Seismicity and stress in the vicinity of Mount Spurr volcano, south central Alaska

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Abstract
Focal mechanism solutions and hypocenters for earthquakes near Mount Spurr volcano in south central Alaska reveal spatial perturbations in the regional stress field and in the maximum depth of seismicity. At the volcano, practically all the shocks during the dormant decade 1981–1991 have depths shallower than 5 km and are characterized by nearly pure normal slip. In contrast, earthquakes located outboard of the volcano mostly concentrate in the depth range 3–18 km and exhibit predominantly strike slip, oblique reverse slip, or reverse slip. The regional stress field is characterized by subhorizontal maximum principal stress directed N30°W, concordant with the direction of convergence between the North American and Pacific plates, whereas the maximum principal stress is probably more nearly vertical beneath the volcano. We suggest that the shoaling of seismicity beneath the volcano reflects localized elevation of the depth to the midcrustal transition from brittle failure to plastic flow. We attribute this localized elevation to relict magmatic heat associated with past volcanic eruptions. The available data are not sufficient to determine the cause of the suggested rotation of the maximum principal stress axis beneath the volcano; possible mechanisms include an increase in the vertical stress from the weight of the Mount Spurr massif and a decrease in the maximum horizontal stress associated with doming of the shallow crust from magmatic processes.