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Preliminary survey of fragile geological features for use as ground motion constraints, southern Oregon

Devin McPhillips, & Katherine M. Scharer

Published August 15, 2019, SCEC Contribution #9736, 2019 SCEC Annual Meeting Poster #290

Fragile geological features, which are extant on the landscape but vulnerable to earthquake ground shaking, provide rare geological constraints on the intensity of prior shaking. These features may be especially valuable in regions such as the Pacific Northwest that have not experienced a major historic earthquake. Here, we describe our preliminary survey of fragile geological features (FGFs) in southern Oregon. Our objective is to assess the potential of FGFs to constrain earthquake ground motions; we do not present sufficient data for comparison with USGS hazards curves or maps. We documented 64 features that appeared sufficiently fragile to warrant further investigation. Because our aim was to discover FGFs over the largest possible area, we characterized the shape and strength of these features using a limited number of photographs, elevation surveys, and Schmidt Hammer measurements. Most of the FGFs are sea stacks distributed along the coast for 130 km from Bandon to Brookings. We also identified precariously balanced rocks, and, farther inland, evaluated speleothems and volcanic plugs. Sea stacks have advantages for use as ground motion constraints: (1) they are widely distributed along the coast; and (2) they are formed by sea cliff retreat, meaning that their ages may be coarsely estimated as a function of distance from the coast. Nearly all of the surveyed sea stacks appear to be at least several thousand years old, and therefore have likely experienced multiple Cascadia megathrust earthquakes. Using a quasi-static analysis, we estimate the minimum ground accelerations that could fracture the sea stacks at their bases. This analysis indicates that 65% of surveyed sea stacks (n=28) are vulnerable to ground accelerations of <1.0 g and 94% to <2.0 g. Inland, on the western slope of the Cascades, at least 5 FGFs yield minimum ground accelerations of <0.2 g. Although these values are subject to change with more sophisticated data and analysis, they indicate that FGFs can constrain prior earthquake shaking intensity in southern Oregon. The presence of sea stacks elsewhere along the coast of Oregon and Washington also suggests that there is the potential to investigate shaking variability along the length of the Cascadia Subduction Margin.

Key Words
fragile geological features, ground motion, geomorphology

McPhillips, D., & Scharer, K. M. (2019, 08). Preliminary survey of fragile geological features for use as ground motion constraints, southern Oregon. Poster Presentation at 2019 SCEC Annual Meeting.

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