abstract

The June 30, 1975 Yellowstone Park Earthquake (ML 6.1, Ms 5.9) occurred on the north-central boundary of the Yellowstone caldera. Intensity studies show the felt area north of the epicenter to be substantially larger than the felt area south of the epicenter. This asymmetry may be due to attenuation of seismic waves propagating through the caldera. Aftershocks define a 10-km northwest-trending zone of seismicity centered on the main shock. A second zone of seismicity (active since March 1974) located 5 km west of and subparallel to the zone containing the main shock, increased in length during the aftershock sequence. No faults are mapped in the region of either zone, but north to northwest trending normal faults are found north of the caldera boundary. Focal mechanisms indicate both normal and oblique-slip faulting, but normal faulting predominates. The T axes are oriented northeast (perpendicular to the two seismic zones) suggesting block faulting. Elevation changes of 12 cm (determined by first-order level lines) between Madison Junction (up) and Mammoth Hot Springs (down) between 1960 and August 1975 are interpreted as a long-term trend with a local perturbation in the Norris Junction area which may have been caused by the June 30 earthquake.

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