A swarm of more than 70 small earthquakes shook the ground near Old Faithful geyser earlier this month. The largest was a magnitude 2.4, barely enough to be felt.
The swarm of 74 quakes lasted several hours Oct. 14, according to information released by the Yellowstone Volcano Observatory.
The tight cluster of earthquakes was moderate compared with others in Yellowstone's past, including one in April 2004 in which more than 400 earthquakes were recorded over three days.
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"It piques our curiosity, but it's not out of the range of normal behavior," Henry Heasler, Yellowstone's principal geologist, said of the Oct. 14 activities.
Park officials said the earthquakes were more likely caused by the underground movement of hot water and gas, rather than the migration of magma.
The largest swarm recorded in Yellowstone was in the fall of 1985 when about 1,800 earthquakes were registered, ranging in magnitude from 1 to 4.9.
Around the same time, the huge Yellowstone caldera stopped slowly rising and began slowly falling.