GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 Invited talk The Borah Peak Earthquake and the Lost River Fault (Idaho, M w 6.9): A Case Study for Earthquake Geology and Seismogenesis S. Bello 1,2 1 DiSPuTer, University “G. d’Annunzio”, Chieti, Italy 2 CRUST – InteRUniversitary Center for Tridimensional Seismotectonic analysis, Italy Following the observations made in survey campaigns along the Lost River Valley (Idaho, USA), different datasets were integrated to obtain an extremely detailed segmentation of the fault that released the 1983 Borah Peak earthquake (M w 6.9). The earthquake ruptured the topographic surface with a normal-oblique faulting mechanism, activating two SW-dipping segments (Thousand Springs and Warm Springs; Fig. 1) and a branching SSW-dipping fault (Arentson Gulch Fault), and producing coseismic surface ruptures with throw up to 3 m. The 1983 earthquake knowledge were augmented by investigating and interpreting high-resolution topography and scarps mapping (e.g., Fig. 2) obtained through structure-from-motion (Bello et al., 2020, 2021; Scott et al., 2020). On the high-resolution topography a large dataset of vertical separation data both on 1983 ruptures and on long-term scarps was acquired. These data, combined with rupture zone width (RZW) measurements, new fault/slip data, and an analysis of major and minor structural-geometric complexities, highlights a partition of the deformation and a segmentation of the fault up to the detail of four orders (i.e., segments, sections, subsections, sectors), providing new useful details of the earthquake and new constraints for paleoseismic and seismotectonic studies. The fault/slip data show variations along the fault-strike that we interpret from the perspective of a kinematic partitioning, which supports segmentation (Fig. 3). In 1983, the two main activated segments had completely different rupture behaviours, with important RZWs in the southern portions and with the deformation concentrated along the main fault trace in the northern portions. Here is shown that 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%).