An overlooked San Andreas Fault strand could be the next big one

Part of the San Andreas Fault in southern California may be on shakier ground than previously thought.

The 800-mile-long San Andreas Fault is one of the more famous faults in the United States, known as the cause of the Great 1906 San Francisco earthquake. But the southern part of the fault has not had a major earthquake since the 18th century.

However, a new study suggests one part of the fault, east of Los Angeles, is accumulating a lot of strain. In seismology terms, this means it may be the next location of a large earthquake sometime in the near future. Although it is impossible to say exactly when it will occur, knowing where to expect it to burst to the surface is good news for geologists and hazard planners.

This finding was published Wednesday in the journal Science Advances.

What’s new — The research was led by Kim Blisniuk, an associate professor of geology at San Jose State University. Bilsniuk and colleagues calculated what is known as the slip rate, or how fast two sides of a fault are moving relative to each other, for an area of the San Andreas Fault in southern California.

It turns out a specific part of the fault, the Mission Creek strand, has accumulated between 6 to 9 meters worth of elastic strain. This means it has the potential to snap and release that strain, where it will become the site of a large earthquake. This strand runs from Indio to the San Bernadino Mountains.

It was previously thought the Mission Creek strand had been slipping slowly or was completely inactive, Blisniuk tells Inverse.

“This study shows that the Mission Creek strand is actually the main one producing most of the earthquakes in the geologic past,” she says.

Some background — The San Andreas Fault is the boundary between the Pacific plate to the west and the North American plate to the east.

The San Andreas Fault isn’t one single line, it is a wide fault zone, says Wendy Bohon, a geologist and science communication specialist for the Incorporated Research Institutions for Seismology who was not involved in this study.

“It's made up of different strands, and these different fault strands each accommodate part of the motion between the Pacific and North American plates,” Bohon tells Inverse. The Mission Creek strand of southern California is one of these strands in the fault zone.

Through paleoseismic study, geologists can tell the last major earthquake in the southernmost area of the San Andreas Fault was around the year 1726. While a fault can’t really be “overdue” for an earthquake, it has been a long time since an earthquake has occurred on this one, Bohon says. This could mean a rupture is imminent.

“And the longer it is between earthquakes, the more time you have for strain to accumulate, which means the more likely an earthquake is to occur,” Bohon says.

How they did it — To figure out the slip rate of fault, the researchers had to spend years dating the motion of ancient earthquakes. They used field mapping and geochronological dating methods to figure out the movement of the fault strands in the Coachella Valley near the Indio Hills.

Mapping and dating rocks and soil from ancient river channels allowed them to determine how much the faults have moved over a certain period of time, so they can calculate slip rates. This helps understand which fault strands are moving and likely to produce a future earthquake.

What they found — The Mission Creek strand appears to be much more important to earthquakes in the area than previously thought. For the past 100,000 years, Blisniuk says, “you have the most earthquakes on this particular strand.”

The slip rate of the fault strands that were studied, the Mission Creek strand and the Banning strand, are around 22 and 2.5 millimeters per year, respectively. This means the Mission Creek strand is bearing the brunt of the strain accumulated from the motion of the Pacific and North American plates.

Why it matters — This much slip and accumulated strain on the Mission Creek strand means a future earthquake is likely to rupture on it, says Blisniuk.

The Mission Creek strand is dangerously close to a lot of people and infrastructure, including water and power lines that feed the Los Angeles area. California takes earthquake hazard planning seriously, and this new study gives a “better location to where to expect it if the past is the key to the present,” Blisniuk says.

“That said,” she adds, “there is so much we do not know about earthquakes, so who knows what will happen when the big one hits.”

Understanding the motion of parts of this fault in southern California is critical for planning, says Bohon. This is the kind of research, she says, “that's the underpinning for those earthquake hazard models.”

“So every time I see something like this come out… [I think] we're getting one step closer to having better information and a better understanding of this huge, complicated fault system.”

Abstract: The San Andreas fault has the highest calculated time-dependent probability for large-magnitude earthquakes in southern California. However, where the fault is multistranded east of the Los Angeles metropolitan area, it has been uncertain which strand has the fastest slip rate and, therefore, which has the highest probability of a destructive earthquake. Reconstruction of offset Pleistocene-Holocene landforms dated using the uranium-thorium soil carbonate and beryllium-10 surface exposure techniques indicates slip rates of 24.1 ± 3 millimeter per year for the San Andreas fault, with 21.6 ± 2 and 2.5 ± 1 millimeters per year for the Mission Creek and Banning strands, respectively. These data establish the Mission Creek strand as the primary fault bounding the Pacific and North American plates at this latitude and imply that 6 to 9 meters of elastic strain has accumulated along the fault since the most recent surface-rupturing earthquake, highlighting the potential for large earthquakes along this strand.