Seismicity of the Yellowstone volcanic field, northwest Wyoming, is characterized by swarms of earthquakes (MC 3 months beginning in October 1985. The swarm had unusual characteristics indicative of interaction between seismicity and hydrothermal/magmatic activity: (1) the swarm followed the reversal of caldera-wide uplift of up to 1 m from 1923 to 1984 to subsidence; (2) swarm hypocenters occupied a nearly vertical northwest trending zone, and during the first month of activity, the pattern of epicenters migrated laterally away from the caldera at an average rate of 150 m/d; (3) the dominant focal mechanisms of the swarm were oblique-normal to strike-slip contrasting with the normal-faulting mechanisms typical of the region; and (4) the maximum principal stress axis averaged for the swarm events was rotated 90¿ from that of the normal background seismicity, from vertical to horizontal with a trend 30¿ from the strike of the plane defined by the swarm. We examined analytic models that best fit the focal mechanisms and the orientation of the plane defined by the swarm and found that the temporal shift of earthquake activity could be explained by the migration of hydrothermal fluids radially outward from the Yellowstone caldera following rupture of a sealed hydrothermal system within the caldera.