure and could trigger another major earthquake in the region.
Scientists explore Sichuan fault
Durham University expert, Alex Densmore, is to explore the fault lines that caused the May 12th earthquake in China that killed 69,000 people.
Geologists study China earthquake for glimpse into future
The May 12 earthquake that rocked Sichuan Province in China was the first there in recorded history and unexpected in its magnitude. Now a team of geoscientists is looking at the potential for future earthquakes due to earthquake-induced changes in stress.
Fatal mine collapse covered 50 acres
New calculations show that the deadly Crandall Canyon mine collapse – which registered as a magnitude-3.9 earthquake – began near where miners were excavating coal and quickly grew to a 50-acre cave-in, University of Utah seismologists say in a report on the tragedy.
Big quakes spark jolts worldwide
Until 1992, when California’s magnitude-7.3 Landers earthquake set off small jolts as far away as Yellowstone National Park, scientists did not believe large earthquakes sparked smaller tremors at distant locations. Now, a definitive study shows large earthquakes routinely trigger smaller jolts worldwide, including on the opposite side of the planet and in areas not prone to quakes.
Earthquake 'memory' could spur aftershocks
Using a novel device that simulates earthquakes in a laboratory setting, a Los Alamos researcher and his colleagues have shown that seismic waves—the sounds radiated from earthquakes—can induce earthquake aftershocks, often long after a quake has subsided.
Earthquake swarms not just clustered around volcanoes, geothermal regions
An earthquake swarm – a steady drumbeat of moderate, related seismic events – over hours or days, often can be observed near a volcano such as Mount St. Helens in Washington state or in a geothermal region such as Yellowstone National Park in Wyoming.
Silent earthquakes may foreshadow destructive temblors
A team of American geoscientists is urging colleagues around the world to search for evidence of tiny earthquakes in seismically active areas, such as the Pacific Northwest, that are periodically rocked by powerful temblors of magnitude 8 and higher.