Several occurrences of intraplate/intracontinental earthquakes have been observed in the lower crust and upper mantle of the western United States. Such deep events are rare and have been only recently detected and identified due to a significant improvement in seismographic coverage in the past decade. Of the six cases of deep earthquakes (>20 km) investigated in this study, all occurred in regions of low to normal surface heat flow. Based on a set of recently developed continental geotherms and measured surface heat flow, we infer the temperatures in the source areas of the deep crustal earthquakes in the Laramie Mountains, Wyoming, the Sierran foothills and Ventura Basin, California, and to some extent, the Paradox Basin in Utah and Colorado to be within the allowable range for brittle failure, less than 350 ± 100°C, as proposed by previous investigators. The uppermost mantle earthquakes in the Paradox Basin are within the upper limit of 600 to 800°C allowable for brittle failure in the mantle. The Paradox Basin and Crownpoint, New Mexico, earthquakes at depths of 30 to 44 km appear to be occurring at anomalously warm lower crustal temperatures; however, given the uncertainties in heat flow measurements and model parameters assumed for the geotherms, the estimated temperatures may have an uncertainty of as much as ±100°C. Other possible explanations involve an actual location within the uppermost mantle or material composition at these depths which are more mantle-like possibly due to a transitional Moho. The 90-km-deep Randolph, Utah, earthquake is anomalous with respect to mantle temperatures.

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