The 1994 Northridge Earthquake and Prediction of Ground Motions on the Hanging Walls and Foot Wall of Thrust Events

本研究的目的是利用1994年Northridge地震和其他逆冲地震的强震数据，建立逆冲断层周围地震动预测的经验模型，包括上、下盘地震动的差异。已知倾斜断层上盘的地面运动明显大于下盘。这是在1994年北岭地震的地面运动中观察到的，位于北岭断裂上壁上的圣费尔南多谷北部的峰值加速度比位于北部圣盖博山脉的下壁侧同等距离记录的峰值加速度要大。北岭地震的峰值加速度比目前的经验衰减关系预测的要大得多（例如，在距离小于30公里的地方，峰值加速度要大40-50%），甚至考虑到冲推事件预期的20-30%的大运动。了解来自北岭地震的地面运动是否代表了来自北岭地震的地面运动是否代表了未来逆冲和/或盲目逆冲地震中预期的地面运动，对于经验地震动衰减关系的发展以及抗震建筑规范的修订具有重要意义。该项目利用1994年北岭地震和其他地震的强震数据，量化了地震机制对周期为0至4秒、距离为0至50公里、震级为6至7.5级的水平和垂直反应谱的影响。它使用了对断层和斜向地震记录运动的经验分析，并辅以数值模拟。这两种方法相互补充，将产生比单独使用任何一种方法所产生的结果更可靠的结果。断层类型因素的经验模型考虑了地震机制作用的距离和震级依赖性以及上、下盘地震动的差异。数值地震动模拟用于帮助理解地震机制对地震动影响的原因，并指导在回归分析中使用的函数形式的选择，以开发经验模型。它允许单独测试各种条件的影响，例如场地在上盘或下盘上的位置，破裂深度，应力降和破裂指向性。数值模拟还用于评估来自北岭地震的地震动，这些地震动比现有的逆冲地震地震动衰减关系预测的要大，对于一般逆冲事件，特别是对于盲逆冲事件来说，是否是异常的，或者它们是否代表了未来逆冲和/或盲逆冲地震中预期的地震动。＊＊＊

9416451 Somerville The objective of this study is to develop an empirical model for the prediction of ground motions around thrust faults, including differences between ground motions on the hanging wall and the foot wall, using strong motion data from the 1994 Northridge earthquake and other thrust earthquakes. Ground motions are known to be significantly larger on the hanging wall than on the foot wall of dipping faults. This was observed in the ground motions of the 1994 Northridge earthquake, where the peak accelerations in the northern San Fernando Valley, which lies on the hanging wall of the Northridge rupture, were larger than those recorded at equivalent distances on the foot wall side, which lies in the San Gabriel Mountains to the north. The peak accelerations from the Northridge earthquake were much larger (e.g. 40-50% larger at distances less than 30 km)than predicted by current empirical attenuation relations, even accounting for the 20-30% larger motions expected for thrust events. Knowing whether or not the ground motions from the Northridge earthquake are representative of ground motions from the Northridge earthquake are representative of ground motions expected in future thrust and/or blind thrust earthquakes has important implications for the development of empirical ground motion attenuation relations, and on the revision of seismic building codes. The project uses strong motion data from the 1994 Northridge earthquake and other earthquakes to quantify the effect of earthquake mechanism on horizontal and vertical response spectra for periods of 0 to 4 seconds, distances of 0 to 50 km, and magnitudes of 6 to 7.5. It uses an empirical analysis of recorded motions from thrust and oblique earthquakes, supplemented by numerical modeling. These two approaches compliment each other and will lead to more robust results than would result from using either approach by itself. The empirical model for the style-of-faulting factor considers the distance and magnitude dependence of the effect of earthquake mechanism as well as differences between ground motions on the hanging wall and foot wall. Numerical ground motion modeling is used to help understand the causes for the effect of earthquake mechanism on the ground motions and to guide the selection of the functional form used in the regression analysis to develop the empirical model. It permits the influences of various conditions, such as location of the site on the hanging wall or on the foot wall, the depth of rupture, stress drop, and rupture directivity to be tested separately. Numerical modeling is also used to evaluate whether the ground motions from the Northridge earthquake, which are larger than predicted by existing ground motion attenuation relations for thrust earthquakes, are anomalous for thrust events in general, and for blind thrust events in particular, or if they are representative of ground motions expected in future thrust and/or blind thrust earthquakes. ***