Modeling the Long-Period Basin Response in San Fernando and Los Angeles Regions During the 1994 Northridge Earthquake

9422238格雷夫斯目标拟议工作的目的是了解和量化圣费尔南多和洛杉矶沉积盆地对北岭地震期间长周期(1-10秒)地面运动响应的影响，并为在今后的地震危险性分析中纳入盆地影响调查提出建议。北岭地震及其余震产生了有史以来有记录的单一地震序列中最全面的一组强震数据。这些数据允许进行广泛的研究，包括对洛杉矶地区内的震源、路径和场地影响进行分析和建模。一个重要的关注领域是了解圣费尔南多和洛杉矶地区下方的深部沉积盆地对这次地震期间长周期(1-10秒)地震能量的放大和聚焦的影响。以前的工作表明，与洛杉矶和圣费尔南多盆地地质结构有关的波传播效应对观测到的地面运动有重大影响，特别是在长于约1秒的周期内。在许多情况下，这些盆地诱导的效应在1-10秒的周期范围内主导着地面运动。在北岭地震期间，观察到了无法用常规衰减关系解释的局部地区强烈的地面震动和相关破坏(例如，圣莫尼卡，I-10高速公路坍塌)。重要的是要了解这些现象的原因并解决它们对工程设计的影响，特别是对对长周期能量最敏感的大型结构(如高层建筑、高速公路桥、基础隔震结构)。方法和预期结果这项调查涉及分析北岭地震及其余震的地面运动记录，以便系统地绘制圣费尔南多和洛杉矶地区内观测到的盆地效应图。这种分析的一个关键要素是使用三维弹性有限差分技术对这些数据进行数值建模。有限差分方法很有吸引力，因为它允许灵活地合并强烈不均匀的速度模型，以及使用有限的、可变滑移源模型。这个项目的目标是：(1)在圣费尔南多和洛杉矶地区的精细空间网格上产生长时间(1-10秒)的时程；(2)以地图格式显示结果，以说明所产生的地面运动的空间分布(例如，波场时间片、峰值速度分布和震动持续时间)以及这些运动与地下地质的关系；(3)将这些结果与现有的经验衰减关系以及一维速度模型的计算结果进行比较，以便量化盆地响应的影响，以便在未来的地震危险性分析中使用。***

9422238 Graves Objective The objective of the proposed work is to understand and quantify the effects that the San Fernando and Los Angeles sedimentary basins had on the long-period (1-10 sec) ground motion response during the Northridge earthquake, and to formulate recommendations for the inclusion of basin effects investigations in future seismic hazard analyses. Significance The Northridge earthquake and its aftershocks have produced the most comprehensive set of strong motion data ever recorded for a single earthquake sequence. These data allow for a wide range of research involving the analysis and modeling of earthquake source, path and site effects within the Los Angeles region. One important area of concern is to understand the effect that the deep sedimentary basins which underlie the San Fernando and Los Angeles regions had on the amplification and focusing of long-period (1-10 sec) seismic energy during this earthquake. Previous work has shown that wave propagation effects related to the geologic structure of the Los Angeles and San Fernando basins have a significant impact on the observed ground motions, particularly for periods longer than about 1 sec. and in many cases, these basin-induced effects dominate the ground motions in the period range 1-10 sec. During the Northridge earthquake, localized areas of intense ground shaking and associated damage were observed that cannot be explained by conventional attenuation relations (e.g., Santa Monica, I-10 freeway collapse). It is important to understand the cause of these phenomena and address their implications for engineering design, particularly for large structures (e.g., tall buildings, freeway bridges, base-isolated structures), which are most sensitive to long-period energy. Method and Anticipated Results This investigation involves the analysis of ground motion recordings of the Northridge earthquake and its aftershocks in order to systematically map out the observed basin effects within the San Fernando and Los Angeles regions. A key element of this analysis is the numerical modeling of these data using a 3D elastic finite-difference technique. The finite-difference approach is attractive because it allows flexibility in incorporating strongly heterogeneous velocity models, as well as using a finite, variable-slip source model. The goal of this project is to: (1) produce long-period (1-10 sec) time histories calculated on a fine spatial mesh throughout the San Fernando and Los Angeles regions; (2) display the results in map format, which will illustrate the spatial distribution of generated ground motions (e.g., wave field timeslices, distribution of peak velocity and duration of shaking) and the correlation of these motions with the subsurface geology; and (3) compare these results with those obtained using existing empirical attenuation relations, as well as the results calculated for 1D velocity models, in order to quantify the effects of the basin response for use in future seismic hazard analyses. ***