The small quakes include three more Friday that measured stronger than magnitude 3.0. The University of Utah Seismic Stations say the strongest was 3.5. Several hundred quakes centered under the northern end of Yellowstone Lake have now occurred since Dec. 26. No damage has been reported.

Earthquake swarms happen fairly often in Yellowstone. But scientists say it's unusual for so many earthquakes to happen over several days. Yellowstone lies mostly in northwestern Wyoming and is the caldera of a volcano that last erupted 70,000 years ago. Scientists have not concluded what is causing the earthquakes.

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