Lacustrine and Terrestrial Paleoseismic Records of the Twin Lakes Fault near Mount Hood, Oregon, USA

Oblique convergence along the Cascadia Subduction zone has induced long-lived and ongoing clockwise rotation of the western plate margin of North America, yet few recognized active faults at the latitude of northern Oregon accommodate resulting upper-plate deformation. Directly south of the Cascade volcano Mount Hood (Oregon), an uphill-facing scarp of the Twin Lakes fault impounds drainages of the Frog Lake and Lower Twin Lake basins, suggesting a history of late Pleistocene to Holocene surface ruptures. Investigations of terrestrial and lacustrine sedimentation at the fault strand reveal morphological, geophysical, and stratigraphic evidence of repeated major earthquakes on the Twin Lakes fault since the Last Glacial Maximum (LGM). Sediment cores extracted from these two fault-dammed basins capture rapid sedimentation event (RSE) deposits in shallow lake stratigraphy. I have ranked and quantified event indicators and interpret two RSE deposits in Lower Twin Lake as earthquake-triggered. Using calibrated radiocarbon dates from confining stratigraphic units, I model age constraints for the two events and find that both interpreted earthquakes post-date local age constraints for the LGM. New high-resolution bathymetry and sub-bottom seismic surveys of both lakes reveal graben structures against the eastern lakeshores and stratigraphic on-lapping at the main fault scarp. Improved geologic mapping finds glacial deposits offset across the Twin Lakes fault and paleolacustrine deposits uplifted from modern lake levels on the eastern footwall. At an uphill-facing scarp between the two basins, a hand-dug paleoseismic trench investigation captures offset of LGM and colluvial wedge deposits across a 2.5 m-wide zone of faults and fissures, indicating at least two post-LGM earthquakes. This new terrestrial and lacustrine evidence supports that the Twin Lakes fault has produced multiple late-Quaternary tectonic-related surface-rupturing earthquakes, designating it a local seismic hazard.