Forget San Andreas Fault; Biggest Earthquake Threat Is On This Overlooked Part

As if the San Andreas fault wasn't scary enough, new evidence reveals that a portion of it travels much faster than scientists historically believed.

(Photo: David Paul Morris/Getty Images)
PARKFIELD, CA - SEPTEMBER 30: The Parkfield Coalinga bridge crosses over the San Andreas fault on the Parkfield Coalinga Road on September 30, 2004, Parkfield, California. The tiny central California town with a population of 19, which claims to be known as "The earthquake capital of the world," was hit with a 6.0 earthquake on Tuesday, September 28, 2004.

It's known as the Mission Creek strand, according to KCET.org. It stretches from Indio to the San Bernardino Mountains, passing across Desert Hot Springs.

San Andreas Fault: What Is It?

Geology said the San Andreas fault, which stretches almost the entire length of California, can generate a huge earthquake known simply as "the Big One."

According to Los Angeles Times, San Andreas is also a major border marker between the Pacific and North American plates. The two sides of the fault pass past one another as the plates move past one another.

However, friction traps the fault, and as the two sides turn, pressure builds up until it is freed by earthquakes. The southern San Andreas bears about half of the pressure caused by plate motion, which may be as much as 25 millimeters (about one inch) a year.

However, not every aspect of the fault is similarly strained. The San Andreas fault line in Southern California comprises several smaller "strands," making it impossible for earthquake experts to determine which areas of the fault system are more vulnerable to rupture.

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San Andreas Fault: Mission Creek Isn't Active

Kimberly Blisniuk, a geochronologist at San Jose State University and lead author on the study, this particular strand of the San Andreas fault has "not been very active."

"Therefore, it has the highest likelihood of a large magnitude earthquake to occur on it in the future," Blisniuk added.

Experts believe Mission Creek's slip rate was about 14 millimeters per year. Still, the study, titled "A Revised Position for the Primary Strand of the Pleistocene-Holocene San Andreas Fault in Southern California," claims that it is currently around 21.6 millimeters per year.

They found that the southern San Andreas fault accounted for the overwhelming majority of the burden at that point.

The Banning strand, on the other hand, had a slip average of just 2.5 millimeters per year, according to their estimates.

A few millimeters does not seem like much. Still, when giant tectonic plates are pressing up against each other, the tension adds up quickly.

Blisniuk claimed the southern San Andreas fault is likely to undergo ground-rupturing earthquakes at a rate of one every 215 years or so. The last one in the southernmost segment occurred in 1726.

Researchers said some six to nine meters of elastic strain have collected along the fault since the previous one. It implied that the earth would move about 20 to 30 centimeters when it eventually releases. It's unclear whether a single quake or a series of them is needed to travel that distance, according to Blisniuk.

San Andreas Fault: More Studies Needed

All of this suggests that this strand of the San Andreas is more dangerous than previously thought. What is the additional risk? Experts must evaluate it.

Given that offsets of up to 30 feet may occur in the event of a major quake, any utilities in that region, such as water or gas lines that pass over the fault itself, would need to be examined with caution.

We know that large earthquakes on the San Andreas are likely, regardless of what occurs on the Mission Creek strand.