A microearthquake study carried out in the vicinity of Mount Katmai, Alaska, revealed disappearence of shear waves along certain paths crossing the volcanic range. A close geographical correlation of these paths and active volcanoes strongly suggests that magma or zones of partial melting exist and are responsible for S-wave shadowing. Three shallow (< 10 km) and one deeper (about 20 to 30 km) magma chambers are proposed. Additional evidence for the magma chamber is provided by the high attenuation of high-frequency P waves. From this attenuation an estimate of viscosity in a magma chamber of 108 cgs units is made.

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