ShareThisScientists have known for years that an earthquake on one fault can trigger movement on nearby faults.
In 1992, the 7.3 Landers quake in California not only caused simultaneous movement on small faults nearby, but triggered a 6.5 aftershock two hours later in Big Bear Lake, 17 miles away.
U.S. Geological Survey geophysicist Ken Hudnut says that, under the right conditions, the phenomenon could be devastating in the inland reaches of the state.
Speaking at a recent meeting of the Earthquake Research Affiliates at the California Institute of Technology in Pasadena, Hudnut showed the latest evidence of what is called the cascade effect, including photos of ruptures along the Pescadores and Borrego faults in Baja California near the epicenter of the 7.2 quake southwest of Mexicali on April 4.
"We recognize a rupture can skip from one fault to the next," Hudnut said in an interview after the conference. "If the distance is less than 5 kilometers (about 3 miles) and the earthquake is big enough, the earthquake seems to be able to skip across. It seems like gaps that are more than 5 kilometers, it's really hard to jump across those, but it does happen sometimes."
The phenomenon could be of concern to inland residents if a quake were to erupt along either the San Jacinto or San Andreas faults where the two nearly merge in the Cajon Pass.
"You could have an earthquake running down the San Andreas and skip onto the San Jacinto or vice versa," he said. "We haven't had a clear case where that has happened. It's been suggested in the 1857 (Ft. Tejon) earthquake, but there hasn't been good evidence for it."
While that scenario may be unlikely, scientists have observed that major quakes can influence minor activity on faults far away. That phenomenon was observed following the April 4 Cucapah quake.
Data presented by Caltech geophysicist Egill Hauksson showed spikes in minor earthquakes along the Elsinore and San Jacinto faults immediately following the Cucapah quake.
"They were associated in time. That seemed to imply they were triggered (by the larger quake)," he said.
But the activity was minor enough that scientists weren't concerned, he said.
Hudnut was just one of several speakers at the Caltech meeting. Earthquake scientists discussed the deadly, destructive quakes in Chile, Haiti and Baja California earlier this year. Lucy Jones, chief scientist for the U.S. Geological Survey's multi-hazards project, said that although earthquakes have gotten a lot of attention this year, the activity is not unusual.
Jones said seismologists expect 13 to 16 earthquakes of 7.0 or greater per year.
"So far this year, we have had seven," she said, "slightly above average, but no way that this is a significant increase."
USGS geophysicist Susan Hough described stronger-than-expected earthquake motion along ridgelines in Haiti seen during aftershocks of the 7.0 magnitude quake of Jan 12. The same phenomenon was observed in Tarzana, Calif. during 1994's Northridge earthquake and at Pacoima Dam in a 1971 quake, she said.
The shaking observed on the ridgeline she studied in Haiti was four to five times greater than at a reference site situated on hard rock.
For hilltop residents in southern California, she said, "There is some reason for concern. How strong the effect is going to be, we don't have an answer for that now. We can't say here's the increased hazard you can expect, so it's hard to know what to do."
The earthquake experts at the Caltech conference said they hope to have a new tool for studying the effects of earthquakes within a few years, one that could help them pinpoint damage and guide emergency response teams after a major quake in Southern California.
Mark Simons, a Caltech professor of geology and geophysics, said he expects NASA to launch the Deformation, Ecosystems and Dynamics of Ice mission in five to seven years.
Simons said current radar satellites used by researchers offer images of a particular spot on the earth only once every 45 days. The new satellite would close that window to eight days, and ground systems could provide analysis of earthquake damage a day or two.
Scientists also expect to monitor volcanoes, landslides, glacier movement and deforestation.
(E-mail reporter Mark Muckenfuss at mmuckenfuss(at)PE.com.)
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)
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