The current swarm peaked in intensity on Wednesday night, when a pair of earthquakes of magnitude 3.7 and 3.8 were recorded at 11:01 p.m. and 11:16 p.m. Since then there’s been more very small quakes, most under 2 on the Richter scale; the exception was a magnitude 3.1 earthquake recorded last night. The quakes are being recorded in an area 10 miles northwest of the Old Faithful area and some nine miles southeast of West Yellowstone.
These two earthquakes are part of an ongoing swarm in Yellowstone National Park that began January 17, 2010 (1:00 PM MST). The largest earthquake in the swarm as of 12 PM, January 21, 2010, was a magnitude 3.8. There have been 901 located earthquakes in the swarm of magnitude 0.5 to 3.8. This includes 8 events of magnitude larger than 3, with 68 events of magnitude 2 to 3, and 825 events of magnitude less than 2. There have been multiple personal reports of ground shaking from observations inside the Park and in surrounding areas for some of the larger events (for felt reports, please visit http://earthquake.usgs.gov/eqcenter/dyfi/). Earthquake swarms are relatively common in Yellowstone.
The consensus in the scientific community: the swarm of earthquakes is caused by tectonic shifts, not anything to do with the Yellowstone caldera. These same shifts are causing more severe earthquakes in areas like Haiti and Oklahoma. Other monitoring stations indicate no change at all in activity with the caldera and the Yellowstone supervolcano.
Almost 80 swarms have been recorded in Yellowstone since 1995.