Nervous Californians are again watching their feet as a powerful earthquake zone begins to stir after a 500-year sleep.

Earlier this year, a rapid staccato of earthquakes pulsed through south and central California. Most were in a dry, mountainous area some 200km north of Los Angeles.

The Ridgecrest fault is no stranger to activity. This year it's been especially busy, producing tens of thousands of quakes since July. Among them were the largest in Southern California for two decades.

But the nature of this sequence of events drew the attention of US geologists.

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A study released in the journal Science late last week reveals why.

The shot-gun effect of this spate of shakes appears to have destabilised other nearby faultlines. Most significant is the long-dormant Garlock fault, which runs some 250km along the northern Mojave Desert between the Sierra Nevada mountain range and Death Valley.

Satellite radar imaging reveals it is starting to bulge.

"This is surprising because we've never seen the Garlock fault do anything. Here, all of a sudden, it changed its behaviour," the lead author of the study, Caltech assistant professor Zachary Ross, said in a statement.

"We don't know what it means."

But Garlock's destructive potential is known.

During its 500-year-long slumber, it's built up the potential for an earthquake of magnitude 8 to tear through densely inhabited areas.

A person looks at an earthquake warning application on their phone in Los Angeles. Photo / AP
A person looks at an earthquake warning application on their phone in Los Angeles. Photo / AP

It may even have the potential to spur "The Big One", a release of the enormous San Andreas Fault running through the heart of California.

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DEMON UNCHAINED

The epicentres of Ridgecrest earthquake swarm push up against its southern edge where it intersects the Garlock fault line.

On July 5, Ridgecrest let loose a 7.1 event. This tore through 55km in just 22 seconds — following an unusual crisscrossing pattern of previously unidentified faults. And the split ended only a few kilometres from the Garlock line.

"We actually see that the (quake swarm) simultaneously broke faults at right angles to each other, which is surprising because standard models of rock friction view this as unlikely," Ross says.

"(It) ended up being one of the best-documented earthquake sequences in history and sheds light on how these types of events occur."

The US Geological Survey now warns the Ridgecrest swarm has raised the chances of a 7.5 or higher magnitude quake in the nearby Owens Valley, Blackwater, Panamint Valley — and Garlock — faults. And one could come within the next year.

If Garlock lets loose, it could send shockwaves through heavily urbanised areas, including San Fernando Valley — part of Los Angeles County — and home to 1.8 million people.

US geologists are scrambling to make sense of what's happening along California's complex web of faultlines.

Lives and billions of dollars of infrastructure are at stake.

One possible explanation is that the Ridgecrest quakes have released pressure on a big block of the earth to the north of Garlock.

That pressure may have previously provided a "clamp-down" effect to keep Garlock dormant.

But this park-brake has now been released.

Radar images show some 30km of the sleeping giant is on the move for the first time in 500 years.

TRIGGER EFFECT

Any big quake along the Garlock fault has the potential to sever vital water aqueducts, critical roads and flatten critical facilities such as Edwards Air Force Base.

But geologists warn that may not be the worst of it.

Garlock itself runs up against another, more significant, fault line.

The infamous San Andreas Fault.

And, just as the recent Ridgecrest quakes could strain Garlock, Garlock may produce a similar effect on San Andreas.

A big quake may not necessarily be coming, geologists advise. Historically, while fault lines may "creep" due to nearby shocks, they are only rarely triggered.

But the awakening of Garlock has their attention.

It's not following the established sequence of events.

Usually, significant quakes of 7 and above are caused by the release of single long fault lines. And it's the length of that line that determines the maximum size of quake it can produce.

But the Ridgecrest quake cluster and its domino-effect presents an alternate scenario:

Small faults can link-up in a web-like network, and between them set-off an unexpectedly powerful event.

"Over the last century, the largest earthquakes in California have probably looked more like Ridgecrest than the 1906 San Francisco earthquake, which was along a single fault," Ross said.

"It becomes an almost intractable problem to construct every possible scenario of these faults failing together — especially when you consider that the faults that ruptured during the Ridgecrest Sequence were unmapped in the first place."

BETWEEN THE LINES

While Garlock has drawn relatively less attention than its more restive relatives, its known history suggests a highly erratic monster may lurk beneath.

USGS research places a 7.0 or higher quake in the central Garlock fault, on average, every 1200 years.

But averages can be deceiving.

The fine print reveals the line can let loose after 2000 years, or just 200.

And researchers believe the last time Garlock released a big quake was some 500 years ago.

Geologists say more research is needed to understand what is happening.

Is the Garlock bulge being caused by surface movements? Or is something stirring much further below?

These questions hold the key as to how much pent-up seismic energy needs to be released.

"It's going to force people to think hard about how we quantify seismic hazard and whether our approach to defining faults needs to change," Ross says.

"We can't just assume that the largest faults dominate the seismic hazard if many smaller faults can link up to create these major quakes."