The Yellowstone caldera was created by an enormous super-volcanic explosion 640,000 years ago. Ever since, the area has been the the site of earthquake activity. Indeed, a map of the historic seismicity of the region over the last 20 years shows swarms of dots much like so many busy bees.
On Monday, a swarm of nearly 500 earthquakes on the western edge of Yellowstone National Park began. Though they increased in frequency and intensity Tuesday, experts who monitor the area say they did not warrant emergency measures. A little over one year ago, there was some concern over a series of quakes centered around the north shore of Yellowstone Lake, which overlays some of the ancient, yet still simmering, Yellowstone Caldera.
The latest activity is further west, about 10 miles north of Old Faithful Geyser and nine miles from the gateway entrance town of West Yellowstone, Montana. One quake at 2:31 p.m. Mountain Time registered a 3.7 magnitude. It followed five earlier quakes of at least magnitude 3.
Rather than being triggered by lava-flow murmurings deep in the Yellowstone Caldera, the latest quakes are believed to be the result of shifting plate tectonics, according to a statement issued by the Yellowstone Volcano Observatory (YVO), which is operated by the University of Utah and the U.S. Geological Survey. "At this time the Yellowstone Volcano Observatory does not consider the swarm to be unusual and the earthquakes are likely related to tectonic fault sources," the statement said. "Also there is no indication of premonitory volcanic or hydrothermal activity, but ongoing analyses will evaluate these different sources."
The YVO acknowledged the quakes were being felt by residents and winter visitors to the park: "There have been multiple personal reports of ground shaking from observations inside Yellowstone National Park and in neighboring communities in Montana and Idaho for some of the larger events." However, the observatory downplayed public concerns. "Earthquake swarms of this nature are relatively common in Yellowstone National Park."