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.

脆弱地质地貌（Fragile geologic features, FGFs）指在足够强烈的地震地面震动作用下易遭受破坏的地表地貌单元。因此，对现存FGFs的观测可约束过往地震地面震动的最大烈度。McPhillips与Scharer（2022，《美国地震学会通报》（Bulletin of the Seismological Society of America）112(1)刊载的《美国南俄勒冈州脆弱地质地貌及其准静态地震地面运动约束》）已证实，可从美国太平洋西北地区的海蚀柱等岩塔中获取有效的地面运动约束数据。本次发布的数据集将McPhillips与Scharer（2022）的调查范围扩展至俄勒冈州与华盛顿州海岸沿线及局部内陆区域。研究团队通过免费公开的斜拍航空影像（https://www.oregonshorezone.info/ 和 https://apps.ecology.wa.gov/shorephotoviewer/Map/ShorelinePhotoViewer，最后访问时间：2022年9月16日）及未按分类进行滤波处理的激光雷达（lidar）点云（https://portal.opentopography.org/datasets，最后访问时间：2022年9月16日）对岩塔开展遥感调查，共计识别出78处全新的、具有潜在脆弱性的地貌。研究团队采用McPhillips与Scharer（2022）提出的方法，从激光雷达数据中提取了这些地貌的几何参数。基于几何参数，结合McPhillips与Scharer（2022）提出的第2式与第3式，分别计算得到了各地貌的准静态破坏加速度与一阶共振模态。上述公式还需要输入地貌的材料属性参数，本次遥感调查采用了McPhillips与Scharer（2022）给出的平均值：体积密度2.4克每立方厘米、抗拉强度1.7兆帕、杨氏模量（Young's Modulus）8.0吉帕。本次调查涉及的海蚀柱与南俄勒冈州的岩性相似，包括海相砂岩、玄武岩及玄武岩混杂岩（https://www.dnr.wa.gov/geologyportal 和 https://gis.dogami.oregon.gov/maps/geologicmap/，最后访问时间：2022年7月10日）。2022年4月，研究团队在华盛顿州Toleak点对砂岩开展了现场实测，以验证体积密度与抗拉强度的估算值：采用回弹仪按McPhillips与Scharer（2022）的方法进行原位抗拉强度测试，得到的结果约为1.5兆帕；通过测量砾石的位移（体积）与质量估算体积密度，得到的结果约为2.3克每立方厘米。上述实测值佐证了采用McPhillips与Scharer（2022）给出的平均值的合理性。在本次调查的78处地貌中，有55处的形成年代足够久远，可能至少经历过2次巨逆冲地震（megathrust earthquake）。参照McPhillips与Scharer（2018）的方法，研究团队以距海崖的距离作为函数估算海蚀柱的年龄，本次研究将阈值设为大于94米，并假设所有内陆地貌均满足足够久远的条件。在这55处地貌中，平均破坏加速度为2.29 g，20百分位值为0.77 g。McPhillips与Scharer（2022）已指出，单个破坏加速度估算值的不确定性通常超过50%，本次发布的数据预计也存在类似的不确定性。本次调查得到的平均一阶共振模态周期为0.11秒。本次发布的脆弱性加速度可用于与一阶共振模态周期附近的地震地面震动模拟所得的谱加速度进行对比；本数据集给出的为最大约束条件，不应将其视为最可能的震动烈度。