Southern California earthquake swarm takes an unexpected turn, and that’s reason to worry


If you live in Fontana, you wouldn’t be blamed if you felt a case of the jitters.

A swarm of earthquakes has shown remarkable staying power in the area around the Southern California city. Caltech seismologist Egill Hauksson said the chance that the series of tremors will turn into a large and destructive quake isn’t particularly high.

But that doesn’t mean residents shouldn’t be on their toes. The likelihood of a larger seismic event, given how many quakes that have occurred over such an extended period, is higher than normal, the scientist said.

“People ought to be concerned,” said Hauksson. “This is probably the most prolific swarm in that area of the Fontana seismic zone that we’ve seen in the past three decades.”


There have been more than 700 earthquakes recorded in the Fontana area since May 25, ranging from magnitude 0.7 to magnitude 3.2, recorded Wednesday at 5:20 p.m., according to Caltech staff seismologist Jen Andrews. Three of the quakes have been of magnitude 3 or greater.

The swarm initially moved northward, but something unusual began Friday when the swarm turned around and went south, back toward the middle of the activity and the 60 Freeway.

“This is somewhat of an unexpected evolution,” Hauksson said Friday evening. Furthermore, an analysis of the earthquakes shows that activity as of late Friday was fading pretty slowly — slower than would be expected for a typical sequence of aftershocks following a main shock, he said.

“That would suggest it’s going to continue for — I don’t know — at least several weeks,” Hauksson said Friday. “We’re watching what’s happening and trying to track that activity.”

By Saturday afternoon, earthquake activity had decreased significantly. “This is difficult to interpret, but it suggests that the sequence is now decaying somewhat similar to an aftershock sequence,” Hauksson said. “There will be fewer and fewer events as time goes on.”

If there is significant renewed quake activity, it’ll probably be from a new seismic sequence, he said.


Get ready for a major quake. What to do before — and during — a big one »

Hauksson said that, given all the seismic activity, residents should be ready and make sure to store “plenty of water, make sure there’s nothing that can fall on them.”

In any home in seismically risky areas of California, experts recommend removing heavy objects around beds, strapping bookcases and dressers to walls, anchoring flat-screen televisions to walls, installing toddler safety latches on kitchen cabinets, and ensuring picture frames are attached to walls with quake-friendly hooks. The last fatality in a California earthquake was from a blow to the head from an unstrapped television.

Homeowners should also make sure their water heaters are properly secured to reduce the chance of a house fire. Homes that sit a few steps off the ground and built before 1979 should be evaluated by a structural engineer to determine whether they need to be braced and bolted to the foundation to prevent them from sliding off when shaken. Apartment owners with carports or garages on the ground floor should also consider having their buildings evaluated to determine if a retrofit is needed.

Hauksson said the timing of the earthquakes is worth noting: shaking, followed by a lull and then a spurt of new quakes.

The earthquakes have been relatively shallow — beginning just 1 to 2½ miles under the surface. As a result, the shaking has been widely felt, Andrews said.

Residents in the Fontana area said there is talk about how industrial wastewater ponds built in the last several years off of Jurupa and Beech avenues were drained recently, right around the time the quakes began.

Hauksson said there could be a connection if the pond water was pumped into the ground to a significant depth of at least 0.6 miles. But if the water was trucked away, he would not expect a relationship.

“This will require more research,” he said. It’s already been shown in Oklahoma that earthquakes can be triggered by water being pumped into the ground.

Seismic swarms can be worrisome — especially when they occur near faults that experts know are capable of unleashing huge, catastrophic earthquakes.

That was the case almost three years ago.

On Sept. 26, 2016, a rapid succession of small earthquakes — three measuring above magnitude 4.0 — began rupturing under the Salton Sea, close to the San Andreas fault, continuing for more than 24 hours. The swarm increased the likelihood of a major quake in Southern California, at least temporarily: Experts said the chance of a 7.0 or greater quake on the mighty fault increased significantly, from 1 in 6,000 in any given week to as much as 1 in 100 during that particular week.

No large quake occurred. But the warnings did prompt some residents to prepare.

With the latest swarm, there is one silver lining: Two of the most fearsome faults in Southern California — the San Andreas and San Jacinto — are not particularly close. That means the chance of a truly large quake — the kind they make Hollywood movies about — resulting from the long-running cluster of tremors is not especially high.