Early in 2016, without any fanfare, a swarm of earthquakes silently revved up in Southern California. The quakes didn’t occur every day, and most were too small for humans to feel, but month after month the tremors continued to rumble and multiply. By the spring of 2018, thousands of little quakes were striking each month, some large enough to set lamps swaying and put residents of nearby towns on edge. Over the past four years there have been more than 22,000 temblors. Yet the source behind all this activity has been a mystery.

Now, in one of the highest resolution looks at a seismic swarm yet, scientists have zeroed in on a likely cause. The results may help geologists