The southernmost stretch of the San Andreas Fault is more than 150 years overdue for a large quake.
The San Andreas Fault stretches 800 miles from the Salton Sea at the southern end of California to the ocean just off Eureka in the north.
Skirting the foggy Bay Area, the fruitful heartland, and the bustling urban center of Los Angeles, it waits quietly.
For The Big One. An earthquake of 7.5 magnitude or greater that is certainly coming.
The southern portion of the San
Andreas Fault is more than 150 years
overdue for a large earthquake.
Credit: Robert E. Wallace, USGS
Experts say the normal probability for a large earthquake on any significant portion of the fault is once every 150 years; yet the southern portion has not “snapped” since 1690.
“We believe it’s accumulated more strain than other parts, like in San Francisco, where it broke with a large earthquake in 1906,” said San Diego State University seismologist Kim Bak Olsen.
The peacefulness, marked by smaller infrequent quakes, worries Olsen and his colleagues.
With extreme natural disasters – Hurricane Katrina, the Indonesian tsunami, the recent Peruvian earthquake – this calm before the storm can give rise to a lack of preparedness. The result is injury and death on a large scale when catastrophe strikes.
The prediction of natural disasters is still far from an exact science. However, Olsen and fellow SDSU seismologist Steven Day are working to learn more about the major scenarios the San Andreas is likely to produce, so Californians can prepare even if there is no answer to the question of when.
Their work may mean never having to say, “if only we had known,” when it comes to the powerful jolt the long-expected Southern California earthquake will deliver.
“Simulation helps us characterize the biggest events, which are also the rarest, the ones that really put a stress on society,” said Day. “We are trying to reduce that level of surprise.”
‘A gift’
Using extensive data on the geography of areas off of the fault, Olsen and Day have been working with the San Diego Supercomputer Center to create finely detailed video simulations of the fault’s most probable seismic activity and how these ruptures would unfold.
Olsen was the first person to do three-dimensional simulations of these kind 12 years ago while working toward a doctorate in geophysics at the University of Utah.
SDSU seismologist Kim Bak Olsen was the first person to do
three-dimensional earthquake simulations 12 years ago.
“It’s a relatively new thing to use supercomputers for these kinds of simulations, because they haven’t been around for that long,” Olsen said. “It’s like a gift for us seismologists; it’s a really big computational problem that hasn’t been possible to solve until now.”
Their latest round of simulations, dubbed TeraShake for the power of calculations, are so vividly detailed they reveal the direction, magnitude and duration of the shaking -- and some bad news for Los Angeles.
If the southern portion of the fault unzips from south to north, the very factors that have historically made L.A. such a desirable place to settle — majestic mountains and flat valleys, would cause it to shake violently for more than two minutes.
The energy from the quake would radiate outward from the likely epicenter near the Salton Sea, but not in stacked, even circles, as earthquakes are graphically conveyed on the evening news. Channeled toward L.A. by the San Gabriel Mountains, the waves would become trapped by the sedimentary basins situated underneath the city.
“The waves would bounce back and forth inside the basin, kind of like waves in a bathtub,” Olsen said. “This specific earthquake seems to be one of the worst-case scenarios.”
Other scenarios simulated in TeraShake include a north-to-south quake, in which Mexicali would experience the strongest shaking.
The trade-off
The information provided by TeraShake is potentially useful to engineers and policymakers in making decisions about new development in Southern California.
“Cities can take the information and basically not build hospitals and universities on those areas,” Olsen said.
If only it were that simple.
Because there is still no way to accurately predict the timing of natural disasters, politicians often find themselves torn between spending funds on catastrophic event preparation or on more clearly defined near-term needs.
According to Day, the results of this trade-off are particularly evident in Third World countries, where catastrophes of enormous magnitude are common. Funding preparedness means diverting money from urgent development projects.
“Our work doesn’t solve the political and economic problems entirely; it provides a scientific foundation for that,” Day said. “Ultimately, it depends on society’s willingness to pay the price for preparedness.”
Until major preparation measures don’t compete financially with other pressing public needs, Olsen and Day are doing what they can to define earthquake risks and raise public awareness for decision-makers and concerned citizens alike.
Even without sweeping changes to building codes and city planning, individuals can still take small – and potentially life-saving – measures to prepare for a major earthquake.
“If the public is aware of fault lines and their proximity to home or the workplace, they may be more likely to respond to the shaking of a large earthquake,” Olsen said. “Letting the public know they should secure bookshelves to the wall, and seek shelter under a solid table could save many lives during a significant shake.”
Olsen has even found a way to entertain as he educates. He contributed to the animated earthquake sequences for the IMAX Film “Forces of Nature” and the National Geographic film “Anatomy of an Earthquake.”
Related information
Credits
- Story by Lauren Coartney
- Graphics by John Signer
- Photographs by Tom Farrington, Instructional Technology Services
- Banner photograph courtesy of FEMA News Photo
- Edited by Coleen L. Geraghty
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