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Most of the quakes occur in a relatively constant fashion throughout the year. But sometimes earthquakes are concentrated into a particularly short time period. Such a series of earthquakes is called a “swarm.” Scientists think these swarms may be caused by underground movement of magma and movement of hot water and gases beneath the ground.

When the movement of the caldera switched from uplift to subsidence in 1985-86, Yellowstone experienced the largest recorded earthquake swarm so far—over 3,000 tremors in a period of three months. In April 2004 there was another swarm of earthquakes in the northwestern part of the park.

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