SOURCE PROPERTIES AND DEFORMATION ANALYSIS OF THE 2008-2009 YELLOWSTONE LAKE EARTHQUAKE SWARM

Yellowstone National Park experienced the second largest earthquake swarm in its recorded history in Dec. 2008-Jan. 2009. The swarm consisted of ~1000 earthquakes, 0.5<M<3.9, in a two week period, with 21 events of M>3. Hypocenters were initially located by a real-time automated system with follow-up analysis by seismic analysts for emergency response. Several of the events were felt throughout the surrounding region. The events were then relocated using a 3D velocity model that revealed a well-defined N-S 12 km alignment of hypocenters extending from central Yellowstone Lake to Fishing Bridge. Focal depths ranged from ~1 to 12 km, most concentrated in the 2 to 8 km range. Notably, swarm epicenters migrated north over the 12 day period and maximum hypocenter depths abruptly shallowed from ~12 km to ~3 km at the time of rapid cessation of activity on Jan. 7. Preliminary results show the dominance of E-W extensional double couple focal sources. Yellowstone averages 1,000 to 3,000 earthquakes per year and the common mode of seismic release are in swarms that account for 40% of earthquakes in Yellowstone. This presentation will discuss source studies using moment tensor analysis to evaluate possible hydrothermal-magmatic fluid involvement that will incorporate data from the Yellowstone PBO borehole strainmeters and downhole seismometers. Data from the Yellowstone GPS network will also be evaluated.