DIGITAL GEOLOGIC AND STRUCTURE MAPPING OF THE HEBGEN DAM AND MOUNT HEBGEN 7.5- MINUTE QUADRANGLES, SW MONTANA, USING GLOBAL POSITIONING SYSTEM (GPS) AND GEOGRAPHIC INFORMATION SYSTEM (GIS) APPLICATIONS

The August 17, 1959 Hebgen Lake – Red Canyon earthquake sequence (Ms7.5; intensity X) was the largest earthquake in the U.S. intermountain region in historic time. The earthquake resulted in an extensive system of fault scarps that mark the Hebgen and Red Canyon normal faults and a massive landslide in the canyon of the Madison River. Despite the fact that the 1959 Hebgen-Red Canyon earthquake was the largest historic event in the Intermountain West, detailed geologic mapping of the area has not been done at a resolution finer than 1:62,500, nor has a detailed structural analysis been conducted using the methods of modern structural geology.

A new generation of geologic mapping of the Hebgen Dam and Mount Hebgen 7.5-mintue quadrangles was completed for a modern structural analysis and to test various models of reactivation of preexisting Laramide faults. Field mapping was done at 1:12,000 and then compiled at 1:24,000. Mapping was aided by GPS-based digital mapping utilizing a Trimble Geo XH GPS receiver. Also, traditional “paper-based” geological mapping was combined with digital aerial photographs and digital topographic maps using Trimble TerraSync for attribute data collection and ESRI GPS Pathfinder Office software for post-processing. A structural data dictionary was generated prior to mapping to record bedrock attitudes and outcrop characteristics in detail. Fault-scarp profiling of the Hebgen and Red Canyon normal faults was completed using the Trimble Geo XH GPS receiver with a Zephyr antenna; surface offset and related measurements were then calculated using a spreadsheet developed by the USGS, thus allowing calculations of scarp degradation based on 50 years of footwall erosion since the main event. Preliminary mapping results include the observation that the Hebgen fault scarp is more laterally extensive than previously mapped; the Red Canyon fault scarp follows the structural attitude of bedrock units in a remarkably consistent manner unit it encounters surficial deposits to the southeast; newly discovered slumps and landslides have been mapped at the north end of Red Canyon scarp; and fault scarp degradation in surficial deposits is consistent with a “basin and range” diffusion model calibrated for more precipitation in SW Montana.