Science2016-2017Southern California, USAlow confidence

The Cahuilla earthquake swarm began in 2016 but went unnoticed until 2017, when a citizen emailed the Southern California Seismic Network about a cluster of tiny earthquakes.

Notes on verification

Confirmed by National Geographic's original reporting and independently corroborated by Smithsonian Magazine, Science News, and a National Academies article quoting the lead researcher directly.

Sources