New Map Shows Seismic Hot Spots

Exactly seven years after the Northridge earthquake and just days after two short, sharp temblors shook the region, geologists on Tuesday released the first report identifying Southern California’s seismic “hot spots”--areas expected to shake the hardest during tremors.

Compton, just south of downtown, sits in the center of one seismic hot spot, as does an area in the northern San Fernando Valley where the Foothill Freeway meets the Golden State Freeway. Other regions of concern include the area south of downtown that stretches into northern Orange County, the Riverside area, and the Ventura basin, near Oxnard.

The report and map created by the Southern California Earthquake Center are considered by some to be as important as detecting a major new fault. They could help shape plans for building construction and retrofitting for the next several decades and could also greatly speed rescue efforts after a major quake by identifying regions of greatest hazard.

“This is one of the best kinds of earthquake prediction: predicting the effects of earthquakes and allowing engineers and rescue personnel to prepare,” said James H. Whitcomb of the National Science Foundation, an agency that helps fund the earthquake center.

The need for such tools was made even more clear by Saturday’s earthquake in El Salvador, which has killed at least 660 people.

The report does not forecast precisely when or where the next earthquake might occur--something still beyond the reach of seismologists. Instead, it focuses on where damage might be worst if an earthquake occurs anywhere in the region. The results can help explain why houses on one side of a street might collapse while those on the other side withstand tremendous shaking.

In general, the amount of shaking is determined by the magnitude of an earthquake and the distance from the fault. The new study examines other “site effects,” such as the type and depth of sediments, to determine why some areas fare so much worse during earthquakes.

At a news conference describing the results, lead scientist Ned Field of the U.S. Geological Survey emphasized that people living in hot spots should not panic, especially because many sites not considered hot spots could also shake violently if nearby faults rupture. “I don’t think it’s anything that [should cause] someone to consider moving,” he said.

Lucy Jones, who runs Southern California’s earthquake hazards program for the Geological Survey, added: “To move away from the earthquake hot spots, you have to move into the . . . mountains,” which carry a higher risk of wildfires and mudslides. “Let’s keep this in perspective.”

Scientists have also learned that each earthquake will have a unique pattern of hot spots--one that cannot yet be predicted accurately, said Kim Olsen, a seismologist at UC Santa Barbara who is developing computer models to make such predictions.

Instead of a mass exodus from hot spots, state geologist James F. Davis is hoping the new maps will motivate homeowners--especially those in homes built before more stringent building codes went into effect in 1975--to retrofit their homes to mitigate quake damage. “I feel that’s the weak link,” he said, noting that many residents do not have an accurate perception of their seismic vulnerability.

The idea that hot spots exist is not new. They were first noted in an 1898 scientific paper. But the new report is the first to quantify hot spots and predict exactly where they will occur.

The magnitude 6.7 Northridge quake in on Jan. 17, 1994, killed 57 people and caused more than $20 billion in damage. By causing an alarming amount of shaking in distant Santa Monica, it also alerted seismologists that their models of shaking behavior were awry.

Tuesday’s announcement was the first formal presentation of work that seismologists initially discussed publicly in April at their annual meeting in San Diego.

The new study melded two factors known to amplify earthquake waves and trigger increased shaking. First, scientists included a new state map developed by engineering geologist Chris Wills, which describes the sediments in the Earth’s top 100-foot layer.

The softest sediments, which shake the most, are in valleys. The hardest materials, rocks, are in mountainous areas. The softest sites, Field said, can shake 2 1/2 times as much as harder sites.

Second, the coalition of geologists examined the depth of sediments and found that shaking was as much as two times worse where sediments are deepest: those areas include the four-mile-deep Los Angeles Basin south of downtown, a six-mile basin on the northern edge of the San Fernando Valley, and the Ventura Basin, which could be deeper than 10 miles.

Such basins, dangerous because they contain the deepest, softest sediments, also include some of the region’s oldest housing, noted Harold Magistrale, a geologist at San Diego State. These areas were settled first because of their fertile land and accessible water and because it is easier to build houses on flat land, he said.

Predicting areas with the worst shaking could save lives by leading rescuers directly to the scene of most devastation, said Larry Collins, a Los Angeles Fire Department captain who leads Urban Search and Rescue Team 103. During the Northridge quake, Collins helped save victims at the collapsed Northridge Meadows apartments. “The epicenter’s not necessarily where the worst shaking is,” he said.

Ellis M. Stanley Sr., general manager of the city’s emergency preparedness department, said the study could mobilize funds and the political will for the massive retrofitting that the city’s older buildings require.

“In my business, we have small windows of opportunity,” Stanley told the crowd of reporters. “I dare say if we hadn’t had a small shake Saturday or did not have [the] El Salvador [quake], half of you wouldn’t be here.”

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Quake Hot Spots

A study released Tuesday is the first to pinpoint Southern California’s earthquake hot spots. Orange and pink show areas expected to have the most shaking during earthquakes. They are characterized by soft, deep sediments. Rocky areas, shown in purple, will shake less.

Source: Southern California Earthquake Center