The 1986 Eruption of Mount St. Augustine: Field Test of a Hazard Evaluation

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Abstract
The 1986 eruption of Mount St. Augustine, an island volcano in Cook Inlet of southcentral Alaska, provided a good test of previously prepared volcanic hazard models for that volcano. The 1986 eruption was generally similar to other historic eruptions. An initial vent‐clearing explosive eruptive phase in late March of 1986 removed part of the dome emplaced at the end of the last eruption (1976). Pyroclastic flows reached the sea during this phase, and airborne ash spread over much of Cook Inlet, including Anchorage. The initial explosive phase was followed by dome growth which continued over much of the spring and late summer of 1986. Pyroclastic flows, much less intense than those of March 1986, accompanied dome growth. Volcanic hazards from the 1986 eruption were generally within the limits set by previous hazard studies. Pyroclastic flows were confined to a very high hazard zone on and immediately surrounding Augustine Island. Airborne ash and volcanic gases presented a moderate hazard over much of Cook Inlet and presented a serious hazard to aircraft encountering the ash plume. The potential for a large debris avalanche that could enter Cook Inlet and result in a tsunami existed during the 1986 eruption. Previous work on Mount St. Augustine underestimated the landslide‐related hazard, and more work is needed to assess fully the potential of this hazard. These new data, when incorporated into existing hazard assessments, will improve an already successful hazard evaluation for Mount St. Augustine. Assuming that Augustine Island will never be developed, far‐field hazards associated with ashfalls and tsunamis are the most serious. Real‐time satellite and radar imagery can potentially provide adequate warnings about ash plumes, but the warning time for tsunami hazards is short. Improvements in the tsunamis warning system are needed to reduce the hazard.