A geophysicist says an earthquake swarm that occurred under Yellowstone Lake a month ago was the second most intense swarm on record in Yellowstone.

More than 800 earthquakes occurred between Dec. 26 and Jan. 8. Many of those quakes were too small to be felt. Even so, University of Utah geophysicist Robert Smith says it was the most intense swarm recorded in Yellowstone since a swarm that rattled the West Yellowstone area in 1985.

Smith and his team have identified 70 to 80 well-defined earthquake swarms in Yellowstone since 1984.

The strongest quake in the recent swarm was a 3.9. No damage was caused and not many people were in the park to feel the larger quakes.

• Click here to visit FOXNews.com's Natural Science Center.

"It's really died down," Smith said of the recent event. "That's one of the surprising things to me. The swarms usually last much longer."

Smith theorizes the quakes were caused by hydrothermal fluids expanding along a fault zone. A similar event is thought to have caused the 1985 swarm.

Smith said the recent swarm is helping scientists understand how tectonic and volcanic forces can work together during an earthquake, and how earthquakes can interact with one another over long distances.

Scientists wonder, however, how the swarm might affect the park's thermal features.

The earthquakes could even change the routine rise and fall of the Yellowstone Caldera. The caldera, a 37- by 25-mile volcanic feature at the center of the park, rests upon a magma plume that extends roughly 400 miles beneath the Earth's surface.

The 1985 swarm coincided with the start of several years of caldera subsidence. The caldera had been gradually rising for decades before 1985.