89-6 Quantifying Coseismic Deformation from the 1983 M6.9 Borah Peak, Idaho, Earthquake Using Topographic Differencing of Historical Aerial Imagery and Modern Lidar

Session: Advancing Earthquake Geology and Surficial Deformation from Geologic Provinces to Political Entities through Multidisciplinary High-Resolution Data

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The 1983 M6.9 Borah Peak, Idaho, earthquake is one of the largest historical normal fault earthquakes in the western US. We quantified meter-scale vertical deformation along the 35-kilometer-long surface rupture by differencing a digital surface model derived from 1966 aerial imagery with 2019 lidar-derived topography. The coseismic deformation, including a maximum vertical displacement of 2.6 meters, is significantly obscured by georeferencing errors, flight-line stripes, and decades of non-tectonic change. We applied error correction methods designed to be insensitive to the earthquake signal, reducing the noise by approximately 50%. Using the corrected data, we calculated vertical separation along most of the rupture and measured a maximum of 2.02±0.46 meters at Double Springs Pass. These measurements are broadly consistent with previous studies based on post-earthquake field surveys and topographic data collected decades following the earthquake. We interpret this agreement as evidence that differencing legacy aerial photographs with modern lidar collected more than five decades apart can resolve centimeter-to-meter-scale tectonic deformation with at least moderate accuracy. Despite the general agreement, our vertical separation values are on average 0.4 meters lower than those reported in earlier studies. We suggest this variability reflects the ability of vertical differencing to isolate deformation from the 1983 earthquake. Fault slip measurements made in the field or from topographic data following the earthquake may not fully distinguish new fault scarp breakage from the pre-existing faults, leading to an overestimate of the fault slip. Our study demonstrates that revisiting historic earthquakes using geodesy can provide new insights into the magnitude and patterns of coseismic deformation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8247

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