The seismic response of the Los Angeles basin, California
The seismic response of the Los Angeles basin, California
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DOI：
10.1785/bssa0880020337
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发表时间：
1998-04
影响因子：
3
通讯作者：
D. Wald;R. Graves
中科院分区：
文献类型：
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作者：
D. Wald;R. Graves
利用1992年兰德斯地震（Mw 7.3）在洛杉矶地区记录的强震数据，我们已经测试了现有的三维（3D）速度模型的准确性，在洛杉矶盆地和周围的圣费尔南多和圣加布里埃尔山谷的长周期（102秒）地面运动的模拟。首先，在观测中确定了盆地长期激发的总体模式和程度。在洛杉矶盆地内，记录到的振幅比盆地外的场地大大约三到四倍;圣费尔南多和圣加布里埃尔山谷内的振幅比周围基岩场地大近3倍。然后，使用3D有限差分数值模拟方法，我们分析了3D地球结构的变化如何影响模拟波形，振幅，以及与观测到的放大模式的拟合。在我们测试的南加州的3D速度模型中存在显著差异（Magistrale等人，1996; Graves，1996 a; Hauksson和Haase，1997）。模型的主要差异包括假设背景模型的速度;洛杉矶盆地的深度;以及较小盆地的深度、位置和几何形状。模型响应的最大差异出现在圣费尔南多谷和洛杉矶盆地的最深处。这些变化在很大程度上是由于盆地结构的变化，特别是有效深度范围的变化，这主要是由于对盆地沉积物填充性质的替代假设。一般地面运动特性匹配的3D模型模拟，验证使用3D建模与地质为基础的速度结构模型。然而，在扩大的总体模式和长期反应的持续时间方面存在重大缺陷。所讨论的用于再现记录的地面运动的模型的成功和局限性为地球结构模型的必要改进提供了基础和方向，无论是地质学的还是断层摄影学的。测试的地球模型响应的差异也转化为成功模拟数据的能力的可变成功，并在给定输入“场景”震源模型的情况下增加盆地响应估计的不确定性。
Using strong-motion data recorded in the Los Angeles region from the 1992 (Mw 7.3) Landers earthquake, we have tested the accuracy of existing three-dimensional (3D) velocity models on the simulation of long-period (≧2 sec) ground motions in the Los Angeles basin and surrounding San Fernando and San Gabriel Valleys. First, the overall pattern and degree of long-period excitation of the basins were identified in the observations. Within the Los Angeles basin, the recorded amplitudes are about three to four times larger than at sites outside the basins; amplitudes within the San Fernando and San Gabriel Valleys are nearly a factor of 3 greater than surrounding bedrock sites. Then, using a 3D finite-difference numerical modeling approach, we analyzed how variations in 3D earth structure affect simulated waveforms, amplitudes, and the fit to the observed patterns of amplification. Significant differences exist in the 3D velocity models of southern California that we tested (Magistrale et al., 1996; Graves, 1996a; Hauksson and Haase, 1997). Major differences in the models include the velocity of the assumed background models; the depth of the Los Angeles basin; and the depth, location, and geometry of smaller basins. The largest disparities in the response of the models are seen for the San Fernando Valley and the deepest portion of the Los Angeles basin. These arise in large part from variations in the structure of the basins, particularly the effective depth extent, which is mainly due to alternative assumptions about the nature of the basin sediment fill. The general ground-motion characteristics are matched by the 3D model simulations, validating the use of 3D modeling with geologically based velocity-structure models. However, significant shortcomings exist in the overall patterns of amplification and the duration of the long-period response. The successes and limitations of the models for reproducing the recorded ground motions as discussed provide the basis and direction for necessary improvements to earth structure models, whether geologically or tomographically derived. The differences in the response of the earth models tested also translate to variable success in the ability to successfully model the data and add uncertainty to estimates of the basin response given input “scenario” earthquake source models.