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Seismicity and Seismotectonic Issues of Western Utah
We describe seismicity, both historical and instrumental, and discuss seismotectonic issues of western Utah within a roughly triangular study area that extends from 15 km north of Interstate Highway 80 to 36.75° N. and from 15 km east of Interstate Highway 15 to 114.25° W. Seismicity in the study area occurs chiefly along the western margin of the Intermountain seismic belt (ISB), a broad zone of mostly shallow (< 20 km) extensional earthquake activity that transects Utah generally following the course of Interstate Highway 15. Throughout the rest of the study area, west of the ISB, seismicity is sparse and scattered. Using the University of Utah’s earthquake catalog as our primary source, we describe the available earthquake database and significant earthquakes for the study area. Because our present study area significantly overlaps that of Arabasz and others (2007), in which we recently described at length the salient features of the ISB in central Utah, we focus here on the seismicity in the West Desert and on a seismotectonic feature of particular interest in the Escalante Valley of southwestern Utah. Sparse seismicity in Utah’s West Desert relates to an abrupt eastward change in earthquake density in Great Basin seismicity at ~114° W. longitude (at the Nevada-Utah border). We suggest that this change in seismicity, together with other geophysical contrasts between eastern Nevada and western Utah, is influenced by either “lithospheric block” structures or upper-mantle dynamics related to toroidal mantle flow around the edge of the subducting Gorda-Juan de Fuca slab. In the Escalante Valley, a linear band of seismicity trending east-northeast to west-southwest forms part of the divergence from the northerly trend of the ISB. From available focal mechanisms, we observe a migration from the average approximately east-west least-principal stress direction of the ISB to a more northwest-southeast direction. Although there is a correlation with pre-existing crustal structures, such as the Sevier thrust belt and mapped crustal transform structures, we suggest that the seismicity pattern is the result of the same mantle dynamics affecting the West Desert.
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