THE MOST RECENT EARTHQUAKE ON THE MOUNT HOOD FAULT ZONE, NORTH-CENTRAL OREGON: IMPLICATIONS FOR CASCADING EARTHQUAKE, LANDSLIDE, AND FLOOD MULTI-HAZARDS IN THE COLUMBIA RIVER GORGE

The Mount Hood fault zone (MHFZ) is a N-trending, ~55 km-long zone of active faulting along the western margin of the High Cascades graben at the crest of the Oregon Cascade Range. We report new results from geologic mapping, fault scarp analysis, and a hand-dug paleoseismic trench on the Gate Creek fault (GCF), the northernmost segment of the MHFZ. The northern ~8 km of the GCF is a NNW-striking fault scarp. Vertical separation across this scarp varies along strike from ~80 m where it apparently cuts ~0.5-Ma Mount Defiance lava flows, to ~25 m in glacial deposits possibly dating to ca. 140 ka, to 1–2 m in glacial deposits likely from the last glacial maximum ca. 20 ka. These observations indicate a steady vertical offset rate of 0.1–0.2 mm/yr for the last ~0.5 Myr. Deflection of drainages incised into Miocene to Quaternary geologic units and secondary faults observed in the trench suggest a right-lateral component of slip (transtension) for this part of the MHFZ. Larger (23-80 m) scarps are cut by sharp, 1-2 m-high scarps, indicating the most recent earthquake (MRE) was quite recent. The paleoseismic trench, ~5 km south of the Columbia River, revealed indurated lodgement till overlain by ~1 m of unconsolidated meltout till and a ~10 cm-thick A/O-horizon soil. The contact between glacial units is vertically offset 1.2 m across a 20–30 cm-wide fault zone, identical to the ground surface offset. On the downthrown (east) side of the fault, the A/O-horizon soil contains abundant large charcoal fragments and is buried by a single, undeformed colluvial wedge deposit we interpret to post-date a single surface-rupturing earthquake. An OxCal model of 15 ¹⁴C ages brackets soil burial and the MRE to AD 1363–1495 (2σ). The nearby 15 km² Bonneville landslide blocked the Columbia River 75 m deep between AD 1426–1448 (¹⁴C 2σ). The impounded lake then partly breached the landslide dam, causing downstream flooding in the Portland basin prior to AD 1479–1482. New ground motion modeling of a ~M7 MHFZ earthquake suggests strong shaking at Table Mountain, the headscarp of the Bonneville landslide ~15 km west of the MHFZ, could have initiated the landslide. Thus, it is possible a catastrophic hazard cascade of earthquake, earthquake-triggered landslide, and then flood from breaching of the landslide dam affected the Columbia River Gorge in the 15^th century.