Yellowstone is the most seismically active area in the Intermountain West. Approximately 1,000 to 3,000 earthquakes occur each year in the Yellowstone area; most are not felt. They result from the enormous number of faults associated with the volcano.
Earthquakes occur along fault zones in the crust where forces from crustal plate movement build to a significant level. The rock along these faults becomes so stressed that eventually it slips or breaks. Energy is then released as shock waves (seismic waves) that reverberate throughout the surrounding rock. Once a seismic wave reaches the surface of the Earth, it may be felt. Surface waves affect the ground, which can roll, crack open, or be vertically and/or laterally displaced. Structures are susceptible to earthquake damage because the ground motion is usually horizontal.
In Yellowstone, earthquakes help to maintain hydrothermal activity by keeping the “plumbing” system open. Without periodic disturbance of relatively small earthquakes, the small fractures and conduits that supply hot water to geysers and hot springs might be sealed by mineral deposition. Some earthquakes generate changes in Yellowstone’s hydrothermal systems. For example, the 1959 Hebgen Lake and 1983 Borah Peak earthquakes caused measurable changes in Old Faithful Geyser and other hydrothermal features.
Sometimes Yellowstone experiences an “earthquake swarm”—a series of earthquakes over a short period of time. The largest swarm occurred in 1985, with more than 3,000 earthquakes recorded during three months. Hundreds of quakes were recorded during swarms in 2009 (near Lake Village) and 2010 (between Old Faithful area and West Yellowstone). Scientists are confident these swarms are due to shifting and changing pressures in Earth’s crust, not to any change in volcanic activity.
Earthquakes help us understand the sub-surface geology around and beneath Yellowstone. The energy from earthquakes travels through hard and molten rock at different rates. Scientists can “see” the subsurface and make images of the magma chamber and the caldera by “reading” the energy emitted during earthquakes. An extensive geological monitoring system is in place to aid in that interpretation.