DENSE SURFACE FAULTING DATA FOR THE STUDY OF COSEISMIC AND LONG-TERM EXTENSIONAL RUPTURE: THE CASE OF THE LOST RIVER FAULT (IDAHO, USA)

We present dense mapping and measurements of surface faulting along the Lost River fault (LRF – Idaho, USA), a normal fault activated in the 1983 (M_w 6.9) earthquake. The fault is composed of six NW-SE trending segments. Two of these failed in the 1983 Borah Peak earthquake, rupturing ~35 km of fault length with a maximum throw of ~3 m.

In spring 2019, we imaged key areas along both segments activated and not activated by the earthquake. In our campaign, we acquired ~10,200 photographs using an Unmanned Aerial Vehicle (UAV) that flew at ~50-110 meters above ground level. In total, we covered ~20 km along the trace of the fault at 13 different areas. New point clouds, digital surface models (DSMs), and orthomosaics (5 to 30 cm-pixels) were produced to map the complex coseismic rupture pattern and the observable Quaternary fault scarps, and with these, to systematically measure fault vertical separation (VS) producing the most comprehensive VS dataset along the fault.

We were able to map on a GIS platform 757 coseismic ruptures (CoRs) and Quaternary fault scarp traces (Qfs) (~51 km), both synthetic and antithetic with respect to the LRF. To make the VS measurements, we created a MATLAB algorithm (https://github.com/cpscottasu/FaultVerticalSeparation) to generate 1295 fault-perpendicular topographic profiles with a 25 m spacing. The integrated use of these products (high-resolution topography, mapped faults, MATLAB algorithm) allowed us to measure 2053 VS, to quantify rupture zone width, and to collect other statistical and geometrical information (e.g. distribution of the ruptures between the HW and the FW of the main trace).

Our database highlights a large number of multi-scale en-echelon normal and normal oblique fault traces, showing a substantial similarity between synthetic and antithetic CoRs and Qfs (73% / 76%, and 27% / 24% respectively), suggesting a recurrence of the subdivision of the surface coseismic deformation for similar events in ~ ¾ on synthetic structures and ~ ¼ on antithetic structures. The database is relevant to the study of other earthquakes to better understand faulting processes and contribute to global probabilistic hazard approaches. Our novel dataset advances scientific knowledge about this fault system and refines scaling laws for intra-continental faults.