YELLOWSTONE NATIONAL PARK, Wyo. -- A swarm of more than 70 small earthquakes shook the ground near Old Faithful geyser earlier this month.

The Yellowstone Volcano Observatory says the largest was a magnitude 2.4, barely enough to be felt. The swarm of 74 quakes lasted several hours on October 14th.

The tight cluster of earthquakes was moderate compared with others in Yellowstone's past. In April 2004, more than 400 earthquakes were recorded over three days.

Park officials says the earthquakes this month were more likely caused by the underground movement of hot water and gas, rather than migration of magma.

The largest swarm recorded in Yellowstone was in the fall of 1985. About 1,800 earthquakes, ranging in magnitude from 1 to 4.9, shook the park.

Around the same time, a huge caldera stopped slowly rising, and began slowly falling.