New structural data from the Novarupta basin, Katmai National Park, Alaska, site of the largest volcanic eruption of this century (1912), provide limits for the location of magma chambers associated with this eruption. To investigate the subsurface structure of the 1912 vents, and to support an interdisciplinary study of this young volcanic system, a project of geologic mapping of surficial and bedrock structures in the vent region of the 1912 eruption has been undertaken.Landslide scarps, arcuate grabens, a monoclinal fold, and truncated ridges circumscribe the Novarupta basin, marking the inferred outer rim of the vent. A set of radial fissures crosses the southern margin of the basin, striking ∼140°, subparallel to the dominant bedrock joint set. These fissures and joints, along with the local plate-motion vector and the inferred regional stress orientation, are consistent with a feeder dike propagating from a reservoir beneath Trident volcano to the eruptive vent.

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