Strong earthquakes along the San Andreas fault in southern California are more frequent than previously thought, so the dreaded "Big One" could be just around the corner, US researchers said in a study.
University of California at Irvine and Arizona State University scientists examined the geological record stretching back 700 years along the fault line 160 kilometers (100 miles) northwest of Los Angeles.
They found that strong earthquakes -- between 6.5 and 7.9 magnitude -- shook the area every 45-144 years, instead of the previously established 250-400 years.
Since the last big 7.9 magnitude earthquake struck southern California in 1857, or 153 years ago, scientists believe the next "Big One" could happen at any time.
The scientists on Friday provided an abstract of their study, which will be published in full in the September 1 issue of the magazine Geology.
"What we know is for the last 700 years, earthquakes on the southern San Andreas fault have been much more frequent than everyone thought," said the study's lead author Sinan Akciz.
"Data presented here contradict previously published reports," he added.
With 37 million people living in southern California, chiefly in the cities of Los Angeles, San Diego and Anaheim, a major earthquake could kill between 2,000 and 50,000 people and cause billions of dollars in damage, scientists said.
UCI seismologist Lisa Grant Ludwig, the study's chief investigator, said people in the area should already be taking precautions.
"There are storm clouds gathered on the horizon. Does that mean it's definitely going to rain? No, but when you have that many clouds, you think, "I'm going to take my umbrella with me today." That's what this research does: It gives us a chance to prepare," she said.
For individuals, that means having ample water and other supplies on hand, safeguarding possessions in advance, and establishing family emergency plans.
For regulators, Ludwig advocates new policies requiring earthquake risk signs on unsafe buildings and forcing inspectors in home-sale transactions to disclose degrees of risk.
Some things, she added however, remain unpredictable, especially Los Angeles' troublesome highway grid, which in the best of times gets hopelessly choked in traffic.
Ludwig said the new data "puts the exclamation point" on the need for state residents and policymakers to be prepared.
Study co-author Ramon Arrowsmith stuck a more optimistic note.
"While earthquakes may be more frequent, they may also be smaller (starting at 6.5 magnitude). That's a bit of good news to offset the bad," said the geology professor at Arizona State.
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