A different kind of shaking
Yellowstone is one of the most seismically active regions in the United States, experiencing 700 to 3,000 earthquakes annually. A significant portion of this activity, about 50%, occurs in the form of earthquake swarms. Unlike a standard earthquake sequence with a large mainshock followed by smaller aftershocks, a swarm is a flurry of many earthquakes, often small, concentrated in one area over a period of days, weeks, or even months.
The ground beneath Yellowstone is constantly in motion. The largest swarm ever recorded happened in 1985 on the northwest side of the park, with over 3,000 earthquakes logged over three months. More recently, a 2010 swarm between the Old Faithful area and West Yellowstone produced nearly 2,000 quakes. Most of these tremors are too weak for a person to feel, typically registering below magnitude 3.0. These quakes result from movements along the dense network of faults crisscrossing the region, driven by the volcanic and tectonic forces deep underground.
Water, rock, or magma?
The central question for geologists studying these swarms is what triggers them. There are two primary explanations, and the evidence often points toward water. The first, and most common, cause is the movement of hydrothermal fluids—hot water and steam through the fractured rock of the Earth's crust. As these pressurized fluids move they can lubricate pre-existing faults, causing them to slip and generate a series of small earthquakes. The vast plumbing system that feeds Yellowstone's famous geysers and hot springs is kept open by this constant seismic activity.
The second, more concerning, possibility is that the swarms are caused by the movement of magma. An intrusion of molten rock into the shallow crust would also stress the surrounding rock and trigger earthquakes. To distinguish between these causes, scientists at the Yellowstone Volcano Observatory (YVO) use a sophisticated network of seismographs and GPS stations. These instruments can detect ground deformation—subtle swelling or sinking of the land surface. While the ground at Yellowstone does "breathe," rising as much as 7 centimeters in a year (as it did starting in 2004), most earthquake swarms are not accompanied by significant ground deformation, suggesting a fluid-driven cause rather than a magmatic one.
The constant monitoring by the YVO, a partnership led by the U.S. Geological Survey, provides a real-time feed of seismic and deformation data. This allows scientists to analyze each swarm and assess whether it signals a change in the behavior of the supervolcano.