Why Central Coast California is the earthquake capital of the world

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Read more from Life on San Andreas: 4-part series

This is a 4-part series on life in Parkfield, California, the science of its seismology, and its distinction as the “Seismic Capital of the World.”

Little Parkfield, California, has not earned the title by being hit by the biggest or even the most earthquakes, the “earthquake capital of the world.”

It has earned the distinction of being located on top of a unique section of the San Andreas Fault that consistently produces a strong earthquake every two decades. Over the past 164 years, the Central Coast town in the southern province of Monterey has experienced a record seven earthquakes of magnitude 6.0.

This seemingly predictable pattern impressed geologists, who flocked to the area in the 1980s and 1990s hoping to catch an earthquake as it hits. But the error prevailed, and the next big earthquake didn’t arrive until 2004.

“This is a place that has probably been studied more intensively than anywhere else in the world,” said Justin Rubinstein, a seismologist with the USGS.

Over the years, scientists have sampled, drilled, measured, and prepared San Andreas in this area, watching for any indication it might rock again.

While they wait, they learn how earthquakes work.

How do faults cause earthquakes?

Earth’s outer layer is a mixed mess – cracked and deformed rocky crust, stretched and compacted over millions of years. Its jigsaw pieces slowly groan on top of each other along fissures that stretch miles below the surface.

Although the imperfections are fairly straight, they are by no means smooth surfaces. Land masses do not generally slide gracefully over each other.

Bumps on cracked surfaces generate friction, making them sticky.

The bug basically works like two pieces of coarse sandpaper. Slowly rubbing those high-friction surfaces together results in a jerky slip between the pieces.

The constant force of the changing Earth’s crust causes the rocks on either side of the fault to move intermittently when this force overcomes the friction, and each time it results in an earthquake. The more the fault surface area slips, the larger the earthquake.

When friction is high, faults are held in place between earthquakes.

But fault surfaces sometimes contain very weak minerals – often the byproduct of a chemical reaction between water and some rocks.

Like a sprinkle of talcum powder, these minerals soften the fault and allow the rocks to slowly “crawl” over each other without causing earthquakes.

San Andreas has locked and crawler sections along an 800-mile stretch through California.

San Andreas is actually many flaws

North of San Juan Bautista, San Andreas is mostly closed.

The last time it subsided significantly was in the 1906 San Francisco earthquake. The 7.9 magnitude earthquake and subsequent fires destroyed much of the city.

In that earthquake, the fault slid 32 feet over a 296-mile stretch from San Juan Bautista to Cape Mendocino, according to the USGS.

The 1906 San Francisco earthquake and subsequent fires destroyed much of the city, as seen in this photo of the city from Sacramento Street near Powell Street toward the bay. (Collection of the Court House Museum)

San Andreas is also closed south of Parkfield in Southern California.

In 1857, a total of 225 miles of fault—from South Parkfield to Cajon Pass on Highway 15—slid down during the Fort Tejon earthquake. Researchers believe that this 7.9-magnitude earthquake may have started near Parkfield and moved south as the sloping area grew.

Between these two sections, the San Andreas River crawls at a speed of approximately one inch per year. It doesn’t produce really big earthquakes that hit the locked sections.

This does not mean that the entire section cannot slip in a major earthquake. “We don’t know for sure. It’s not likely that anything will happen,” said Bill Ellsworth, a seismologist at Stanford University.

So, what’s the deal with Parkfield?

Ellsworth said Parkfield is at the southern end of the creeping section, a transitional zone between the closed and creeping fault.

Here, he said, the error is like a vertical chessboard made up of tiny, creeping, hanging spots. South of Parkfield, the rift is a giant stuck spot.

Scientists are still trying to figure out why San Andreas behaves this way.

“We think geology is the key control,” Ellsworth said.

When the fault is strong, it is locked. When weak metals reduce friction on its surface, the bug crawls.

The presence of serpentinite – the state rock of California – appears to dictate where the fault is weak, according to the USGS and several scientific publications.

Part of the drilling core was taken from the San Andreas Fault deep in the Earth. Green rock is a serpentine that reacts with other rocks to form weak minerals that make the fault slide. EarthScope Repetition of earthquakes on a chessboard

In the small stuck patches, scientists have observed thousands of earthquakes of about magnitude 2 that repeatedly strike the same enclosed area every two to three years, Ellsworth said.

He added that the size of the patch controls the exact size of the earthquake.

“We think the Parkfield Main Shock (magnitude 6.0) is one of those frequent earthquakes that happen every 25 years or so,” Ellsworth said.

This incomplete region is somehow fractal, the Stanford researcher said. Small chessboard squares form large chessboard squares, which form larger squares.

However, these larger earthquakes are not exact copies of each other, as smaller repeaters seem, but share many of the same characteristics.

The frequency of large and small earthquakes isn’t unique to the Parkfield area, Ellsworth said, but it doesn’t happen everywhere.

“In general, we see replicates in areas where we are seeing active encroachment,” Rubinstein said.

Parkfield Café is the city’s only restaurant and describes the city’s reputation as the “earthquake capital of the world”. David Middlecamp [email protected] Parkfield earthquakes are regular, but unpredictable

Until recently, 6.0-magnitude earthquakes near Parkfield struck fairly regularly — about every 22 years. The most recent one, in 2004, came a full 38 years after the previous year. Attempts by the US Geological Survey to predict the earthquake that were scheduled for around 1988 failed.

“I think the last one showed us that it’s not as regular as we really thought,” Rubinstein said.

Despite extensive monitoring of the area, no single instrument had recorded a signal of an impending earthquake prior to the 2004 earthquake. In this fault, the most studied fault in the world, earthquake prediction remains elusive at this point.

But scientists are still learning from the hundreds of small earthquakes that rock the region each year as they continue to monitor the fault.

Data and rock samples collected from a major drilling experiment in the early 2000s led scientists to trace fault creep to weak minerals along the fault surface.

They learn how earthquakes start, how they trigger, and how the forces causing earthquakes are distributed.

“Now that we have this huge amount of observations. It would be a huge loss if you didn’t keep digging in there,” Rubinstein said.

Related stories from the San Luis Obispo Tribune

Jane Schmidt is an AAAS Fellow covering natural hazards and science news. PhD completed. She received her PhD in Earth Sciences at Lehigh University in 2018. She is an NSF-sponsored Postdoctoral Fellow at Temblor Inc. , where it operates Temblor Earthquake News.

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