SCARP MORPHOLOGY, SLIP DISTRIBUTION, AND SHALLOW FAULT STRUCTURE OF THE MW 7.2 1959 HEBGEN LAKE EARTHQUAKE RUPTURE FROM AIRBORNE LIDAR

We use airborne lidar topography to examine scarps of the M_w 7.2 1959 Hebgen Lake, Montana earthquake, gaining an improved understanding of the rupture morphology, slip distribution, and shallow structure of what remains one of the largest continental normal faulting events on record globally. We exploit a 50 cm-resolution terrain model from a lidar survey flown in 2014 by the National Center for Airborne Laser Mapping. ~700 new throw measurements, derived from topographic scarp profiling, in places greatly exceed vertical slip surveyed at the fresh scarp free face after the earthquake (Witkind, 1964). Modern scarps along the rangefront-forming Hebgen and SE Red Canyon faults must variably capture slip not only in the 1959 rupture but also in one or two preceding Holocene events (known from trenching), yet they appear simple in profile, seldom exhibiting bevels characteristic of composite landforms. This has wider implications for how morphologically-simple scarps are interpreted: with no knowledge of the 1959 event, the scarps could easily be mistaken as generated by a single paleo-earthquake, whose magnitude would then be overestimated (from scarp heights and rupture length) as M_w ~7.4. Next, we examine the roughness of slip distributions along the three major rupture strands. Surface slip along the rangefront Hebgen and SE Red Canyon faults is quantifiably rougher than along the intramontane NW Red Canyon fault. The latter fault exploits weak bedding planes in rupturing to the surface, which may have enhanced the uniformity of its scarp heights. Finally, we use the 3-D intersection of the rupture with topography to estimate the dip of the faulting, using simple planar fitting that samples length scales of 100s of meters and depth scales of 10s of meters. This proves more challenging than for other recent (strike-slip) earthquakes analyzed in a similar manner, perhaps indicating that the Hebgen and Red Canyon faults are highly-segmented over short spatial scales. However, average segment dips are consistently ~30^o - 45^o, significantly lower-angle than values of 45^o - 60^o from earlier seismological and geodetic models, which in contrast sample the full seismogenic zone. This apparent anti-listric geometry may reflect the influence of gently-inclined Laramide structures in guiding the modern normal faulting in the near-surface.