The South Napa earthquake that struck northern California on Sunday has cracked open new fears among state residents that “the big one” is imminent. The 6.0-magnitude earthquake was the largest to strike the region in 25 years, and offers a painful reminder that the San Andreas Fault is overdue for a powerful rupture. Experts caution that it’s impossible to predict exactly when the big one will occur, but there’s a good possibility that it could happen within the next three decades.

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“The area surrounding the epicenter of the main shock is continuing to experience a number of aftershocks,” the U.S. Geological Survey notes as part of an ongoing effort to monitor aftershocks in and around Napa Valley. “As of Tuesday Aug. 26, 4 PM PDT, there have been more than 80 aftershocks,” the agency adds. But “only four of these have had magnitudes greater than 3.”

The U.S. Geological Survey warns that there is a 12 percent chance that a “strong and possibly damaging aftershock” that is M5 or greater could hit the same region within the next seven days. But Sunday’s earthquake and subsequent aftershocks have not taken place on the San Andreas Fault that fills Californians with such dread.

This earthquake was caused by the West Napa Fault, which is like a “lesser version of the San Andreas Fault,” Thomas Heaton, professor of geophysics at the California Institute of Technology, told NBC News.

Still, there is cause for concern.

Related: 6.0-magnitude Earthquake Hits Northern California

Segments of time occur when no large earthquakes take place on major plate boundaries like the San Andreas Fault, according to another U.S. Geological Survey report. These lapses are called “seismic gaps” and can be used to loosely forecast impending catastrophic earthquakes.

“Geologic studies show that over the past 1,400 to 1,500 years large earthquakes have occurred at about 150-year intervals on the southern San Andreas fault,” the authors write. The last major earthquake on the southern end of the San Andreas Fault occurred in 1857, or 157 years ago.

Despite the likelihood that we will see a major earthquake along the San Andreas fault during our lifetime, at this point, California’s historic drought is probably more likely to cause an exodus of residents than an earthquake.

Via NBC News

Golden Gate Bridge image via Shutterstock / Napa Valley image via Shutterstock