RECURRENCE RATES OF BASALTIC VOLCANISM IN SP CLUSTER, SAN-FRANCISCO VOLCANIC FIELD, ARIZONA

A new compilation of 23 K-Ar and Ar-40-Ar-39 radiometric ages, stratig raphic 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 yo unger than 1700 ka, Relatively steady-state volcanic activity (1 volca no per 15 k,y,) has characterized the SP cluster since 780 ka, This ac tivity has been concentrated in an similar to 250 km(2) area since abo ut 300 ka, We estimate that the chance of an eruption in the SP cluste r 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 km(2) are a of the SP cluster within the next 22-26 k,y,, a rate of activity sig nificantly greater than predicted by field-wide averages. Thus, spatia l variations in recurrence rate of basaltic volcanism, such as those r ecognized in the SP cluster, should be considered in the formulation o f hazard assessments.