Experts say a bigger earthquake along the lesser-known fault that gave southern California a moderate shake a few days ago could do more damage to the region than the long-dreaded "big one" from the more famous San Andreas fault.
The Puente Hills thrust fault, which brought Friday night's magnitude-5.1 shock centred in La Habra and well over 100 aftershocks by Sunday, stretches from northern Orange County under central Los Angeles into Hollywood, which is a heavily populated area.
A magnitude-7.5 earthquake along that fault could prove more catastrophic than one along the San Andreas, which runs along the outskirts of metropolitan southern California, seismologists told the Los Angeles Times.
The United States Geological Survey (USGS) estimates that such a quake along the Puente Hills fault could kill between 3,000 and 18,000 people and cause up to US$250 billion in damage. In contrast, a larger magnitude-8 quake along the San Andreas would cause an estimated 1,800 deaths.
In 1987, the fault caused the Whittier Narrows earthquake. Still considered moderate at magnitude-5.9, that quake killed eight people and did more than US$350 million in damage.
Part of the problem with the potential damage is that the fault runs near so many vulnerable older buildings, many made of concrete, in central Los Angeles and Hollywood. And because the fault, discovered in 1999, is horizontal, heavy reverberations are likely to be felt over a wide area.
The shaking from a 7.5 quake in the centre of urban Los Angeles could be so intense it would lift heavy objects in the air, like the 1989 Loma Prieta earthquake in northern California, where the shaking was so bad "we found an upside-down grand piano", USGS seismologist Lucy Jones said.
That would "hit all of downtown ... and everywhere from La Habra to Hollywood", she said.
About 150 aftershocks, including one of magnitude-4.1, have been felt since Friday night's quake, which forced several dozen people in the Orange County city of Fullerton out of their homes after firefighters discovered foundation problems that made the buildings unsafe to enter, authorities said.
Fire crews closed 20 apartment units after finding a major foundation crack, but residents have since been allowed to return. Structural woes, including broken chimneys and leaning, were uncovered in half a dozen houses, which were also deemed unsafe to occupy until building inspectors could clear the structures.
About two dozen residents remained displaced, down from more than 80 after the initial quake.
Another 14 residential structures around the city suffered lesser damage, including collapsed fireplaces.
A water-main break flooded several floors of Brea City Hall, and the shaking knocked out computers and displaced ceiling tiles, Stokes said.