Just read this from the Yellowstone observatory page.
Yellowstone volcano Observatory
YELLOWSTONE VOLCANO OBSERVATORY INFORMATION RELEASE
Tuesday, January 6, 2009 00:39 MST (Tuesday, January 6, 2009 07:39 UTC)
YELLOWSTONE VOLCANO (CAVW#1205-01-)
44.43°N 110.67°W, Summit Elevation 9203 ft (2805 m)
Volcano Alert Level: NORMAL
Aviation Color Code: GREEN
Yellowstone Lake Earthquake Swarm Summary as of 6 January 2009
Through 5 January 2009, seismic activity has markedly decreased. It is possible that the swarm has ended, though a return of activity may occur as
Yellowstone swarms of the size usually last for tens of days to many weeks.
About 500 earthquakes occurred between Dec. 26 through Jan. 1. Three hundred of the earthquakes (including all >M2.0) have been reviewed by
seismologists. There have been 86 earthquakes with M > 2.0 and 16 earthquakes > M3.0. About 200 smaller earthquakes have yet to be reviewed. Depths
are difficult to determine accurately. The best located earthquakes have depths on the order of 3 to 10 km (1.8 to 6.0 miles). From Dec. 26 through
Jan 2, the earthquake hypocenters appear to have migrated northwards, starting southeast of near Stevenson Island, with many of the latest events
occurring near Fishing Bridge.
The recent swarm is well above typical activity at Yellowstone. Nevertheless it is not unprecedented during the last 40 years of monitoring. Swarms
are the typical mode of occurrence of earthquakes within the Yellowstone caldera, with magnitudes ranging to > 4.0. The 1985 swarm on the northwest
rim of the caldera lasted for three months, with earthquakes up to M4.9 and over 3000 total events recorded.
Magnitudes of earthquakes in this swarm range from zero to 3.9. Seismologists categorize those of magnitude less than 3.5 as generally not felt by
persons. For perspective, earthquakes of magnitude 3.4 to 4.5 are often felt, as several of the events in this swarm have. A magnitude 5 or greater is
generally required to produce damage to buildings or other structures.
Improved volcano and seismic monitoring at Yellowstone gives us a greater ability to locate earthquakes, understand their source process and identify
anomalous sources of seismic activity. New equipment including precise measurements of ground motion by GPS receivers and borehole strainmeters
provided by the National Science Foundation's EarthScope and Continental Dynamics Program have been used extensively during the last week of intense
earthquake activity. Ground motions accompanying the swarm, from the GPS instruments will take two or more weeks to fully process. It is worth noting
that in 2004 the Yellowstone caldera began a period of accelerated uplift measured by GPS instruments that was as large as 7 cm/yr (2.7 inches/yr),
three times as fast as recorded in the recorded history but has now reduced to about a maximum rate of 4 cm/yr. Scientists have modeled this
deformation as due to magmatic recharge of the Yellowstone magma chamber at a depth of ~10 km (6 miles). The area of the swarm is on the eastern side
of the uplift area.
Earthquakes at Yellowstone are caused by a combination of geological factors including: 1) regional stress associated with normal faults (those where
the valleys go down relative to the mountains) such as the nearby Teton and Hebgen Lake faults, 2) magmatic movements at depth (>7 kms or 4 miles),
and 3) hydrothermal fluid activity caused as the groundwater system is heated to boiling by magmatic heat.
At this time, no one has noted any anomalous changes in surface discharges (hot springs, gas output, etc.).
YVO staff from the USGS, University of Utah and Yellowstone National Park continue to carefully review all data streams that are recorded in
real-time. At this time, there is no reason to believe that magma has risen to a shallow level within the crust or that a volcanic eruption is likely.
The USGS Volcano alert level for Yellowstone Volcano remains at Normal/Green.
Yellowstone National Park is evaluating infrastructur