ABSTRACT

Since late Tertiary time (Miocene?), the geologic pattern of the Hebgen Lake area has involved the repeated dropping and tilting of fault blocks. At 11:37 p. m. on August 17, 1959, the pattern was repeated. Two fault blocks east of the Madison Range, and one west of the Range, dropped and tilted, synchronously reactivating the marginal normal faults. Geodetic data are not available to indicate how the core of the range was deformed. Release of the accumulated strains resulted in a major earthquake that was felt over 600,000 square miles.

Prominent fault scarps appeared coincident with or closely parallel to the reactivated faults. The major scarps, north of Hebgen Lake, face valleyward (southward), have maximum displacement of about 20 feet near their midpoints, and offset surficial debris, chifly colluvium.

The subsidence and northward tilting of the fault block containing Hebgen Lake displaced the lake northward, and caused the surface of the lake to oscillate in great waves. The seiche thus created lasted for 11½ hours, and initially was able to crest Hebgen Dam four times. When the lake quieted, the north shore was submerged, and the south shore was emerged.

Damage to Hebgen Dam, an earthfill structure with a concrete core, was extensive. The concrete core was cracked and twisted out of line, the earthfill slumped unevenly, and several concrete piers in the spillway were cracked or knocked askew.

During or shortly after the earthquake, parts of former Highway 287 slid into Hebgen Lake. In places along the north shore, a mixture of sand and water was extruded from fissures to form sand spouts. About a week after the main shock, an earthflow in Kirkwood Canyon began to move, felling trees and draining a small lake.