A dangerous L.A. fault system rivaling the San Andreas tied to recent earthquakes

Monday's magnitude 4.4 earthquake centered four miles northeast of downtown Los Angeles was modest but packed quite a jolt.

Although no major damage was reported, experts say the temblor was in the general area of a dangerous fault system — one they have long feared is capable of producing a catastrophic earthquake in the heart of the city.

The quake ruptured on a small fault strand associated with the Puente Hills thrust fault system , which has long been cited as a major seismic hazard for Southern California because it runs through heavily populated areas and is capable of a huge quake .

“It’s a reminder that this is actually our most dangerous fault,” earthquake expert Lucy Jones said, surpassing the San Andreas fault.

The length of the fault strand that ruptured Monday, resulting in the magnitude 4.4 quake, was relatively tiny, perhaps only a few hundred feet across. Small earthquakes happen all the time on relatively tiny fault strands and most of the time are not followed by anything larger.

Still, Angelenos have been feeling this part of L.A. — the Eastside neighborhood of El Sereno — rumble since early June. And it's a reminder of the seismic threats that are too often ignored in Southern California.

Monday’s earthquake, centered about 1,100 feet southwest of the intersection of Huntington Drive and Eastern Avenue, occurred in the same general area as a pair of earthquakes in early June — a magnitude 3.4 on June 2 and a magnitude 2.8 on June 4 , also associated with the Puente Hills thrust fault system. There also was a magnitude 2.9 quake in the same area on June 24.

“All of these earthquakes are closely spaced in three dimensions, just beneath the main Puente Hills thrust [fault] plane,” USC earth sciences professor James Dolan said. “They’re all associated with the same cluster of small events.

“But the key thing is, they are very small events. These are very small earthquakes that don’t necessarily mean anything in terms of potentially being the harbinger of a future large magnitude earthquake on the Puente Hills thrust,” Dolan said.

What is the Puente Hills thrust fault system?

It 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.

Why are scientists concerned about it?

The biggest concern is geography because the fault runs through highly populated areas, including older parts of L.A. that have concrete frame buildings that could crumble in the event of a massive quake.

Another problem with a quake along the Puente Hills thrust fault system is that the soft sediment beneath the L.A. Basin amplifies the quake’s energy.

Experts say a major quake — say of magnitude 7.5 — on that fault would be catastrophic. It could kill 3,000 to 18,000 people, according to the U.S. Geological Survey and Southern California Earthquake Center. And the economic loss could be up to $252 billion, which would be the costliest disaster in U.S. history.

That’s worse than the hypothetical death toll of 1,800 people from a plausible magnitude 7.8 earthquake that begins on the southern San Andreas fault near the Mexican border and unzips all the way to the mountains of Los Angeles County.

How often does this fault see a big quake?

Scientists believe the Puente Hills fault has a major quake roughly every few thousand years — but they don’t know when the last one was. The San Andreas fault has quakes more frequently; it ruptures on average, in a very rough sense, every century or so. In Southern California, the last major earthquake on the San Andreas fault was in 1857, estimated at somewhere around a magnitude 7.8.

But even moderate quakes can cause major problems.

In 1987, the magnitude 5.9 Whittier Narrows earthquake left old brick buildings in Whittier’s downtown area battered. The quake also damaged some freeway bridges. More than 100 single-family homes and more than 1,000 apartment units were destroyed. It caused more than $350 million in damage. Eight deaths were reported.

In 2014, a magnitude 5.1 earthquake in La Habra struck along the fault system. Residents within 10 miles of the epicenter reported toppled furniture, broken glass and fallen pictures. Several water mains broke, and a rock slide in Carbon Canyon caused a car to overturn, leaving those inside with minor injuries. Officials said more than a dozen homes and apartments were red-tagged because of possible structural damage.

When was it discovered?

The Puente Hills thrust fault system was previously unknown to scientists when it ruptured in 1987. Scientists discovered the fault in 1999.

Five years earlier, the magnitude 6.7 Northridge earthquake hit on another “invisible” fault — completely underground, without coming to Earth's surface — that scientists didn’t know about.

What are the takeaways?

Los Angeles has widely felt two quakes in a week: Monday's magnitude 4.4 in El Sereno and last week's magnitude 5.2 about 18 miles southwest of Bakersfield . In both cases, the state's early warning system sent out alerts.

People should heed the lessons from this modest earthquake, Dolan said, and take action to prepare for future ones, such as buying extra water for their homes and workplaces and securing spaces, such as fastening bookshelves to walls.

“If it inspires even a few people to do that ... that’s a good thing for L.A.,” Dolan said.

“People really need to be ready for a very, very large earthquake, or earthquakes, in L.A.’s future," he said. "It’s going to happen. We don’t know when. We don’t know exactly which fault is going to generate those earthquakes, but they are going to happen.”

This story originally appeared in Los Angeles Times .