Morphotectonic anatomy and segmentation pattern of the 1983, Mw 6.9 Borah Peak earthquake (Idaho, USA)

Following survey campaigns along the Lost River Fault (Idaho, USA), we integrate different datasets to obtain a detailed segmentation of the Borah Peak earthquake (1983, Mw 6.9). The latter ruptured with a normal-oblique kinematics producing coseismic ruptures with throw up to 3m. High-Resolution Topography and a large dataset of vertical separation data, combined with rupture zone width (RZW) measurements, new fault/slip data, and an analysis of structural-geometric complexities, highlight a deformation partitioning and a clear multiscale segmentation of the fault, providing new constraints for paleoseismic and seismotectonic studies. In 1983, the two main activated segments had completely different rupture behaviours, with important RZW in the southern portions and with concentrated deformation along the northern portions. The distributed ruptures, in addition to being a large percentage of all deformation in terms of a total length of the ruptures (~19.5 km vs 31 km in total for the main ruptures) also accommodate most of the surface deformation (~66%). We also show that 83% of the deformation in terms of length of the surface faulting is located at the hangingwall of the main rupture, while at the footwall it is located the remaining 17%. The fault/slip data highlight the control of obliquity and kinematic partitioning in the surface expression of the earthquake propagation. We interpret the coseismic and long-term behavior, showing that the two activated segments had similar cumulated behaviours in distributing the deformation over time, even with different geometries (Bello et al., 2021, 2022).