Monthly updates from the Yellowstone Volcano Observatory (YVO) summarize seismic activity (table 1) and ground deformation at Yellowstone caldera. Earthquake activity remained at low levels during the majority of the reporting interval (November 2006 through February 2010). There were several earthquake swarms during this time, including significant events in December 2008-January 2009 and January-February 2010. The early 2010 events comprised the second largest earthquake swarm recorded at Yellowstone, second only to the fall 1985 swarm (BGVN 17:03). The swarm that began in December 2008 was the third largest swarm recorded.

Table 1. Seismic data for Yellowstone organized by month, including the number of recorded earthquakes, the largest magnitude recorded, and earthquake swarm information. Note the large swarms during December 2008-January 2009 and during January-February 2010. Data courtesy of the USGS.

Earthquake swarm, December 2008-January 2009. An earthquake swarm from 26 December 2008 to 5 January 2009 was centered beneath the N end of Yellowstone Lake. The event consisted of ~ 900 earthquakes with magnitudes ranging up to 3.9; 19 events had magnitudes greater than 3.0, while 141 had magnitudes between 2.0 and 2.9.

Earthquake swarm, January-February 2010. The January-February 2010 earthquake swarm was centered about 16 km NW of Old Faithful, on the NW edge of the caldera. The event began with a few small earthquakes on 15 January and began to intensify on 17 January. A 3.7 magnitude earthquake was recorded at 2301 on 20 January, followed by a magnitude 3.8 event at 2316. The events were felt throughout the park and surrounding communities in Wyoming, Montana, and Idaho. By 25 February, YVO had recorded a total of 1,809 earthquakes, with 14 reaching magnitudes of over 3.0 and 136 with magnitudes between 2.0 and 2.9. By the end of February activity had returned to background levels.

The University of Utah Seismology Research Group stated that the total seismic energy released by all the earthquakes in this swarm corresponded to one earthquake with an approximate magnitude of 4.4. YVO emphasized that while this was an unusually large event, it did not indicate premonitory volcanic or hydrothermal activity. Rather, the swarm earthquakes were likely the result of slip on pre-existing faults.

Geological Summary. The Yellowstone Plateau volcanic field developed through three volcanic cycles spanning two million years that included some of the world's largest known eruptions. Eruption of the over 2,450 km3 Huckleberry Ridge Tuff about 2.1 million years ago created the more than 75-km-long Island Park caldera. The second cycle concluded with the eruption of the Mesa Falls Tuff around 1.3 million years ago, forming the 16-km-wide Henrys Fork caldera at the western end of the first caldera. Activity subsequently shifted to the present Yellowstone Plateau and culminated 640,000 years ago with the eruption of the over 1,000 km3 Lava Creek Tuff and the formation of the present 45 x 85 km caldera. Resurgent doming subsequently occurred at both the NE and SW sides of the caldera and voluminous (1000 km3) intracaldera rhyolitic lava flows were erupted between 150,000 and 70,000 years ago. No magmatic eruptions have occurred since the late Pleistocene, but large hydrothermal events took place near Yellowstone Lake during the Holocene. Yellowstone is presently the site of one of the world's largest hydrothermal systems, including Earth's largest concentration of geysers.

Information Contacts: Yellowstone Volcano Observatory, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA (URL: http://volcanoes.usgs.gov/yvo/).