La Habra quake a reminder about dangerous Puente Hills fault

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Greg Sebourn, Fullerton’s mayor pro tem, confers with a woman whose house on Juanita Place was red-tagged after a magnitude 5.1 earthquake hit the area. (Irfan Khan / Los Angeles Times)

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Cracks are apparent in a house on Juanita Place in Fullerton that suffered structural damage in the 5.1 earthquake. (Irfan Khan / Los Angeles Times)

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Fullerton firefighters check for damage to an apartment complex in the 2600 block of Associated Road after a 5.1 earthquake hit the community. (Irfan Khan / Los Angeles Times)

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Jessine Luna, 21, describes how she held her 3-month-old daughter, Faith Perea, and started praying as a 5.1 earthquake hit La Habra. A friend, Joseph Brackett, 21, left, and her father-in-law, Jeff Perea, 49, look on. (Irfan Khan / Los Angeles Times)

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Whitney Urien holds a broken photo frame after it was knocked off the wall in a magnitude 5.1 earthquake that hit Fullerton. She lives in an apartment complex on Associated Road that sustained cracks in the temblor. (Irfan Khan / Los Angeles Times)

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Caltrans workers and Brea police officers inspect a BMW that was overturned in a rock slide in Carbon Canyon after a magnitude 5.1 earthquake. (Irfan Khan / Los Angeles Times)

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Richard Rodriguez, left, and Guillermo Valencia board up a broken window at Towne Cleaners in La Habra, the epicenter of Friday’s 5.1 earthquake. (Allen J. Schaben / Los Angeles Times)

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Shampoo and soap containers are strewn about in the 99 Cent Only store on Imperial Highway. (Robert Gauthier / Los Angeles Times)

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Ray Aguirre cleans up his greeting card shop on Whittier Boulevard in La Habra, the 5.1 quake’s epicenter. (Brian van der Brug / Los Angeles Times)

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A BMW is damaged from a rock slide in Carbon Canyon after a 5.1 earthquake. The road was closed because of the risk of an aftershock. (Francine Orr / Los Angeles Times)

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Cesar Zamora, night manager at the 99 Cent Only store on Imperial Highway, looks over aisles of fallen goods. (Robert Gauthier / Los Angeles Times)

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Fullerton police stand by as a city employee fixes a water main break at the intersection of Gilbert St. and Rosecrans Ave. (Allen J. Schaben / Los Angeles Times)

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An employee surveys fallen hair and body products at a CVS pharmacy on Whittier Boulevard after the 5.1 earthquake near La Habra. (Brian van der Brug / Los Angeles Times)

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Sean Higa, general manager at the Lebard and Underwood motorcycle dealer on Imperial Highway in La Habra, surveys damage after the quake. (Allen J. Schaben / Los Angeles Times)

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A Chevron station in La Habra is a bit of a mess after Friday’s swarm of quakes. (Allen Schaben / Los Angeles Times)

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Products are strewn about at the 99 Cent Store on Imperial Highway in Brea. (Robert Gauthier / Los Angeles Times)

By Rong-Gong Lin II

March 29, 2014 7:46 PM PT

The magnitude 5.1 earthquake that rattled Southern California on Friday was a 10-second reminder of a fault that seismologists believe can produce a catastrophic disaster.

The Puente Hills thrust fault is so dangerous because of its location, running from the suburbs of northern Orange County, through the San Gabriel Valley and under the skyscrapers of downtown Los Angeles before ending in Hollywood.

Experts say a major, magnitude 7.5 earthquake on the fault could do more damage to the heart of Los Angeles than the dreaded Big One on the San Andreas fault, which is on the outskirts of metropolitan Southern California.

The size of Friday’s quake was considered moderate, but it packed a punch. Residents within 10 miles of the epicenter in La Habra reported toppled furniture, broken glass and fallen pictures. Several water mains broke, and a rock slide in Carbon Canyon caused a car to overturn, leaving those inside with minor injuries.

Officials said more than a dozen homes and apartments were red-tagged because of possible structural damage.

Preliminary checks by the U.S. Geological Survey after the quake show it erupted around the Puente Hills thrust fault system, said Caltech seismologist Egill Hauksson. Further research is underway.

In 1987, another “moderate” quake on that fault killed eight people and caused more than $350 million in damage. The magnitude 5.9 Whittier Narrows quake left old brick buildings in Whittier’s downtown area battered and also damaged some freeway bridges. More than 100 single-family homes and more than 1,000 apartment units were destroyed.

Friday night’s earthquake was caused by the underground fault slipping for half a second, said USGS seismologist Lucy Jones, prompting about 10 seconds of shaking at the surface.

But a 7.5 quake on the Puente Hills fault could cause the fault to slip for an entire 20 seconds — and the shaking could last far longer.

The Puente Hills fault could be especially hazardous over a larger area because of its shape. Other local faults, like the Newport-Inglewood and Hollywood, are a collection of vertical cracks, with the most intense shaking occurring near where the fault reaches the surface. The Puente Hills is a horizontal fault, with intense shaking likely to be felt over a much larger area, roughly 25 by 15 miles.

According to estimates by the USGS and Southern California Earthquake Center, a massive quake on the Puente Hills fault could kill from 3,000 to 18,000 people and cause up to $250 billion in damage. Under this worst-case scenario, people in as many as three-quarters of a million households would be left homeless.

One reason for the dire forecast is that both downtown L.A. and Hollywood are packed with old, vulnerable buildings, including those made of concrete, Jones said.

By contrast, a magnitude 8 “Big One” on the San Andreas fault — more than 30 miles from downtown L.A., on the other side of the San Gabriel Mountains — would cause up to 1,800 deaths, according to estimates.

The shaking from a quake in the center of urban L.A. would be so intense that it could lift heavy objects into the air, Jones said. It has happened before, near the epicenter of the 1989 Loma Prieta earthquake in the Santa Cruz Mountains. There, the shaking was so bad, “we found an upside-down grand piano.”

“That’s the type of shaking that will hit all of downtown. And everywhere from La Habra to Hollywood,” Jones added.

The violent motion would be amplified by the soft soil underneath the Los Angeles Basin and the valleys, which produces a jello effect as shaking waves wobble off the basin.

Scientists believe the Puente Hills fault has a major quake roughly every 2,500 years but don’t know when the last one was. The San Andreas has quakes more frequently (both the Loma Prieta and 1906 San Francisco quakes were on this fault).

The Puente Hills fault was discovered relatively recently — in 1999. Five years earlier, the magnitude 6.7 Northridge earthquake hit on another “invisible” fault — completely underground — that scientists didn’t know about.

“When Northridge happened,” Jones said, “it was very sobering for us to think we could have that big of an earthquake that doesn’t come to the surface.”

So scientists launched a major study to discover more of these invisible faults. They strung thousands of sensors across the Los Angeles region and set off small explosions underground.

“From that, we saw this Northridge-like structure sitting under downtown L.A., which is horribly sobering,” Jones said.

Video simulations of a rupture on the Puente Hills fault system show how energy from a quake could erupt and be funneled toward L.A.’s densest neighborhoods, with the strongest waves rippling to the west and south across the Los Angeles Basin.

By contrast, the Northridge earthquake, which killed 57, channeled its strongest shaking north to the more sparsely populated Santa Susana Mountains.

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