The second largest earthquake swarm ever recorded in Yellowstone National Park occurred during the two weeks from 27 December 2008 and 7 January 2009 and included more than 1000 earthquakes. Analysis of the swarm suggests epicenters migrated north over the 12 day period and maximum hypocenter depths abruptly shallowed from 12 km to 3 km depth at the time of rapid cessation of activity on Jan. 7.
http://www.sciencedaily.com/releases/2009/04/090409134747.htm