Shallow landslides are significant natural hazards in Oregon, and identification of areas susceptible to future landslides is a critical step in reducing risk. Recent advances in identification of areas susceptible to shallow landslides are mostly based on geographic information system (GIS) calculations of the slope stability using the infinite slope equation. This technique was further improved with high-resolution light detection and ranging (LiDAR)–based digital elevation models (DEM) converted to very accurate slope data as input into the GIS models. However, these models still underestimate and overestimate the susceptibility in certain areas compared to past landslide events and field observations. One significant overestimation we noted occurs in regionally flat areas with isolated steep slopes that have very little relief. We developed a method to remove these isolated overestimated areas using a neighborhood analysis with a maximum relief of 1.22 m (4 ft). Because landslides that originate on the steep slope may extend back into the flat area above the slope, or out onto the flat area at the toe of the slope, we applied a 9 m (30 ft) buffer (twice our defined depth to failure for shallow landslides) for all of the areas with a calculated factor of safety (FOS) less than 1.5. We tested the methods on three landslide inventory databases examining two main criteria: (1) capture rate (overall and individual landslides) and (2) reduction in total map area susceptibility coverage while maintaining a high capture rate. We found the two methods maintained a capture rate between 90% and 99% while at the same time reducing the total map area susceptibility zones from 64% to 42%.

The pattern of deformation in the western part of the Columbia River flood basalt province contains two key components: (1) anticlinal uplifts of the Yakima Fold Belt with east-northeast to west-southwest trends, and (2) strike-slip fault zones with dominantly northwest trends. It is the abundance and regional extent of the latter that distinguish this area from other parts of the province. There are many northwest-striking, right-lateral, strike-slip faults in the interval from the Willamette Valley eastward to Umatilla (123°W to 119°W longitude). Some of these faults are only a few kilometers long, whereas others are of regional extent (>100 km). Conjugate northeast-striking, left-lateral, strike-slip faults have also been identified but are far less numerous. Local variations in the stress field within basins have produced sets of subsidiary structures by transtension and transpression. These occur where fault zones change trend with respect to the NNW-SSE–oriented maximum principal compressive stress. Strike-slip faulting was active early in the history of the Yakima Fold Belt uplifts, at least by emplacement of the Columbia River Basalt Group lavas, but after the Yakima Fold Belt uplift, spacing had already been firmly established. It is probable that many of these faults are episodically reactivated basement structures that have repeatedly undergone cycles of emergence, burial by flood basalts, and reemergence. Strike-slip deformation appears to have happened simultaneously within the Yakima Fold Belt uplifts and adjacent synclinal basins. However, the pattern and magnitude of deformation differ significantly in the basins compared to the uplifts. The Yakima Fold Belt uplifts have been segmented and shifted many kilometers by strike-slip faults, while displacements within adjacent basins are orders of magnitude less. Within Yakima Fold Belt uplifts, reversals of vergence sometimes occur wherein the frontal (forelimb) thrusts and fold asymmetry switch from one side of the uplift to the other. These changes are accommodated by cross-trending, right-lateral, strike-slip faults of regional extent. The pattern of strike-slip deformation as mapped within basins in many cases appears to be immature and lacking in interconnection. Eruptive vents in the Simcoe backarc volcanic field and Boring lavas are often aligned along strike-slip faults. Pliocene-age Simcoe lava flows have been deformed by both folding and strike-slip faulting within the Klickitat Valley basin. Pleistocene-age deposits are known to be cut by both the Luna Butte and Portland Hills faults. Strike-slip earthquake focal mechanisms have also been determined for some faults.

ABSTRACT Basalt flows of the Columbia River Basalt Group (CRBG) host a series of regionally extensive aquifers between western Idaho and the Pacific Ocean that serve as an important source for domestic, municipal, agricultural, and industrial water supply throughout much of this area, and are the sole source for some communities in the Willamette Valley. Rapid growth and increased pumping have resulted in significant water level declines in some locales in the Willamette Valley, forcing some communities to develop other water sources, and/or develop aquifer storage and recovery projects to store water in CRBG aquifers. The CRBG generally consists of multiple concordant, tabular sheet flows. The primary water-bearing horizons within the CRBG are associated the vesicular and/ or brecciated flow top and flow bottom (pillow/hyaloclastite) structures that form the interflow zone between two flows. The interiors of the CRBG flows typically have limited vertical permeability and act as aquitards, creating a series of layered confined aquifers. The dominant groundwater flow pathway in the CRBG aquifer system is along these individual, laterally extensive, interflow zones. Tectonic structures may modify the dominant flow regime in the CRBG by offsetting or otherwise disturbing originally laterally continuous interflow zones. Faults result in a wide spectrum of effects on flow in the CRBG aquifers depending on the nature of the fault. The hydraulic properties inherent to CRBG aquifers, including high degree of confinement, low bulk permeability and limited recharge have led to overdraft conditions in many areas. Conversely, these characteristics create favorable conditions for aquifer storage and recovery system development in the central Willamette Valley and Tualatin Basin.

ABSTRACT Miocene flood basalts of the Columbia River Basalt Group inundated eastern Washington, Oregon, and adjacent Idaho between 17 and 6 Ma. Some of the more voluminous flows followed the ancestral Columbia River across the Cascade arc, Puget-Willamette trough, and the Coast Range to the Pacific Ocean. We have used field mapping, chemistry, and paleomagnetic directions to trace individual flows and flow packages from the Columbia River Gorge westward into the Astoria Basin, where they form pillow palagonite complexes and mega-invasive bodies into older marine sedimentary rocks. Flows of the Grande Ronde, Wanapum, and Saddle Mountains Basalts all made it to the ocean; at least 33 flows are recognized in the western Columbia River Gorge, 50 in the Willamette Valley, 16 in the lower Columbia River Valley, and at least 12 on the Oregon side of the Astoria Basin. In the Astoria Basin, the basalt flows loaded and invaded the wet marine sediments, producing peperite breccias, soft sediment deformation, and complex invasive relations. Mega-invasive sills up to 500 m thick were emplaced into strata as old as Eocene, and invasive dikes up to 90 m thick can be traced continuously for 25 km near the basin margin. Mega-pillow complexes up to a kilometer thick are interpreted as the remains of lava deltas that prograded onto the shelf and a filled submarine canyon southeast of Astoria, possibly providing the hydraulic head for injection of invasive sills and dikes at depth.

Topography and ground conditions were important factors in controlling the distribution of individual Columbia River Basalt Group (CRBG) flows in western Oregon. The Columbia trans-arc lowland, the Yakima fold belt, the Portland Hills–Clackamas River structural zone, and Cascadian volcanism largely controlled the distribution of CRBG flows across the Miocene Cascade Range. The first flows to cross the Miocene Cascades into the Willamette Valley encroached onto a low-relief topography generally consisting of eroded Tertiary-age marine sedimentary rocks deformed along northwest-trending structural zones, volcanic highs, and estuaries. No north-south trough affected the distribution and thickness of the CRBG in the Willamette Valley, but an incipient Coast Range acted as a leaky barrier to the Oregon coast. Water-saturated sediments rapidly extracted heat from advancing CRBG lava flows, producing narrow, abnormally thick lobes extending along existing topographic lows. Deformation along the northwest-trending Portland Hills–Clackamas River structural zone produced a major topographic barrier early and late in the incursion of CRBG flows. The CRBG thins across this zone from 600 to 150 m. This zone diverted the earliest Grande Ronde flows into and through the Portland Basin. Some of the succeeding R 2 and N 2 Grande Ronde flows were able to cross this zone and followed another structural low, the Sherwood trough, to the Oregon coast. The total thickness of CRBG along the Sherwood trough is approximately 300 m, about twice that on either side. Paleodrainage developed during time intervals between emplacement of CRBG flows. The positions of these drainage courses were influenced by the position of the CRBG flow margins and/or structural lows. A longer hiatus between flows (> 100,000 yr) enabled rivers to develop major canyons by headward erosion, which served to channelize subsequent CRBG flows.