Seismic refraction profiles of the ash flow in the Valley of Ten Thousand Smokes, Katmai National Monument, Alaska

Abstract

Two seismic profiles across the 1912 pyroclastic flow in the south and central branches of the Valley of Ten Thousand Smokes give P-wave velocities within the flow of from 0.38 to 0.92 km/sec, indicating considerable lateral variation in compaction and/or welding along the traverse lines. Average P-wave velocities of 2.8 km/sec were obtained for the underlying strata, providing a sharp discontinuity at the base of the ash flow. The bedrock underlying the south branch of the valley is displaced by a normal fault at the east end of the profile. A fissure fumarole line overlies this fault on the surface of the ash flow, indicating a deep rather than ‘rootless’ origin for these particular fumaroles. Maximum thicknesses of 50 and 25 meters were determined for the pyroclastic flow along traverses across the south and central branches of the valley, respectively. These depths are in approximate agreement with those proposed by Sbar, Matumoto, and Kienle on the basis of gravity and refraction surveys along similar profiles, but in strong disagreement with the estimated thicknesses up to 300 meters suggested by other workers.