Surface Rupture Morphology and Vertical Slip Distribution of the 1959M_w7.2 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.

Variable normal-fault rupture behavior, northern Lost River fault zone, Idaho, USA

“…For profiles with heavy vegetation, we manually selected and removed profile points above the ground surface prior to defining footwall and hanging-wall surface projections. Because footwall fault exposures (preserved free faces) along the 1983 rupture are rare, we calculate VS rather than fault throw, which requires assumptions about near-surface fault dip (e.g., Johnson et al, 2018). Although slope steepness and fault dip can enhance the discrepancy between actual fault throw and measured VS (e.g., Mackenzie and Elliott, 2017), our VS measurements are likely ~80%-90% or greater of throw because steep (60°-90°; Crone et al, 1987) near-surface faults dip in the same direction as moderately (<15°) sloping geomorphic surfaces (Caskey, 1995).…”

“…1). As one of the largest intraplate normal-faulting earthquakes recorded historically and an example of the complex rupture of a multisegment normal fault system (Haller and Crone, 2004), the Borah Peak earthquake rupture offers an important opportunity to relate spatial and temporal patterns of surface displacement to fault-rupture processes (e.g., Wesnousky, 2008;Nissen et al, 2014;Haddon et al, 2016;Delano et al, 2017;Personius et al, 2017;Johnson et al, 2018).…”

Revisiting the 1959 Hebgen Lake Earthquake Using Optical Image Correlation; New Constraints on Near‐Field 3D Ground Displacement

“…Since the field measurements are usually collected across a very narrow aperture that does not extend beyond the immediate proximity of the fault scarp, they are not able to capture the total deformation accommodated across the wider fault zone. Similarly, Johnson et al (2018) estimated fault offset from the relative height difference in the LiDAR between the footwall and the hanging wall topography. While this method can capture some OFD, it is possible that a large proportion of OFD remains unaccounted for, perhaps due to surface modification (e.g., erosion).…”

The 2017–2018 Maple Creek Earthquake Sequence in Yellowstone National Park, USA

“…The beachball is the focal mechanism of the largest (M w 4.4) earthquake in the sequence. The red line is the surface rupture of the 1959 M w 7.2 Hebgen Lake earthquake (Johnson et al, 2018), black lines show regional faults, and the purple line is the boundary of the 2.1-Ma caldera. (b) A-A′ cross section with depth relative to sea level.…”

“…The V P /V S decrease we observe would require a porosity increase of~80%, which might be expected if there were increased damage in the northwestern segment of Cluster n (i.e., Cluster n-II) because of its closer proximity to the Hebgen Lake rupture zone. Johnson et al (2018) noted significant scarp height in the southeastern portion of the rupture zone, and because of the relatively low strain rates, it is plausible that the damage in the rupture zone has not yet had time to heal. The increased epicentral scatter in Cluster n-II is also consistent with a highly fractured region.…”