Recurrence Rates of Basaltic Volcanism in SP Cluster, San Francisco Volcanic Field, Arizona

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A new compilation of 23 K-Ar and 40Ar-39Ar radiometric ages, stratigraphic and paleomagnetic correlations, and cone morphology from the SP cluster, San Francisco volcanic field, Arizona, is used to illustrate a quantitative approach to long-term volcanic hazards assessment for basaltic volcanic fields. SP cluster is a group of 67 basaltic cinder cones, tuff rings, spatter cones, and lava flows, all predominantly younger than 1700 ka. Relatively steady-state volcanic activity (1 volcano per 15 k.y.) has characterized the SP cluster since 780 ka. This activity has been concentrated in an ∼250 km2 area since about 300 ka. We estimate that the chance of an eruption in the SP cluster during the next 1000 yr has an upper bound (95% confidence level) of 13%. Spatial and temporal probability models further indicate with 90% confidence that an eruption will take place within this 250 km2 area of the SP cluster within the next 22–26 k.y., a rate of activity significantly greater than predicted by field-wide averages. Thus, spatial variations in recurrence rate of basaltic volcanism, such as those recognized in the SP cluster, should be considered in the formulation of hazard assessments.

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Geology, v. 26, issue 7, p. 655-658