Abstract

On the basis of a limited historical record, the seismicity of the Colorado Plateau appears to be of very low level, with diffusely distributed earthquakes of small to moderate magnitude (ML ⩽ 5.5). From December 1978 to January 1980, a spatially and temporally isolated swarm of earthquakes was observed near Capitol Reef National Park in south-central Utah. This sequence of at least 38 earthquakes, ML 1.0 to 3.6, represents the highest level of seismicity historically observed within the interior of the Colorado Plateau physiographic province, with the exception of coal-mining-induced seismicity in the Book Cliffs of east-central Utah. The earthquakes occurred in a structurally complex area near the juncture of Waterpocket fold and the Caineville monocline just west of the Henry Mountains. The location of the swarm suggests a possible association with the Laramide basement faults that controlled the development of the fold and monocline. A focal mechanism for one of the events exhibited (1) normal faulting trending northwest and (2) crustal extensional stress in a northeast-southwest direction. These observations are inconsistent with the expected reverse or strike-slip faulting and the compressional tectonic stress field of the Colorado Plateau interior. The observed Capitol Reef seismicity may therefore (1) represent an occurrence of Basin and Range extensional tectonism along a reactivated Laramide structure and (2) refine Zoback and Zoback's boundary of the Basin and Range-Colorado Plateau transition zone.

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