Here is the latest on the earthquake swarms at Yellowstone:
1) The Yellowstone Volcano Observatory put out an update yesterday evening
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.
The University of Utah operates a seismic network in Yellowstone National Park in conjunction with the National Park Service and the U.S. Geological Survey. These three institutions are partners in the Yellowstone Volcano Observatory. Seismic data from Yellowstone are transmitted to the University in real-time by radio and satellite links from a network of 28 seismographs in the Yellowstone area and are available on the web.
Seismologists continue to monitor and analyze data from this swarm of earthquakes and provide updates to the NPS and USGS and to the public via the following web pages.
Information on U.S. earthquake activity including Yellowstone can be viewed at the U.S. Geological Survey web site: http://earthquake.usgs.gov/eqcenter/recenteqsus/.
Information on earthquakes can also be viewed at the University of Utah
Seismograph Stations web site: http://www.seis.utah.edu/.
Seismographic recordings from Yellowstone seismograph stations can be
viewed online at: http://www.quake.utah.edu/helicorder/heli/yellowstone/index.html.
2) Here is an amazing attempt at visualizing the earthquake swarm.
3) Here is a bit of what Scientific American has to say on the topic:
In recent years, Yellowstone's caldera has been rising thanks to uplifting magma beneath it—leading to more cracks, hot springs and even more frequent eruptions of Steamboat Geysers. Paired with the earthquakes, such magma movement might presage an eruption—either big or small. Unfortunately, scientists can't really predict when the next such eruption will happen, and the range of possibilities is large: from later today to a million years from now.
How will we know if we should start worrying? The real warning signs will be rapid changes in the shape of the ground as well as volcanic gases leaking from the ground, neither of which have been sighted—yet.
"Eruptions are far enough apart that there is a very low probability of the next eruption happening in our lifetimes or anytime soon," Daniel Dzurisin of the USGS told me in 2006. "The flipside is: [Yellowstone] has been active for millions of years and it's going to erupt again sometime."
4) Here is a Google Map mashup of the swarm.
5) Slashdot also has a very active discussion about the Yellowstone quakes.