PRELIMINARY VIRTUAL NEOTECTONIC ANALYSIS OF FLATHEAD LAKE AND MISSION FAULTS, MONTANA

Flathead Lake and Flathead Valley sit in a topographically low position with respect to the neighboring Mission Range; the Mission and Flathead Fault system, a ~100km long set of young, segmented normal faults, form their boundary. Our preliminary study of the Flathead Lake fault system was part of a one-week-long project in a four-week-long virtual field course at the University of Houston in the summer of 2021. We used the following publicly available data and the following methods: 1) A literature review was conducted to understand the basic geology, offshore faults, and paleoseismology. 2) LiDAR imagery, obtained through the Montana Department of Natural Resources and Conservation, was used to make hillshades via ArcGIS, which allowed us to map this ~100km long fault system with an eye focused on trying to map active fault scarps. The northern portion of the Mission and Flathead Fault system consists of multiple segments in the Flathead Lake basin; the southern portion is a single, long fault cutting through the Mission Valley. Several of these faults cut lateral moraines and drumlins that formed during the last glacial maximum. 3) Fault scarp profiling and diffusion analysis was done using LiDAR-derived DEMs. 4) GPS data from the University of Nevada-Reno Geodesy Lab was used to measure regional movements, assess velocity gradients and elastic strains; V_e versus longitude plots show no detectable elastic strain accumulation (~E-W extension) across the fault system at the level of 1 mm/yr; V_up versus longitude plots show no detectable vertical elastic strain at the level of 2-3 mm/yr. 5) Finally, earthquake hazard and risk assessments were conducted by examining the history of earthquakes, calculated recurrence, and using our geodesy constraints. Understanding fault rupture history and present-day neotectonics is crucial due to a growing population as communities expand into the Flathead Valley.