The magnitude-5.1 La Habra earthquake Friday – and the aftershocks since – are a reminder of the enormous complexity of the fault systems below Los Angeles, the scope of which scientists have only begun to understand during the past 20 years.
The full extent Puente Hills fault that is thought to have caused Friday's earthquake was not known until 1999, when seismologists were given access to private oil-company data. Meanwhile, the fault responsible for the 1994 Northridge earthquake, which caused more than $20 billion in property damage, was completely unknown before the quake hit.
The sheer variety of faults beneath Los Angeles was underscored earlier this month, when a magnitude-4.4 earthquake on a fault that ran beneath the Santa Monica Mountains jolted residents awake the morning of St. Patrick's Day. But the the fault at issue this weekend is of particular concern for Los Angeles.
For decades, as southern Californians have considered the possibility of the "Big One," attention has centered on the San Andreas fault, which runs from southern California all the way up to the San Francisco Bay Area. But the San Andreas runs well to the east of the city on the other side of the San Gabriel Mountains.
By contrast, the fault system that includes Puente Hills runs through the Santa Ana Hills in northern Orange County before descending directly into the Los Angeles Basin.
"In terms of location, it couldn't be much worse," James Dolan, a professor at University of Southern California's department of earth sciences, told National Geographic in 2003. "Downtown L.A. is sitting on top of this thing."
The concern is twofold: First, much of the Los Angeles Basin is filled with soil, which would amplify the motion of a large quake on the fault. Buildings tend to withstand quakes better if they're built on rock. Second, a number of old concrete buildings in central Los Angeles are not adequately seismically retrofitted.
A US Geological Survey map of the faults beneath Los Angeles looks like a knitting pattern. The map counts 60 in all, including the Whittier fault system, which includes Puente Hills. The urgency to find hidden faults was underscored by the Northridge quake.
In 2001, scientists put a network of 250 global positioning system sensors across southern California. The sensors allow scientists to measure distances with great precision. If they notice two points moving farther away or closer together in ways that currently known faults don't explain, that's a hint that other faults could be at work.
But, unfortunately, earthquakes are often the best way to learn more about a little-known fault. The St. Patrick's Day earthquake surprised seismologists because it was the strongest to hit directly under the Santa Monica Mountains in the 80 years "since we started recording earthquakes in Southern California," Caltech seismologist Egill Hauksson told the Los Angeles Times. Before that, only earthquakes with a magnitude of 1 to 3 had been recorded.
Seismologists say magnitude-7.5 earthquakes have hit the Puente Hills fault system, but only in the distant past. A study by Professor Dolan "shows the occurrence of at least four earthquakes with a magnitude of 7.2 to 7.5 on the Richter scale during the past 11,000 years." Before this weekend, the most recent quake on the fault was a magnitude-5.7 quake in 1987.
"The good news is that major earthquakes along this fault are very infrequent, it may not happen again for thousands of years," Dolan told National Geographic. "The bad news is that it could be very strong when it does happen."