Over the past few weeks, as San Diego shook from a series of earthquakes, scientists were able to test a new technology from the University of California, San Diego that allows them to see a quake just minutes after it occurs.
A magnitude 4 quake off the coast of Mission Beach early Sept. 4 set off a series of temblors in the normally quiet network of fault lines. At first, scientists including Anthony Guarino of Cal Tech thought the Sept. 4 temblor might be a precursor, or foreshock to a larger quake.
“We know that statistically, there is a 5 percent chance it could be a foreshock to a [magnitude] 5 or a 6 quake,” Guarino said. “One out of every 20 earthquakes we have is a foreshock to a bigger quake.”
Seismologists at Cal Tech and elsewhere are still concerned about the “big one,” from the San Andreas fault line, which has not moved since 1857. The fault travels from north of San Francisco to the Salton Sea. Experts believe the fault will lock like a chain link fence, starting at one end, and rupture all the way down, Guarino said.
The San Andreas will move both Interstate 5 and Interstate 15 by 20 feet when it ruptures, which experts say will be in their lifetimes, Guarino said.
Now, a new technology lets scientists view a movie of the waves from a particular quake after it happens. Scientists used the Sept. 4 main-quake and its aftershocks to view this new animated technology from UCSD. The “supercomputer” uses sensors in the ground that pick up seismic events. It translates those events into animation, creating a movie of the earthquake.
The process takes about 23 minutes after the event and is available on demand to scientists and others who are signed up beforehand, said UCSD’s Paul Tooby.
Whether the increase in quakes means that pressure is increasing or decreasing, seismologists do not know. The main quake may relieve stress in some areas but increase stress in others, Guarino said.
On Thursday, Sept. 13, a magnitude 3 aftershock off the coast of La Jolla jolted San Diego. Scientists began to watch the area closely.
“This has been a very robust aftershock sequence,” Guarino said. “More active than normal, but nothing we haven’t seen before.”
Once some time elapsed, seismologists at Cal Tech decided the quake was probably not a foreshock. Then, more significant quakes shook the same area “” about 20 in all “” and seismologists changed the classification of the Sept. 4 quake to a main-shock, or the biggest quake, Guarino said.
Generally, there are only one or two “feelable” earthquakes a week, but the past few weeks have been quite different, especially for San Diego, generally a “pretty quiet place, seismically speaking,” Guarino said. Scientists watched the quake cluster closely, to see if an “earthquake swarm” was occurring “” something highly unusual for San Diego.
“Normally, with an earthquake swarm, you have many quakes in a particular location,” Guarino said. “These events are not considered a swarm yet because the quakes following the main shock are the smaller, 3-magnitude quakes.”
San Diego was not the only city in Southern California to wake up jolted over the past few weeks. The supercomputer at UCSD and scientists at Cal Tech in Pasadena saw more earthquakes than usual with larger magnitudes than normal, Guarino said.
“We’ve had increased seismic activity throughout Southern California throughout the past couple weeks,” Guarino said.
Although seismologists can’t predict anything for certain, Guarino and others at Cal Tech said these temblors are a reminder that San Diegans are living in earthquake country, and that a “big one” could rupture at any time.
“This is an example to let people know they need food and water on hand and enough medications to ride out an emergency,” Guarino said. “Enough supplies for three to five days.”
View the earthquake online at: http://ucsdnews.ucsd.edu/newsrel/science/07-07EventDrivenSciencePT-.asp.