The cluster of earthquakes that began Friday night in La Habra appear to have struck on a fault underneath the Puente Hills thrust fault, scientists said Wednesday.
Friday's earthquake has not provoked earthquakes on either the main Puente Hills fault or the nearby Whittier fault.
Friday's magnitude-5.1 quake "was relatively shallow and thus did not significantly perturb the parts of the Puente Hills thrust or the Whittier fault, where we expect a major earthquake to start," said Caltech seismologist Egill Hauksson.
"Further, there were no small quakes triggered on either the Puente Hills thrust or the Whittier, indicating they remain locked," Hauksson said.
Friday's earthquake was a reminder of the significance of the Puente Hills thrust fault, which runs 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.
In 1987, a "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.
The Puente Hills fault could be especially hazardous because of its shape as it slopes underneath a large region. Other local faults, like the Newport-Inglewood and Hollywood, are almost vertical, with the most intense shaking occurring where the fault reaches the surface. Because the Puente Hills fault is relatively more horizontal, intense shaking is likely to be felt over a much larger area, roughly 25 by 15 miles.
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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.
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, U.S. Geological Survey seismologist Lucy 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.