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California Q uakes Reminder of Threat

Two earthquakes that jolted the Los Angeles area in recent weeks have given residents a potent, and frightening, reminder that their city’s next big temblor could come from any of the dozens of seismic faults that crisscross the region.

Early March 17, a magnitude-4.4 earthquake emanating from a fault running through the Santa Monica Mountains shook homes across Los Angeles. Late Friday, a magnitude-5.1 earthquake centered in La Habra, east of downtown Los Angeles, rattled residents again.

Neither quake caused any deaths, although the event in La Habra seriously damaged more than a dozen homes and set off a rockslide in a canyon east of the city that overturned a vehicle, causing minor injuries to the driver and passenger. Los Angeles Dodgers radio announcer Vin Scully said on the air that he felt the quake during the sixth inning of an exhibition game at Dodger Stadium. Aftershocks continued more than a day after the initial events.

But they were relatively strong events after two decades of seismic serenity.

“This is another reminder — we live in earthquake country in Southern California,” U.S. Geological Survey seismologist Robert Graves said at a briefing after Friday’s quake. “This is an earthquake that’s not terribly damaging, but it should be taken to heart that we can have larger, more damaging earthquakes.”

A series of quakes caused significant damage and multiple deaths in the 1980s and 1990s, including the 1987 Whittier Narrows quake, a 5.9-magnitude temblor that killed eight people and damaged hundreds of buildings, and the 1994 Northridge quake, which killed 60 people and destroyed freeways across Los Angeles.

But since then, few earthquakes registering as more than minor tremors have shaken the area. The recent quakes sparked questions about whether the quiet spell is over. Scientists say a few tremors along unconnected fault lines within a short period do not necessarily indicate a more active period, but more earthquakes over the long term will raise their level of concern.

Los Angeles Mayor Eric Garcetti’s office has taken early steps to remind residents that they live in an active seismic zone. The city borrowed Lucy Jones, a local legend known as the “earthquake lady,” from her post at the USGS to conduct a yearlong survey of Los Angeles. Jones is tasked with developing policy recommendations aimed at retrofitting particularly vulnerable buildings; protecting the city’s water supply, essential to fighting post-earthquake fires; and securing communications lines. If telecommunications wires that cross the San Andreas fault are damaged during an earthquake, Internet and cellular networks that first responders need could fail.

“We are more prepared than we were, say, before the Northridge earthquake, because there’s been a big push to educate people,” said Kate Hutton, a staff seismologist at CalTech. “We have a long ways to go. A lot of the buildings that were built in the ’50s and ’60s probably aren’t as strong as we thought they were.”

Both March quakes came from little-known faults that seismologists are still exploring. The Santa Monica earthquake was the strongest along that fault in the nearly 80 years since Charles Richter developed a scale for measuring the magnitude of seismic events. Until St. Patrick’s Day, scientists had recorded quakes there ranging only from 1 to 3 on the Richter scale.

Friday’s earthquake startled scientists because it occurred on the Whittier fault, which runs from northern Orange County all the way to downtown Los Angeles.

A stronger seismic event could imperil thousands of buildings, and hundreds of thousands, if not millions, of people who live and work near the center of the second-largest U.S. city. Estimates by the USGS and the Southern California Earthquake Center suggest that a much larger quake along the Whittier fault could kill more than 3,000 people and cause as much as $250 billion in damage, leaving nearly a million people homeless.

Most Angelenos are worried about the next major seismic event along the San Andreas fault, the boundary between the Pacific Plate and the North American Plate that runs nearly the length of California. But the two March earthquakes show the dangers posed by smaller faults in the Los Angeles area; the USGS has documented 60 faults, ranging from the 800-mile-long San Andreas to faults that run just a few miles.

“We don’t know about the location of all the small faults. The major ones, that are capable of producing ⅛magnitude-⅜ 6’s or greater, we know where those are. We probably have names for them,” Hutton said.

After the 1994 Northridge earthquake, a magnitude-6.7 quake that struck the previously-unknown Northridge Thrust fault, USGS scientists undertook a study of the Los Angeles area, using thousands of sensors to record even the smallest seismic activity, to map fault lines. That study revealed the Whittier fault - which looks just like the Northridge fault, possibly foreshadowing a quake that costs thousands of lives and billions of dollars.

But even today, scientists say they don’t have a complete picture of the Whittier fault, much less the region’s broader seismic makeup.

Asked what scientists knew about the Whittier fault, Jones was blunt: “We don’t know how long it is,” she said.