Experts have for years said that the Big One — an earthquake with a magnitude of 6.7 to 8 or higher — is long overdue in urban Southern California. When it comes, it will be bad: A local 7.8 shaker, the U.S. Geological Survey has estimated, could result in 1,800 deaths, 50,000 injuries, 300,000 damaged buildings, 1,600 fires and perhaps more than $200 billion in total damage.

But new research suggests it could be even worse than that. A study by the Scripps Institution of Oceanography at UC San Diego published in the journal Science says the Big One could trigger a series of aftershocks on separate faults. The effect of those temblors could combine to create a massive “mega-quake,” the scientists concluded.

“These findings have important implications for earthquake hazard–prone regions like California, where ruptures on complex fault systems may cascade and lead to mega-earthquakes,” according to a summary of the research.

Last year, Scripps geophysicist Peter Shearer and Scripps graduate student Wenyuan Fan looked at 48 previously unexamined aftershocks over an 11-year span. Those global quakes were triggered by magnitude 7 to 8 “mainshock ruptures,” they stated.

One magnitude 7 earthquake in Indonesia triggered two large aftershocks more than 120 miles away — the distance between San Diego and L.A. “These aftershocks miles away reveal that stress can be transferred almost instantaneously by the passing seismic waves from one fault to another within the earthquake fault system,” the researchers stated.

There's more bad news: The study also determined that a massive earthquake could trigger aftershocks for months after the main event. The findings could have deep implications for SoCal and the Bay Area. Both regions exist near the Big One–producing San Andreas earthquake fault, and both have a series of smaller, active faults.

“The results are particularly important because of their seismic hazard implications for complex fault systems, like California,” said Fan, lead author of the study. “By studying this type of triggering, we might be able to forecast hosting faults for large earthquakes.”

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