The Ridgecrest earthquake in July was the strongest to strike Southern California in 20 years. And the main quake, along with more than 100,000 aftershocks, caused a major fault to move for the first time, researchers say.
The movement attributed to the quake is less than an inch along the surface of the fault and would be virtually undetectable to an ordinary resident. But it has intrigued researchers for two reasons. They’re not clear what it means, and they’ve never seen this particular fault move, said Zachary Ross, assistant professor of geophysics at Caltech and an author of a study of the fault published Friday in the journal Science.
Researchers from Caltech and NASA recognize the fault shift as creeping. The phenomenon, though, usually occurs without an earthquake, according to the US Geological Survey.
The findings come as scientists continue to warn that the “Big One” — the monstrous earthquake that could potentially level populous Southern California — is overdue.
Remember the July earthquakes? There were 110,000 aftershocks
It started on July 4, when a 6.4-magnitude foreshock struck Southern California, the study said. That gave way 36 hours later to a 7.1-magnitude main shock, sending tremors as far as Arizona and Nevada. The area’s low population density saved it from substantial damage, though nearby naval facilities require billions of dollars to repair.
The main shock ruptured just miles from the Garlock fault, a major fault line that runs more than 180 miles from the San Andreas Fault to Death Valley. It has remained relatively dormant until now and has slipped 2 centimeters since the July quakes, researchers found.
Combining advanced seismometer data with satellite imaging of fault ruptures, the team tracked more than 110,000 aftershocks in the surrounding area over 21 days. The “dominoes-like sequence of ruptures” put heavy strain on the Garlock fault, researchers said.
Southern California has seen a few triggered creeps before, he said. The southern end of the San Andreas fault started creeping after the 7.2-magnitude earthquake in 2010 just south of the US-Mexico border. The creeping then didn’t lead to a significant earthquake.
What does all this mean for the ‘Big One?’
Californians have long feared the “Big One.” But the study proves how much is still unknown about earthquakes and how difficult they are to predict.
The USGS says there is a 31% chance that a 7.5-magnitude earthquake will strike Los Angeles sometime in the next 30 years. LA County is home to more than 10 million people, according to the census.
The southern San Andreas Fault typically sees massive quakes every 150 years. The last one occurred there in 1857, which means that segment is considered a “likely location for an earthquake.”
But the 7.9-magnitude quake in San Francisco in 1906 means there’s a slightly lower chance of a major quake in the northern segment of the fault.
Seismologist Lucy Jones tweeted that there’s a 2% chance of the Big One occurring each year, and a 1 in 20,000 chance every day.
The “Big One” is impossible to predict. So, the USGS urges California residents to stay prepared, retrofit homes to make sure they hold up during a quake, and keep a healthy supply of food, water and fuel in case of an emergency.