PALEOSEISMIC INVESTIGATION OF THE 1959 RED CANYON FAULT, SOUTHWESTERN MONTANA

The 1959 Mw 7.3 Hebgen Lake earthquake produced surface rupture on two closely spaced echelon faults in southwestern Montana; however, the geometry and character of these faults are quite different. The Hebgen fault has long been recognized as a typical range-bounding normal fault. Its 13-km-long trace separates the northern part of Hebgen Lake basin from the mountains to the northeast. In contrast, the 21-km-long Red Canyon fault has a curvilinear trace, nearly two-thirds of it lies within the mountain block, and only the southern part of the fault has characteristics common to range-bounding normal faults. The southern part separates the Grayling Arm of the basin from the low hills to the north. This investigation along with excavations at two sites on the Hebgen fault document the past rupture history of both faults and test the possibility of previous synchronous rupture. Collectively, they represent much of the work that has been accomplished since the post-earthquake characterizations in 1959.

Our Grayling Creek trench site on the Red Canyon fault is located near the middle of the basin bounding, southern part of the fault, about 4 km from the southern end of the 1959 rupture. The site is on a latest Pleistocene fan (11-15 ka based on cosmogenic surface exposure dating by colleagues at Lawrence Livermore National Laboratory). Stratigraphic relations in the trench and the presence of fault scarps at this site on 1954 aerial photographs indicate the occurrence of at least one pre-1959 surface-rupturing earthquake. The height of the scarps formed in 1959 near this site, as reported in the USGS Professional Paper documenting the earthquake, is 50-75 percent of the maximum observed along the Red Canyon fault. However, the site is characterized by offsets that exceed (140%) the average observed scarp heights. Thus, we believe this site should yield results that reasonably represent the slip rate for the fault as a whole.

Our data suggest that one post-15-ka event preceded the 1959 surface rupture at Grayling Creek. The timing of that event is not well constrained but is clearly Holocene. The total surface offset of the faulted latest Quaternary surface, based on a long topographic profile, is only about 2 m, which implies that the Red Canyon fault is characterized by a low slip rate and recurrence intervals that are thousands of years long.