Winter weather brings seismic tremors. A new study reveals how water buildup and runoff throughout the year can increase stress along faults in California, triggering small earthquakes. “This kind of observation is extremely important to constrain our models of earthquakes,” says Jean-Philippe Avouac, a geologist at Caltech who was not involved in the study. Improved models could ultimately help scientists better forecast seismic activity. Snow and rain compress mountain ranges in Northern California several millimeters during wet winter months. But with the weight of the water gone during the dry summers, the landscape lifts back up.