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Evidence of Huge Prehistoric L.A. Quakes Found

TIMES SCIENCE WRITER

Twice in prehistoric times, earthquakes up to 15 times more powerful than the 1994 Northridge temblor occurred on the Sierra Madre fault along the southern flank of the San Gabriel Mountains, researchers said Thursday, offering the first direct evidence that faults so close to Los Angeles are capable of such destructive force.

Researchers investigated the fault where it runs through Loma Alta Park in Altadena, about 11 miles north of downtown Los Angeles. They discovered that two ancient earthquakes as strong as magnitude 7.6 occurred there between 10,000 and 15,000 years ago.

Experts said they have no idea when such earthquakes could happen again on that fault.

Even the possibility of such titanic earthquakes in the urban area should be enough to prompt a reassessment of building codes, safety practices and emergency management plans for the entire region, said earthquake experts at UCLA, Caltech and USC. In that light, the discovery along the Sierra Madre fault offers an essential benchmark in recalibrating the consequences of an urban earthquake, several seismologists said.

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“One of the most important impacts of this study will be on the engineering community, who now have to consider the impact of earthquakes this size on any building within metropolitan Los Angeles,” said UCLA earthquake expert David D. Jackson, who is science director for the Southern California Earthquake Center.

Findings May Alter Safety Standards

When the northwest section of the Sierra Madre fault complex ruptured during the magnitude 6.7 Sylmar-San Fernando earthquake in 1971, it killed 64 people and caused $558 million in damage. The 6.7 Northridge earthquake in 1994 was responsible for about 57 deaths and more than $40 billion in damage, one of the costliest natural disasters in U.S. history.

If anything like the ancient earthquakes were to occur again on that section of the Sierra Madre fault, the consequences to the densely populated Los Angeles area would be far worse than the San Fernando or Northridge disasters, experts said. Not only would the quake itself be so much more powerful, but almost all of its energy would be directed southward into the heart of the city.

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“Unlike the Northridge earthquake that ruptured northward, away from the metropolitan region, a magnitude 7 or greater earthquake on the Sierra Madre fault would rupture southward, directing energy into the densely populated regions,” the research team from Central Washington University, UC San Diego and the consulting firm of S.C. Lindvall, William Lettis & Associates reported today in Science.

Ground shaking would be stronger, last longer and encompass a much broader region of Southern California, said seismologist David Wald at the U.S. Geological Survey in Pasadena, who maps the shaking intensities of Southern California quakes.

In Pasadena, Paul Somerville, a prominent engineering seismologist, suggested that the new findings about the Sierra Madre fault would lead to a revision of standards in the federal National Earthquake Hazard Reduction Program as they apply to California. He said federal regulators might have to strengthen the design specifications because they are based on estimates of the strongest known earthquakes in the region.

Region at Juncture of Tectonic Plates

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However devastating such an earthquake might be to the millions of people living in the Los Angeles area, the scanty historical evidence also suggests that it is an extremely rare local event.

At the same time, other recent research has shown that seismic strain may be building up along the fault at just one-third the rate previously believed.

“The bad news is that these earthquakes seem to happen. The good news is that they don’t happen very often,” said Caltech engineering seismologist Thomas H. Heaton. “It creates a difficult policy issue of how to plan for infrequent but very violent earthquakes.”

It is a problem that, in a sense, arises from the paradox of the Southern California landscape: The same geologic forces that lift up its mountains and sculpt its sun-drenched seascapes also periodically threaten to destroy them.

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Like a New Age crystal in a vise, Southern California is caught between the converging tectonic plates of North America and the Pacific, which are compressing the northern and southern ends of the metropolitan area by about a quarter of an inch every year. That fractures Earth’s crust under the region into scores of faults, many of which pose a serious earthquake threat.

Some surface faults, like the San Andreas, are so well known that they serve as shorthand for the seismic hazards faced by the entire state. Others, like the hidden thrust fault responsible for the Northridge temblor, are secrets in the Earth that only reveal themselves when they trigger a violent earthquake.

Until recently, scientists have only been able to speculate about the threat posed by the faults buried beneath the superhighways and subdivisions of Los Angeles.

“This is the evidence we have all been waiting to find--that such earthquakes are no longer speculation,” said USC earthquake expert James F. Dolan. “It is the first really compelling direct evidence that we have had extremely large prehistoric earthquakes on at least one of our urban faults.”

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Section of Fault Visible in Altadena

To reconstruct the history of the Sierra Madre fault, Charles Rubin, a geologist at Central Washington University who led the study, first had to locate a fracture in the regional rock, which is almost completely obscured by modern urban development.

So, as a first step, Rubin turned to the city’s archives to search out photographs taken when much of the area was still pastures and orange groves. Aerial photographs taken during the 1920s allowed him to pinpoint spots where early earthquakes had scarred the Earth’s surface.

Topographical maps of the San Gabriels compiled during the city’s original search for municipal water supplies also hinted at buried seismic structures.

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“I put those and the photos together and targeted areas along the front of the range to find a site [to dig],” Rubin said. “It was detective work.”

The search led him to Loma Alta Park. There, just north of the public parking lot, his team located an exposed scarp of the Sierra Madre fault. With picks, a backhoe and the permission of Los Angeles County, they dug for most of last winter to expose as much of its past as they could.

“We saw evidence for two events clearly in the trench,” Rubin said. The soil sediments had preserved a precise record of how the two quakes had ruptured the ground.

In the more recent quake, the ground appeared to have slipped by about 12 feet. In the earlier quake, the ground may have slipped even more.

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In all, the ground on both sides of the fault slipped a total of 34 feet, the soil evidence indicated. The amount of slip allowed the researchers to estimate the magnitude of the prehistoric earthquakes: between 7.2 and 7.6. They dated charcoal fragments in the sediments to determine how long ago the quakes occurred.

“We knew this amount of slip was greater than seen on any historical earthquake in the L.A. region,” Rubin said. “This study shows that in the past we have had larger earthquakes than Northridge, which we have not documented before.

“Whether they happen frequently or not, I can’t say,” he said.

Several experts cautioned that more research is needed before assuming the worst about the magnitude of the ancient Sierra Madre fault earthquakes.

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The scientists dug into only one spot along the fault, which stretches for 59 miles along the edge of the San Fernando and San Gabriel valleys. There may be other explanations for the ground shifts they discovered.

“Jumping from the presence of these events on the fault to estimating the size of the events is a huge leap, which they are doing with one shred of data,” said USGS geologist Ross Stein in Menlo Park. “It is possible that smaller earthquakes could have produced what they see.”

Even so, Stein said, the evidence of any major activity at all on this section of the Sierra Madre fault is important.

“They have really upset the tea kettle,” he said. “The evidence that this is an active fault is very strong. And the idea that the fault has been active in the last 10,000 years is new. It is an important and sobering finding.”

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Other earthquake experts said they found the evidence of the large magnitude believable.

Thomas Henyey, director of the Southern California Earthquake Center, said he is confident that the estimate is correct, adding that it is “a wake-up call.”

“I have every reason to believe they have read the record correctly,” Henyey said. “It is the fault that is responsible for the uplift of the San Gabriel Mountains, and that is an impressive range. You need a very active, big fault to do that, so it is not unreasonable to assume that those sorts of earthquakes occurred.”

Times staff writer Kenneth Reich contributed to this story.

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To get information on earthquake preparedness and links to continually updated reports on recent earthquake activity, go to The Times’ Web site at: https://www.latimes.com/quake

(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)

Reconstructing an Earthquake

Geologists detected the massive quakes by studying soil sediment, which records movements. Each wedge of soil was evidence of an ancient quake.

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Earthquake 1

Fault moves, forms scarp

Scarp collapses

Surface erodes, leaving wedge of debris

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*

Earthquake 2

Fault slip forms fresh scarp face

Scarp collapses

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Erosion buries old debris, forming second wedge.

Source: Science

(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)

Fault Locations

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The discovery of two 7.2 to 7.6 quakes does not automatically increase the probability of a big quake. Experts must still study stresses to determine probability. Here are the current highest-likely magnitudes of some of the area’s fastest-slipping faults:

San Andreas: 6.8-8.0

San Jacinto: 6.5-7.5

Whittier-Elsinore: 6.5-7.5

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Sierra Madre: 6.5-7.6

Sources: “Putting Down Roots in Earthquake Country”; Southern California Earthquake Center

researched by JULIE SHEER / Los Angeles Times


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