Remote survey of fragile geologic features for use as earthquake ground motion constraints, Oregon and Washington, USA

Fragile geologic features (FGFs) are elements of the landscape that are vulnerable to destruction during sufficiently strong earthquake ground shaking. As result, the observation of extant FGFs on the landscape may constrain the maximum intensity of past earthquake shaking. McPhillips and Scharer (2022, Survey of fragile geologic features and their quasi-static earthquake ground motion constraints, southern Oregon, Bulletin of the Seismological Society of America 112 (1)) demonstrated the potential to derive useful ground motion constraints from rock towers, such as sea stacks, in the Pacific Northwest region of the United States. The data set presented here extends the survey of McPhillips and Scharer (2022) along the length of the Oregon and Washington coasts and locally inland. Rock towers were remotely surveyed using freely available oblique aerial imagery (https://www.oregonshorezone.info/ and https://apps.ecology.wa.gov/shorephotoviewer/Map/ShorelinePhotoViewer, last accessed 16 September 2022) and lidar point clouds (without filtering by classification; https://portal.opentopography.org/datasets, last accessed 16 September 2022). A total of 78 new, and potentially fragile, features were identified. Geometrical parameters for these features were extracted from the lidar data using methods described in McPhillips and Scharer (2022). The quasi-static failure accelerations and first resonance modes of the features were calculated from the geometrical parameters using Equations Two and Three, respectively, from McPhillips and Scharer (2022). These equations also require for material properties of the features. For the purpose of this remote survey, we used average values from McPhillips and Scharer (2022): 2.4 grams per cubic centimeter, bulk density; 1.7 megapascals, tensile strength; and 8.0 gigapascals, Young's Modulus. The sea stacks in this survey are composed of rocks similar to southern Oregon, including marine sandstone, basalt, and basaltic melange (https://www.dnr.wa.gov/geologyportal and https://gis.dogami.oregon.gov/maps/geologicmap/, accessed 10 July 2022). Bulk density and tensile strength estimates were also measured in the field for sandstone at Toleak Point, in Washington State, in April 2022. There, tensile strength was measured in situ using a rebound hammer, using methods from McPhillips and Scharer (2022), and found to be approximately 1.5 megapascals. Bulk density was estimated by measuring the displacement (volume) and mass of cobbles on the beach and found to be approximately 2.3 grams per cubic centimeter. These values are interpreted to support the choice to use average values from McPhillips and Scharer (2022). Among the 78 surveyed features, 55 are likely old enough to have experienced at least two megathrust earthquakes. Following McPhillips and Scharer (2018), we estimated the ages of sea stacks as a function of distance from sea cliffs, in this case using a threshold of greater than 94 m. We also assumed that all inland features are sufficiently old. Among these 55 features, the average failure acceleration is 2.29 g and the 20th percentile value is 0.77 g. McPhillips and Scharer (2022) showed that the uncertainty for individual failure acceleration estimates is frequently greater than 50%, and similar uncertainties are expected for the data reported here. The average first resonance mode is 0.11 seconds. The fragility accelerations presented here may be suitable for comparison with spectral accelerations derived from earthquake ground motion simulations near the periods of the first resonance modes; these data are maximum constraints, and should not be taken to represent the most likely shaking intensities.