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The ground beneath Southern California has been particularly unsteady as of late, with the region experiencing more moderate-sized earthquakes this year than it has in decades.

What precisely is fueling the sequence of shakers is not entirely clear, and officials warn that prior seismic activity does not necessarily mean more powerful temblors are imminent. But the series of modest shakers have many wondering what is going on.

"Earthquakes pop off around the state, and it's a little bit like popcorn that they hit—sometimes they bunch up for reasons that we don't understand," said Susan Hough, seismologist for the U.S. Geological Survey.

Where the quakes are

By the count of seismologist Lucy Jones, a Caltech research associate, Southern California has felt 15 independent seismic sequences this year, with at least one magnitude 4 or higher earthquake. That's the highest annual total in the last 65 years, surpassing the 13 seen in 1988.

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