Caltech researchers say there is an elevated risk that a long-overdue earthquake, dubbed the ‘Big One,’ is on the way, after recent seismic activity shifted a previously-dormant fault line for the first time in 500 years.

California was rocked by several major earthquakes in July and August. The Ridgecrest earthquake in Southern California was the strongest in 20 years and caused roughly 100,000 aftershocks.

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All of the pressure from these tremors and aftershocks was so extreme that it apparently woke a sleeping giant in the form of the 185-mile-long Garlock Fault, causing it to ‘creep’ almost an inch (a big deal in geological terms if nothing else).

“This is surprising because we’ve never seen the Garlock Fault do anything. Here, all of a sudden, it changed behaviour,” said Zachary Ross, assistant professor of geophysics at Caltech and an author of a study on the fault.

There are now fears that if Garlock keeps creeping it may disturb the San Andreas fault and trigger the long-feared ‘Big One’ which could devastate the entire state and swallow cities like San Francisco and Los Angeles whole. The southern San Andreas Fault experiences major quakes roughly every 150 years, the last of which took place in 1857, meaning the fear of a potentially cataclysmic future megaquake grows every year.

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According to the USGS, there is a 31% chance that the ‘Big One’ (a magnitude 7.5 and above earthquake) will strike Los Angeles in the coming three decades. Roughly 10 million people live in LA county.

The spectre of the ‘Big One’ has loomed large ever since San Francisco was struck by a 7.9 earthquake in 1906 which killed an estimated 3,000 people.

For perspective, however, seismologist Lucy Jones claims there is a 2% chance of the ‘Big One’ striking each year, and a one in 20,000 chance each and every day that the long-anticipated temblor will finally churn the ground beneath California.

If the daily probability of the Big One was 50/50, then the chance it would happen in the next week would be >99%. Real probability is about 2% per yer, or 1/20,000 each day. Your change of being in a car accident today is ~1/7,000. I still wear my seat belt every day. https://t.co/3ovnAfiPFV — Dr. Lucy Jones (@DrLucyJones) July 3, 2019

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