The La Habra earthquake Friday was only a magnitude 5.1. But the fault runs beneath downtown Los Angeles, meaning it could be a bigger concern than other, better-known southern California faults.
By Mark Sappenfield, Staff writer / March 30, 2014
The 5.1-magnitude 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 an 4.4-magnitude earthquake on a fault that ran beneath the San 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 two-fold: 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 (GPS) 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.