Computer Pictures ‘the Big One’
A study of how earthquake waves from the San Andreas fault travel through different types of Southern California soil marks what scientists say is a promising first step in an ambitious effort to pinpoint neighborhoods and even individual city blocks where the shaking would be most severe.
Researchers from the Southern California Earthquake Center hope to duplicate the research on hundreds of faults around the region, producing maps that show specific areas that face the greatest danger from the quake waves.
The scientists simulated two magnitude 7.7 temblors along the San Andreas fault to determine how the waves from the quakes would move across the region’s topography.
They found that the waves from the San Andreas fault funneled northwest into the Los Angeles Basin, moving through the valleys that line the San Bernardino and San Gabriel mountains like water rushing through a trench.
The study identified several areas, including communities at the base of the San Gabriel Mountains, that would experience particularly strong shaking because the local topography would force waves toward the surface.
Researchers are entering data from hundreds of thousands of possible ruptures on other local faults and running computer simulations that map the direction and intensity of the waves.
They hope to have preliminary maps assessing the shaking risk in downtown Los Angeles, Pasadena and Long Beach in coming months.
Thomas Jordan, a USC geophysicist who runs the Southern California Earthquake Center, said the maps could eventually be used by city planners, insurance companies, real estate brokers and others to understand the quake risk of a particular piece of property.
But the prospect of detailed shaking hazard maps also raises questions about how much stock government, private industry and the public is willing to place in research that Jordan and others acknowledge, like most quake preparedness efforts, is speculative.
“We need to have a detailed discussion of how this information is going to be used and how society will respond to it,” Jordan said.
This study was different from previous attempts to map the impact of a quake on the San Andreas fault because its authors applied the laws of physics to the waves and the Earth to get their results, said seismologist Ned Field, who studies earthquake hazards for the U.S. Geological Survey in Southern California.
“What is new here is rather than just throwing up your hands and saying nature is just random ... this more physics-based approach to modeling is telling us for this particular earthquake where the hot spots are,” Field said. “Our models that predict ground shaking have always had a large amount of uncertainty.”
The study, published this week in the scientific journal Geophysical Research Letters, found that a major temblor that starts near the Salton Sea and ruptures north to the Cajon Pass would cause particularly high levels of shaking along the base of the San Gabriel Mountains, all the way into the Los Angeles Basin.
The waves would become trapped between the mountains on one side and hard rocks on the other, rocketing toward Los Angeles in a funnel of soft, sedimentary earth that forms the basis for the region’s many valleys. The waves, slowed by the soft earth, would make up for that slowness by becoming higher, and as a result would cause devastating shaking all along the way.
In metropolitan Los Angeles, the worst shaking would be east of downtown, starting where the 60 and 605 freeways meet, and traveling along the San Gabriel River as it flows through Whittier Narrows and southward to the 91 Freeway near Long Beach.
Parts of the San Fernando Valley would also experience significant shaking from the quake that scientists simulated because of the soft valley floor.
Downtown Los Angeles, Santa Monica and most of Long Beach would shake harder than previously anticipated, but not as badly as the rest of the region.
Joan Fryxell, a structural geologist at Cal State San Bernardino, said the San Andreas study provides a scientific answer to a question that has long puzzled seismologists -- how does shaking from an earthquake in one location turn up in unexpected places?
For example, there was serious damage from the 1994 Northridge quake on the Westside and in parts of South Los Angeles, but not in most other places outside of the San Fernando Valley.
There was a similar effect during the 1992 Landers quake, Fryxell said, because in both cases the waves became higher and more noticeable as they traveled through soft soil.
“The ground shaking was much more noticeable to people out in the Palos Verdes Peninsula than in the intervening areas, because Palos Verdes is built on goo,” she said.
For the study, researchers used a supercomputer to enter more than 2 billion points on a grid representing the topography of Southern California, said San Diego State seismologist Kim Olsen, who performed the study along with two scientists from UC San Diego and Jordan of USC.
The study was organized by the Southern California Earthquake Center, which is based at USC.
“It was running for days on the supercomputer,” Olsen said. “We were able to digitize the geological layers in the basins -- under L.A., the Chino Basin, the San Bernardino Basin -- at high resolution.”
The resulting three-dimensional modeling allowed seismologists to pinpoint localized areas of stronger shaking, Olsen said.
In the future, Jordan and others said, scientists hope to develop computer models that will use all of the area’s known fault zones to compute the risk in a particular community -- down to the level of a city block or a single structure.
For example, Jordan said, the area around the proposed natural gas terminal at the Port of Long Beach could be studied to see if it is at risk for unexpected shaking.
Field, of the U.S. Geological Survey, said researchers have so far attempted to run computer models of quake risks in downtown Los Angeles, Pasadena and Long Beach.
“Right now it’s beyond our grasp to do [models] for the entire region but we are starting to be able to look at a particular site, like downtown L.A., or your house,” Field said.
Information about where the shaking would be highest in a major quake is particularly important for emergency response and urban planning needs, said Ellis Stanley, director of emergency services for the city of Los Angeles.
Stanley said he would use the San Andreas study to press policymakers to ensure that the city’s building codes and seismic retrofitting ordinances reflected the new understanding of risk from a quake on the fault, which experts say may be overdue for a major rupture.
“The springs are tightening on this one,” Jordan said. “It’s the kind of earthquake we need to consider in our emergency planning -- in thinking what might break the system.”
