THE 1959 HEBGEN LAKE, MONTANA, SURFACE RUPTURE AND RECORD OF LATE-PLEISTOCENE-HOLOCENE EARTHQUAKES

The 1959 Mw 7.3 Hebgen Lake, Montana earthquake was one of the largest in the U.S. during the 20th century. Surface rupture occurred primarily on two normal faults, the Hebgen and Red Canyon, with rupture lengths of 13 km and 18 km and surface displacements of 1-3 m and 1-5.5 m, respectively. The large net displacement and short length indicate a high stress drop, which may characterize structurally complex faults with low cumulative throw such as these in the Basin and Range-Rocky Mountain transition (Hecker et al., 2009).

Paleoseismic investigations on the Hebgen fault at Cabin Creek (Pierce et al., 2000) and Section 31 (Hecker et al., 2000, 2002) and on the Red Canyon fault at Grayling Creek (Haller et al., 2000) found evidence of three surface-rupturing earthquakes (including 1959) on both faults. Radiocarbon dates indicate that the penultimate event on each occurred 1-3 ka. Cosmogenic radionuclide dating (26Al, 10Be) of displaced fan and terrace surfaces at these sites (Van der Woerd et al., 2000) coupled with scarp slope modeling show that the pre-penultimate event on the Hebgen fault occurred 10-14.5 ka. Possible evidence of this event was found in the Red Canyon trenches but is undated. The intervals between the past three large events are variable: 1-3 ka between 1959 and the penultimate and 7-13.5 ka between the penultimate and pre-penultimate ruptures. At each site, the net 1959 slip is within 30% of the penultimate displacement.

The scarp associated with the penultimate earthquake has been severely eroded, and in some locations completely removed, by upslope retreat of the 1959 free face. This is particularly evident where the fault crosses steeper slopes and where the 1959 free face was large. At Section 31, remnants of the penultimate event bevel can be seen on photographs taken shortly after the 1959 earthquake. Follow-up photography in 1978 shows the penultimate scarp had been largely eroded. By the time we trenched in 2000, it was gone. The rapid disappearance of the penultimate scarp cautions against assuming that a morphologically simple scarp represents a single paleoearthquake.