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Lost and Found: Evidence of the Penultimate Earthquake on the Hebgen and Red Canyon Faults, Montana

Description: 

The 1959 Mw 7.3 Hebgen Lake, Montana, earthquake, one of the largest-recorded continental normal-faulting earthquakes, occurred on a structurally complex pair of overlapping normal faults (Hebgen and Red Canyon) at the eastern end of the Centennial Tectonic Belt near Yellowstone National Park. In this study, we document from historical photographs the short-lived existence of a steep prehistoric scarp above the 1959 surface ruptures. At several locations, the preexisting scarp was rapidly regraded as the 1959 scarp retreated upslope, and within a few decades, the scarp bevel had largely disappeared. Post-earthquake investigators had recognized that the 1959 scarps near south end of the Red Canyon fault were superimposed on similar older scarps (Myers and Hamilton, 1964); however, it’s unclear the extent to which this observation has affected measurements of 1959 scarp heights and baseline studies of how scarps degrade with time.

A photo taken in 1959 at a site where we trenched the central Hebgen fault (“Section 31” alluvial fan) provides geomorphic confirmation of a displacement event inferred from subsurface relations. We present paleoseismic evidence of this young penultimate earthquake (PUE) and of a relatively long interval separating it from earlier faulting. Recycling of soil organic matter from older colluvial-wedge deposits into the PUE wedge complicates estimation of its age, although several 14C ages (~600 yr B.P.) on detrital charcoal and wood near the base of the PUE wedge are congruent with the youthful steepness (~30-40°) of the scarp as estimated from photography. An older scarp bevel above the 1959/PUE scarp at the Section 31 site is, in contrast, gently sloping (~20°) and stable enough to have a soil B horizon developed beneath it. Evidence on both faults indicates that slip in the PUE was similar to that in 1959, suggesting that event magnitude and pattern of rupture were also similar. The rapid post-1959 loss of morphologic evidence of the PUE scarp demonstrates that a simple fault scarp, even modest in size, should not be assumed to represent a single earthquake.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments - IV

Type: Oral

Date: 5/1/2024

Presentation Time: 05:15 PM (local time)

Presenting Author: Suzanne

Student Presenter: No

Invited Presentation: 

Authors