A string of earthquakes shakes Southern California. Are they telling us something more?

Several small earthquakes have rocked Southern California recently. They created a small tremor but nevertheless they left behind psychological shocks in a region whose seismic vulnerabilities match our willingness to put the dangers out of our minds.

For many, this all led to one question: Is this the start of something bigger?

First, a magnitude 3.6 earthquake in the Ojai Valley sent weak tremors from Santa Barbara to Los Angeles on May 31. Then two small earthquakes struck the eastern LA neighborhood of El Sereno, the most powerful of which was 3.4. Finally, three earthquakes struck the Costa Mesa-Newport Beach border, peaking at a magnitude of 3.6 on Thursday.

After half a dozen earthquakes with a magnitude greater than 2.5 per week, they happen to hit three separate parts of Southern California, all in highly populated areas, which is not common.

But experts say these small quakes have no predictive power over the next major, devastating earthquake in urban southern California, the last of which came 30 years ago.

Read more: Faster alerts for California megaquakes: The early warning system gets a major upgrade

In general, there is a 1 in 20 chance that any earthquake in California will be followed by a larger one, said Susan Hough, a seismologist with the US Geological Survey. Those odds are not high, and usually, the next larger quake would occur in the same area within a week. In addition, if something bigger were to happen, a new templor would likely be only a little bigger, Hough said.

Still, the recent small quakes are a reminder that Southern California is unique and highly vulnerable to earthquakes right next to us. The risk is not limited to the region’s most famous fault, the southern San Andreas, which, apart from running under San Bernardino and Palmdale, is mostly desert and remote mountains but capable of magnitude 8 quakes.

In contrast, last week’s earthquakes exposed nearby fault systems directly beneath our most populous cities and may have caused an even worse death toll than the San Andreas megaquake, targeting our oldest neighborhoods with many non-retrofitted buildings when they collapse.

“All three of these sets of earthquakes occurred near large, potentially dangerous faults,” said James Dolan, a professor of earth sciences at USC. “LA’s urban fault network has been in a seismic lull for the entire historical period, and this lull probably goes back on the order of the last 1,000 years. We know at some point this lull we’re in will end.”

While some cities and government agencies have taken significant steps to protect infrastructure, such as ordering retrofits on older buildings, many others have not, putting pressure on seismic vulnerabilities that will eventually become apparent.

Here’s a look at some of the major fault systems near recent earthquakes that often feature more famous faults in the region:

Puente Hills thrust fault

Three-dimensional mapping of last week’s El Sereno quakes below the Earth’s surface found that they were just below the plane of the Puente Hills thrust fault, a terrifying fault that has received far less attention than the San Andreas but can generate a devastating tremor.

A magnitude 7.5 quake on the Puente Hills fault – which runs under highly populated areas of LA and Orange counties – could kill 3,000 to 18,000 people, according to the USGS and the Southern California Earthquake Center.

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

“This is a very large fault, located in the worst place you could imagine for a fault under LA,” said Dolan.

The Puente Hills thrust fault was discovered only recently — in 1999 — by John Shaw of Harvard University and Peter Shearer of the Scripps Institution of Oceanography at UC San Diego, who concluded that the 1987 Whittier Narrows earthquake, which killed eight, ruptured a small part. of this defect.

The Puente Hills thrust fault is of particular concern when it ruptures in its entirety because of what is on top of it – Downtown Los Angeles, which has many old and non-restored buildings, as well as a wide range in the southeast LA County, San Gabriel Valley and north. Orange County.

“This thing is huge,” Dolan said.

The fault is like an angled ramp deep underground – deepest along the Highway 210 corridor, where it is about 10 miles deep, and shallower about a mile south of USC, where it is 2 miles below the surface.

This orientation is particularly bad if the entire fault ruptures because the shaking would commonly start on the deeper side and move to the west – meaning that the shaking energy would likely be transferred from the suburbs of the foothills towards the town centre.

“Where is all that energy … going to end? It’s going to end up at the top of the ramp,” Dolan said. “That, unfortunately, is right in the heart of the LA metropolitan region.”

The shaking will also hit the edge of the Los Angeles Basin, a 6-mile-deep, bathtub-shaped hole in the bedrock that is filled with loose sand and gravel eroded from the mountains and forms a flat land inhabited by millions of people. The district stretches from Beverly Hills through southeastern LA County and into northern Orange County.

When earthquake energy is sent into these sedimentary basins, Dolan said, it increases the intensity of the shaking — perhaps 10 times worse than one would experience on bedrock — and causes shaking like a bowl of Jell-O, extending the duration.

“So, the Puente Hills thrust is a very dangerous fault for Los Angeles, in terms of its location and geometry,” Dolan said.

There is one silver lining: Unlike the San Andreas fault, which generates a major earthquake on average every 100 years or so, the Puente Hills thrust fault generates large quakes only every few thousand years, Dolan said.

Read more: No shaking

Compton thrust fault

Last week the Newport Beach-Costa Mesa earthquakes were near two fault systems, one of which is the Compton thrust fault.

The discovery of this defect in 1994, by Shaw of Harvard and John Suppe of Princeton University at the time, was once controversial, Dolan said, but now there is evidence that it not only exists, but that it is active.

“It’s had six earthquakes in the last 12,000 years or so,” Dolan said. “All these earthquakes were above magnitude 7.”

A major concern when the Compton thrust fault ruptures is that the center of the LA Basin would be pushed up and southwest. That would mean that the Los Angeles River, which runs from the mountains to the ocean, would start running back around the place where the fault raised the land.

“If you raise the downstream part of the LA River by 5 feet in one of these blind Compton earthquakes, well, the river will flow west,” Dolan said. So will all other plumbing systems that use gravity, from fresh water to sewage systems.

When major earthquakes occur on either the Puente Hills or Compton thrust faults, swaths of land will be suddenly jutted skyward, and the lift will create a trail of destruction perhaps 100 feet wide and 30 miles long. A similar zone of destruction – resulting in what is known as a “folded scarp” – was seen after a magnitude 7.7 earthquake struck Taiwan in 1999.

“Think about what happens to every single building and every piece of infrastructure that’s built across that 30-mile stretch, where things are going to be permanently out of step by several degrees,” Dolan said. “Every building built across that has to be torn down.”

The Newport Beach-Costa Mesa earthquakes also occurred near the Newport-Inglewood fault, which caused the 1933 Long Beach earthquake.

Read more: California has been hit by two series of earthquakes. What is going on along the Mexican border?

Transverse Range thrust fault system

The Ojai Valley earthquake on May 31st was in the same general area as the magnitude 5.1 quake on August 20th – significant because it struck on the same day that Southern California was preparing for the arrival of Tropical Storm Hilary.

Both quakes occurred along the Trans-Rain thrust fault system — which Dolan describes as “a very complex system of large east-west faults” that built the mountains in the area. The system begins at Cajon Pass and extends westward through the San Gabriel Mountains, Santa Monica Mountains, Topatopa Mountains and Santa Ynez Mountains all the way to Point Conception, west of Santa Barbara.

The Channel Islands are the top of mountain ranges that are now mostly underwater but were built by large thrust faults across the Trans-Rain system.

About 10 years ago, San Diego State professor Tom Rockwell and his colleagues made a remarkable discovery: About 900 years ago, a large earthquake on the Trans-Range system raised the beach around Pitas Point – between Ventura and Santa Barbara – almost 30 feet. That kind of activity can only be explained by “very large earthquakes, typically much greater than magnitude 7.5 and possibly greater than magnitude 8,” Dolan said.

And it wasn’t just Ventura-Pitas Point’s fault that could do that; that fault was too small. The only way to explain such massive displacement is for other faults to connect “to generate earthquakes that would be much larger than they would rupture on their own,” Dolan said.

Read more: Are you oblivious to the LA earthquakes? Here’s why you might be a ‘never-feeler’

LA market with tectonic forces

The beauty of Southern California is also shaped by the same seismic forces that generate earthquakes.

The San Andreas fault, with its odd bend north of the Los Angeles area and the movement of the Pacific plate relative to the North American plate, has contributed to the creation of many east-west mountain ridges, including the Santa Monica Mountains.

Earthquake faults also help explain why LA is today — one of the largest cities in the world not centered on a navigable waterway.

In the oldest part of Los Angeles, there used to be a source of fresh water directly above the old town. Its location, Dolan said, was caused by the Hollywood fault, which forced otherwise hidden groundwater to flow up and over a bedrock ridge, a major perennial source of fresh water.

Quakes helped make LA what it is today, but they also threaten its future. And all three thrust fault systems near last week’s earthquakes are sure to rupture again. Seismic lulls like the one we’re experiencing now tend to “end up with clusters of big quakes,” Dolan said.

“We know that these large faults have produced very large earthquakes in the past, and they will do so again in the future,” he said.

“These small earthquakes we’ve had next to these major urban faults are a useful reminder that we need to prepare ourselves for the inevitable future seismic storm, the likes of which hasn’t happened in LA in at least a thousand years.”

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This story originally appeared in the Los Angeles Times.

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