Survey of Fragile Geologic Features and Their Quasi-Static Earthquake Ground-Motion Constraints, Southern Oregon
Survey of Fragile Geologic Features and Their Quasi-Static Earthquake Ground-Motion Constraints, Southern Oregon
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俄勒冈州南部脆弱地质特征及其准静态地震地面运动约束的调查
DOI：
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
K. Scharer
中科院分区：
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作者：
D. McPhillips;K. Scharer
脆弱的地质特征（FGFs），这是现存的景观，但容易受到地震地面震动，可能会提供地质约束的强度，以前的震动。这些经验上的限制在太平洋西北部这样的地区尤其重要，这些地区在有文字记载的历史上没有经历过大规模逆冲断层地震。在这里，我们描述了我们在俄勒冈州南部的FGFs的实地调查。我们记录了58个脆弱的几何特征，确定从现场测量的大小和强度，历史照片，光探测和测距点云。在所调查的FGF中，海堆在用作地面运动约束方面具有特别的优势：（1）它们通常又高又薄;（2）它们广泛分布于平行于海岸的地方，靠近海沟和可能的巨型逆冲断层破裂面;（3）它们是由海崖后退形成的，这意味着它们的年龄可以根据与海岸的距离进行粗略估计。大约40%的被调查的海烟囱似乎在多次卡斯卡迪亚大推力地震中幸存下来。使用准静态分析，我们估计的最小水平地面加速度，可能破裂的岩柱。我们提供的准静态的结果进行比较，从运动学模拟和地面运动预测方程的预测。在足够老的海烟囱已经幸存下来的多个巨型逆冲断层地震（n = 16），8个产量打破加速度低于预测，虽然他们一般重叠的不确定性。断裂加速度最低的FGFs均匀分布在130 km的海岸线上。结果内陆功能，如洞穴，与预测密切一致。我们的结论是，FGFs显示的承诺，调查过去的地震震动和它的空间变化沿着海岸的俄勒冈州和华盛顿，海烟囱往往是普遍的。未来的工作可以完善我们对FGF年龄和进化的理解。
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.