Seismic Evidence for Dilatational Source Deformations Accompanying the 2004-2008 Yellowstone Accelerated Uplift Episode (Invited)

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.