Hundreds of small earthquakes have shaken desert towns east of San Diego in the last few days and geologists are hoping to learn more about why these “earthquake swarms” happen.

Most Californians are used to the typical earthquake pattern: one big quake followed by smaller aftershocks. Earthquake swarms happen in places where there’s hydrothermal activity - hot water moving under the Earth’s crust. "That would explain why you don’t just have one main shock. You have constantly moving fluids that can be constantly triggering earthquakes," says USGS seismologist Jeanne Hardebeck.

Earthquake swarms also happen in an area called the Geysers, 75 miles north of San Francisco. USGS hopes this swarm outside of the town of Brawley will help them understand how earthquake swarms work. "Some of the earthquakes in the swarm are triggering other earthquakes in the swarm and we’d really like to understand better how that happens," says Hardeback.

This weekend's desert earthquakes damaged some property. Swarms can last from several days to a week.