In the epicentral region of the Hebgen Lake earthquake, Montana, late Cenozoic structures marked by tension are superimposed on Laramide structures marked by compression. During the earthquake, several large normal faults northeast of Hebgen Lake, and many small faults and monoclines south of it, were reactivated, and a very large area was abruptly dropped and warped.
New fault scarps are as high as 20 feet. They are mostly in unconsolidated or weakly coherent material but certainly reflect displacements of the underlying bedrock. Offsets produced by refraction of faults, which are steeper in surficial material than in bedrock, and by various types of slumping, generally result in surficial scarps higher than true fault displacements.
Subsidence amounted to as much as 22 feet in the lake basin, as proved by releveling of bench marks by the Coast and Geodetic Survey and by shoreline measurements around Hebgen Lake. Parts of the basin subsided differentially, warping the lake bed and creating a gigantic long-lived seiche. Depth-sounding traverses of the lake recorded warping but gave no indication of major faulting of the lake bottom.
A part of the Madison Valley, west of Hebgen Lake, also subsided, though not as much, and a segment of the Madison Range fault was reactivated. At least part of the highway through Madison River Canyon, which cuts directly across the Madison Range between Hebgen Lake and Madison Valley, subsided also; one group of geologists regards this subsidence as tectonic, and considers the entire range, for which there are otherwise no positive data, to have subsided between Hebgen Lake and Madison Valley. In this view, an eastward trending syncline has been propagated across the northwest-trending Madison Range. Another group considers the subsidence in Madison River Canyon to be due in part to compaction and slumping, and the Madison Range to have changed little in altitude. The pattern of deformation appears to them as the uneven subsidence of two northwest-trending basins, one on either side of the Madison Range.
The earthquake caused widespread slumping and sliding of the surficial mantle; the huge Madison Slide killed 26 people. An old earthflow was reactivated and moved 100 feet or more in the month following the earthquake.