Characterization of the Quaternary Active Twin Lakes Fault, Mount Hood, Oregon

The High Cascades chain of volcanoes results from Cascade Arc volcanism and is responsible for the generation of the highest topography in the state of Oregon. Surrounding Mt. Hood at the crest of the Cascade chain in northern Oregon is the Mt. Hood Fault Zone, an extensional fault zone of generally north-south trending normal faults that down-drop this high volcanic topography. The Twin Lakes Fault is located just south of Mt. Hood and is one of the southernmost faults in the Mt. Hood Fault Zone, previously mapped as approximately 12-13 km in length. This study provides the first in-depth characterization of the Twin Lakes Fault in Oregon's Mt. Hood Fault Zone. The extent of previous work includes only a lidar-derived trace map of the fault and limited field reconnaissance of selected sites. With new lidar data, I utilize GIS to evaluate fault scarps, measure vertical separation across the fault, and revise the existing fault trace mapping. I use a Matlab GUI to analyze 35 topographic profiles taken perpendicular to fault strike to measure vertical separation of correlative surfaces across the fault. Geologic mapping is improved along the length of the Twin Lakes Fault and is paired with fault offsets derived from lidar to investigate whether older units had experienced more offset than younger units. Finally, with empirical fault-scaling relationships, I estimate the potential magnitudes of surface rupturing earthquakes able to be generated by the Twin Lakes Fault. I find that the Twin Lakes Fault is approximately 16 km long, an addition of 3 km compared to the original trace. The updated surficial geology map adds fluvial and lacustrine features and changes glacial-bedrock contacts. I have enhanced its resolution to 1:70k scale from the original 1:100k scale. Offsets between correlative surfaces at scarps are inconsistent within the same units, indicating that the fault may not always rupture along its entire length. An empirical fault-scaling relationship between fault length and earthquake magnitudes suggests a 16 km long normal fault is capable of producing a Mw 6.4 earthquake. Given the fault's proximity to major infrastructure in places such as the city of Portland or the Columbia River Gorge, I propose that the fault is a seismogenic hazard for the region.