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Survey of fragile geologic features and their quasi-static earthquake ground motion constraints, southern Oregon

Devin McPhillips and Katherine M. Scharer
Survey of fragile geologic features and their quasi-static earthquake ground motion constraints, southern Oregon
Bulletin of the Seismological Society of America (September 2021) 112 (1): 419-437

Abstract

Fragile geologic features (FGFs), which are extant on the landscape but vulnerable to earthquake ground shaking, may provide geological constraints on the intensity of prior shaking. These empirical constraints are particularly important in regions such as the Pacific Northwest that have not experienced a megathrust earthquake in written history. Here, we describe our field survey of FGFs in southern Oregon. We documented 58 features with fragile geometric characteristics, as determined from field measurements of size and strength, historical photographs, and light detection and ranging point clouds. Among the surveyed FGFs, sea stacks have particular advantages for use as ground-motion constraints: (1) they are frequently tall and thin; (2) they are widely distributed parallel to the coast, proximal to the trench and the likely megathrust rupture surface; and (3) they are formed by sea cliff retreat, meaning that their ages may be coarsely estimated as a function of distance from the coast. About 40% of the surveyed sea stacks appear to have survived multiple Cascadia megathrust earthquakes. Using a quasi-static analysis, we estimate the minimum horizontal ground accelerations that could fracture the rock pillars. We provide context for the quasi-static results by comparing them with predictions from kinematic simulations and ground-motion prediction equations. Among the sea stacks old enough to have survived multiple megathrust earthquakes (n=16), eight yield breaking accelerations lower than the predictions, although they generally overlap within uncertainty. FGFs with the lowest breaking accelerations are distributed uniformly over 130 km of coastline. Results for inland features, such as speleothems, are in close agreement with the predictions. We conclude that FGFs show promise for investigating both past earthquake shaking and its spatial variability along the coasts of Oregon and Washington, where sea stacks are often prevalent. Future work can refine our understanding of FGF age and evolution.


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 112
Serial Issue: 1
Title: Survey of fragile geologic features and their quasi-static earthquake ground motion constraints, southern Oregon
Affiliation: U. S. Geological Survey, Earthquake Science Center, Pasadena, CA, United States
Pages: 419-437
Published: 20210914
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 98
Accession Number: 2021-065982
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., 1 table, geol. sketch map
N42°45'00" - N44°25'00", W118°07'60" - W116°04'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2022, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202146
Program Name: USGSOPNon-USGS publications with USGS authors
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