We develop ensemble ShakeMaps for various magnitude 9 (⁠M 9) earthquakes on the Cascadia megathrust. Ground‐shaking estimates are based on 30 M 9 Cascadia earthquake scenarios, which were selected using a logic‐tree approach that varied the hypocenter location, down‐dip rupture limit, slip distribution, and location of strong‐motion‐generating subevents. In a previous work, Frankel et al. (2018) used a hybrid approach (i.e., 3D deterministic simulations for frequencies <1 Hz and stochastic synthetics for frequencies >1 Hz⁠) and uniform site amplification factors to create broadband seismograms from this set of 30 earthquake scenarios. Here, we expand on this work by computing site‐specific amplification factors for the Pacific Northwest and applying these factors to the ground‐motion estimates derived from Frankel et al. (2018). In addition, we use empirical ground‐motion models (GMMs) to expand the ground‐shaking estimates beyond the original model extent of Frankel et al. (2018) to cover all of Washington State, Oregon, northern California, and southern British Columbia to facilitate the use of these ensemble ShakeMaps in region‐wide risk assessments and scenario planning exercises. Using this updated set of 30 M 9 Cascadia earthquake scenarios, we present ensemble ShakeMaps for the median, 2nd, 16th, 84th, and 98th percentile ground‐motion intensity measures. Whereas traditional scenario ShakeMaps are based on a single hypothetical earthquake rupture, our ensemble ShakeMaps take advantage of a logic‐tree approach to estimating ground motions from multiple earthquake rupture scenarios. In addition, 3D earthquake simulations capture important features such as strong ground‐motion amplification in the Pacific Northwest’s sedimentary basins, which are not well represented in the empirical GMMs that compose traditional scenario ShakeMaps. Overall, our results highlight the importance of strong‐motion‐generating subevents for coastal sites, as well as the amplification of long‐period ground shaking in deep sedimentary basins, compared with previous scenario ShakeMaps for Cascadia.

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卡斯卡迪亚俯冲带9级地震的Ensemble ShakeMaps

我们针对卡斯卡迪亚特大推力上的各种9级地震（⁠M9）地震开发了整体的ShakeMap。地震动估计是基于卡斯卡迪亚30 M 9地震情景而定的，这些情景是使用逻辑树方法选择的，该方法改变了震中位置，下倾破裂极限，滑移分布以及产生强烈运动的子事件的位置。在先前的工作中，Frankel等人。（2018年）使用混合方法（即，频率小于1 Hz的3D确定性模拟和频率大于1 Hz的随机合成）和统一的站点放大因子，从这30个地震场景中创建了宽带地震图。在这里，我们通过计算太平洋西北地区特定地点的放大因子，并将这些因子应用于从Frankel等人得出的地面运动估计中，来扩展这项工作。（2018）。此外，我们使用经验地面运动模型（GMM）将震撼性估算范围扩展到Frankel等人的原始模型范围之外。（2018）涵盖华盛顿州，俄勒冈州，加利福尼亚北部和不列颠哥伦比亚省南部的所有地区，以促进在整体范围的风险评估和情景规划练习中使用这些集成的ShakeMap。使用这套更新的30 M 9卡斯卡迪亚地震场景，我们给出了中，第二，第十六，第84和第98个百分位地震动强度量度的整体ShakeMap。传统的方案ShakeMap是基于单个假设的地震破裂，而我们的集合ShakeMap利用逻辑树方法来从多个地震破裂方案中估计地震动。此外，3D地震模拟捕获了重要特征，例如西北太平洋沉积盆地中的强烈地震动放大，而在组成传统方案ShakeMaps的经验GMM中却没有很好地表现出来。总的来说，与以前的卡斯卡迪亚方案ShakeMaps相比，我们的结果强调了在沿海地区产生强烈运动的子事件的重要性，以及深部沉积盆地中长周期地震动的放大。