Dilatational source deformations associated with two unusual M 3+ earthquakes in the area of the 2004-2008 Yellowstone, WY, accelerated uplift episode were identified through detailed analysis of moment tensor inversions. Pressurized hydrothermal fluids are suggested to be associated with the dilatational source processes of these unusual earthquakes, which is consistent with the mechanism of the GPS-InSAR derived deformation signal of the uplift modeled as intrusion of a near horizontal magmatic sill at ∼10 km depth beneath the Yellowstone caldera. One unusual earthquake, the 5 November 2007 Mw 3.3 earthquake, occurred in a volume of expected crustal expansion above the inflating magmatic sill. A notable 60% isotropic expansion component was determined for this earthquake with a 3.2 cm opening across an area of 0.12 km2. We propose that the inflation of the magmatic sill activates a high-pressurized fluid migration upward which in turn triggers dilatational deformation inducing this earthquake. Another dilatational deformation earthquake, the 9 January 2008 Mw 3.8 earthquake, occurred on the northern rim of the caldera. The moment tensor solution for this earthquake shows that the source mechanism had a 30% of the energy associated with tensile dislocation corresponding to a 3.3 cm opening crack over an area of 0.58 km2. We suggest that stress changes produced by a collocated Mw 3.4 earthquake may have increased the fracture permeability promoting fluid migration and thus encouraging the dilatational dislocation. These dilatational source earthquakes are the first non-double couple earthquakes to be documented unambiguously in the 35 year recording period of the Yellowstone seismic network.