It would appear that seismologists and geophysicists are still trying to determine what the swarm meant in the first place. Judging from their press release, they are not too concerned about volcanic activity right now; it appears that geothermal activity (i.e. hot water, as opposed to hot lava) may be the culprit. There is a history of that at Yellowstone Lake. In addition to the geothermal features such as geysers, springs, mud pots, and fumaroles around the park, occasionally steam explosions of various sizes rearrange the topography. (Pretty much what it sounds like - steam builds up underground to the point that the rock constraining it shatters.) Parts of Yellowstone Lake's shoreline were carved out by steam explosions. From "Questions About Volcanic Activity at Yellowstone" on the Yellowstone Volcanic Observatory website,
The most likely type of eruption would not be volcanic but, rather, hydrothermal. This type of small, but still explosive eruption can occur from shallow reservoirs of steam or hot water rather than molten rock. These reservoirs are the sources of Yellowstone's famous geysers, hot springs, and fumaroles. Such explosions could blast out shallow craters more than a kilometer wide; as has occurred in the northern Yellowstone Lake Basin, including Mary Bay and nearby Turbid Lake and Indian Pond, and in western Yellowstone National Park north of Old Faithful. Each of these craters was produced by steam blasts within the past few thousand years.So we more likely are seeing the beginnings of a steam explosion, rather than an active volcano. But that is not certain, yet. There's going to be a bunch of scientists spending a lot of hours studying seismograms before we hear any hypotheses.
There is agreement on point - there is nothing to panic about. Yet.
Geology, in action. Bring a lunch.
YELLOWSTONE VOLCANO OBSERVATORY INFORMATION RELEASE
Friday, January 2, 2009 19:30 MST (Saturday, January 3, 2009 02:30 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 Update: 2 January 2008
The University of Utah Seismograph Stations reports that as of 1800 MST on 2 January 2009, seismicity of the ongoing Yellowstone earthquake swarm continues. Over 500 earthquakes, as large as M 3.9, have been recorded by an automated earthquake system since the inception of this unusual earthquake sequence that began Dec. 27, 2008. More than 300 of these events have been reviewed and evaluated by seismic analysts. Depths of the earthquakes range from ~ 1km to around 10 km. We note that the earthquakes extend northward from central Yellowstone Lake for ~10 km toward the Fishing Bridge area, with a migration of recent earthquakes toward the north. Some of the dozen M3+ earthquakes were felt in the Lake, Grant Village and Old Faithful areas. Personnel of the Yellowstone Volcano Observatory continue to evaluate this earthquake sequence and will provide information to the NPS, USGS and the public as it evolves.
This earthquake sequence is the most intense in this area for some years. No damage has been reported within Yellowstone National Park, nor would any be expected from earthquakes of this size. The swarm is in a region of historical earthquake activity and is close to areas of Yellowstone famous hydrothermal activity. Similar earthquake swarms have occurred in the past in Yellowstone without triggering steam explosions or volcanic activity. Nevertheless, there is some potential for hydrothermal explosions and earthquakes may continue or increase in magnitude. There is a much lower potential for related volcanic activity.