Detailed seismic monitoring of the Hebgen Lake-western Yellowstone Park region of Wyoming and Montana has delineated a zone of earthquakes extending 80 km in a direction N80 ¿W from the northwest edge of the Yellowstone calderas to the Madison Valley. The active zone includes the epicentral area of the 1959 Hebgen Lake earthquake (magnitude 7.1). Focal depths ranged from near surface to 15 km in the Hebgen Lake region, whereas near the caldera boundaries the maximum focal depths decreased to 5 km. The abrupt change to shallow focal depths over and near the calderas may be related to increased temperature and pore pressure sufficient to inhibit brittle fracture. Six composite and four single-event fault plane solutions indicate north-south regional extension. Three composite fault plane solutions for earthquakes along the northwest boundary of the Yellowstone calderas indicate northwest-southeast crustal shortening, possibly from uplift on concentric fractures around the calderas.