Collaborative Research: A New 3-D Velocity and Structural Model of the Northern Basins in the Los Angeles Region for Improved Ground Motion Estimates

下一次发生在圣安德烈亚斯断层南部的大地震预计将在加州洛杉矶以北数百公里处产生强烈的地面震动。之所以会发生这种情况，是因为地球表面下的地质结构起到了引导地震波进入洛杉矶市中心的作用。研究表明，人们所能预料到的地面运动可能比地震危险模型所预测的要大4倍。这种差异可能会大大增加地震造成的损害。在规划大洛杉矶地区的地震恢复能力时，也应该考虑到这一点。对地面震动的不准确估计是由于地球模型不完整，没有正确地表示引导地震能量的地质结构。在这里，研究人员建立了该地区更详细的地球模型。他们使用了2017年至2020年间进行的一种新型地震现场实验的记录；它包括沿研究区域的10条线布置700多台现代仪器。这种密集的几何结构使研究小组比以往任何时候都能更好地拍摄到引导地震能量的地下结构。研究人员对密集数据集进行了一整套分析。他们改进和更新了区域地震速度模型。他们模拟了预期的地面运动来解释新发现。新的地球模型通过更新南加州地震中心社区速度模型提供给科学界；这是通过一个新开发的平台来完成的，该平台将高分辨率模型嵌入到更大规模的区域模型中。研究结果改进了洛杉矶地区的地震危险性评估。该合作项目还支持来自一个未被充分代表的群体的一名早期职业女性科学家，以及路易斯安那州立大学和加州理工学院的研究生和本科生。研究人员使用接收函数，利用记录的几个远震事件，沿着每条线构建结构的轮廓。该盆地的三维横波速度将通过基于节点和宽带站（作为调查的一部分部署）以及该地区南加州地震台网站之间环境噪声相关性的面波分析获得。他们利用盆地的重力数据，将剖面和三维横波速度估计结合起来。它们根据环境噪声和局部地震产生衰减（QS和QP）模型。预期结果将是一个具有明确基底界面的盆地的三维地球模型，并以比现有模型更高的分辨率获得穿过沉积层的横波速度。下一步，研究小组将用新模型模拟地震地面运动，并将强烈运动与噪声相关性预测的运动进行比较。该勘探的独特几何形状允许团队尝试更先进的方法，包括水平集反演、自相关方法，以及可能的全波形反演。这推动了其中一些方法首次应用于这种特定类型的数据集。这里开发的全套方法将适用于位于沉积盆地的世界其他主要城市，如旧金山、西雅图、墨西哥城和利马。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The next large earthquake on the southern San Andreas fault is expected to produce strong ground shaking hundreds of kilometers to the north, in Los Angeles, California. This will occur because the geologic structures beneath the Earth’s surface act as a wave guide channeling earthquake waves into downtown Los Angeles. It has been shown that the ground motions one can expect could be a factor of four larger than what earthquake hazard models predict. This difference may significantly increase damages resulting from earthquakes. It should also be accounted for when planning for earthquake resilience in the greater Los Angeles area. The inaccurate estimates for ground shaking result from incomplete Earth models which do not properly represent the geologic structures that channel seismic energy. Here, the researchers build a more detailed Earth model of the region. They use recordings from a new style of seismic field experiment carried out between 2017 and 2020; it consisted of laying out more than 700 modern instruments along 10 lines across the study area. This dense geometry allows the team to image better than ever before the subterranean structures channeling earthquake energy. The researchers carry out a full suite of analyses of the dense dataset. They refine and update models for regional seismic velocity. They simulate expected ground motions accounting for the new findings. The new Earth models are made available to the scientific community by updating the Southern California Earthquake Center community velocity models; this is done using a newly developed platform for embedding high-resolution models into larger-scale regional models. Results of the study improve seismic hazard assessment in the Los Angeles area. This collaborative project also supports a female early-career scientist from an underrepresented group, and graduate and undergraduate students at Louisiana State University and the California Institute of Technology. The researchers use receiver functions to construct profiles of the structure along each of the lines, using several teleseismic events that were recorded. The three-dimensional (3-D) shear wave velocities for the basins will be obtained from surface wave analysis based on ambient noise correlation between the nodes and broadband stations that were deployed as part of the survey, and the Southern California Seismic Network stations in the region. They combine the profiles and 3-D shear wave velocity estimates by using the gravity data across the basins. They produce attenuation (QS and QP) models from ambient noise and local earthquakes. The expected result will be a 3-D Earth model for the basins with a well-defined basement interface and shear wave velocities down through the sedimentary layers obtained at a higher resolution than existing models. As a further step, the team will simulate earthquake ground motions with the new model and compare strong motions to those predicted from noise correlations. The unique geometry of the survey allows the team to try more advanced methods including level-set inversion, autocorrelation methods, and possibly full waveform inversion. This advances the application of some of these methodologies for the first time to this particular type of data set. The full suite of methodologies developed here will be applicable other major cities around the world that are located on sedimentary basins such as San Francisco, Seattle, Mexico City and Lima.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.