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Yellowstone Swarm (Wyoming)

December 26, 2008 – January 8, 2009 Series; Largest: M3.9 Earthquake

Available Information


Beginning on December 26, 2008, a swarm of over 900 earthquakes occurred within the very large (34 miles wide by 45 miles long) Yellowstone Caldera. The largest occurred in a remotely populated area of the park (which is closed for the winter), with a magnitude of 3.9. A second smaller earthquake swarm began on January 9, 2009, with a maximum magnitude of 3.2. The map indicates the first swarm started in Yellowstone Lake and migrated towards the north shore. Earthquake swarms occur nearly every year at Yellowstone Park, although the last time a swarm was this energetic was in 1985.

Increased seismic activity in the Yellowstone area is of great concern because some of the largest volcanic eruptions on Earth have come out of the Yellowstone Caldera. The most recent was about 640,000 years ago and spewed ash over much of what is now the United States. There have been a number of volcanic eruptions since then with large lava flows.

The Yellowstone hotspot is an area where basaltic magma rises from the mantle into the Earth’s crust. Partially molten rock exists at least four miles beneath the surface, releasing heat that fuels the geysers and other thermal features in the park. Earthquakes in the area occur due to normal faulting (such as the devastating 1959 Hebgen earthquake), the movement of magma, and hydrothermal fluid activity.

The Yellowstone Volcano Observatory (YVO) was created in part to provide warnings of potential volcanic activity at Yellowstone. So far, no other geological activity has been observed accompanying this earthquake swarm. Notices from the YVO are available at: .