Surface Rupture Morphology and Vertical Slip Distribution of the 1959 Mw 7.2 Hebgen Lake (Montana) Earthquake From Airborne Lidar Topography

Abstract

The 1959 Mw ∼7.2 Hebgen Lake earthquake is among the largest continental normal faulting events recorded, as well as one of the earliest associated with a multifault rupture. Multimeter vertical slip was observed on three main, morphologically distinct strands: the Hebgen fault and southeastern section of the Red Canyon fault, which both follow sharp topographic rangefronts, and the Red Canyon fault Kirkwood Ridge section, which cuts steep topography in the footwall of the Hebgen fault. We augment early field, seismological, and geodetic studies by investigating the modern surface rupture using newly acquired airborne lidar topography. By estimating throw from scarp profiling of the ∼36.5 km primary surface rupture, we show both that peak 1959 slip occurred at a structurally mature part of the fault and that many 1959 slip minima are associated with clear structural complexities. Vertical slip often substantially exceeds throw measured at the fault free face immediately after the earthquake; the scarps do not conclusively express beveled forms characteristic of repeated slip and degredation, yet must in places capture both the 1959 earthquake (for which we estimate an average throw of 2.64 m) and one or two preceding latest Pleistocene–Holocene events known from trenching. This has wider, cautionary implications for interpreting paleo-earthquake chronologies and deriving magnitudes from morphologically simple scarps. By comparing 1959-only and multievent vertical displacement populations, and considering preliminary paleoseismic data, we suggest that large surface-rupturing earthquakes on the Hebgen and Red Canyon faults involve highly variable slip distributions.