SEISMICITY AND STRESS IN THE VICINITY OF MOUNT-SPURR VOLCANO, SOUTH CENTRAL ALASKA

Focal mechanism solutions and hypocenters for earthquakes near Mount S purr volcano in south central Alaska reveal spatial perturbations in t he regional stress field and in the maximum depth of seismicity. At th e volcano, practically all the shocks during the dormant decade 1981-1 991 have depths shallower than 5 km and are characterized by nearly pu re normal slip. In contrast, earthquakes located outboard of the volca no mostly concentrate in the depth range 3-18 km and exhibit predomina ntly strike slip, oblique reverse slip, or reverse slip. The regional stress field is characterized by subhorizontal maximum principal stres s directed N30-degrees-W, concordant with the direction of convergence between the North American and Pacific plates, whereas the maximum pr incipal stress is probably more nearly vertical beneath the volcano. W e suggest that the shoaling of seismicity beneath the volcano reflects localized elevation of the depth to the midcrustal transition from br ittle failure to plastic flow. We attribute this localized elevation t o relict magmatic heat associated with past volcanic eruptions. The av ailable data are not sufficient to determine the cause of the suggeste d rotation of the maximum principal stress axis beneath the volcano; p ossible mechanisms include an increase in the vertical stress from the weight of the Mount Spurr massif and a decrease in the maximum horizo ntal stress associated with doming of the shallow crust from magmatic processes.