Calibration of the near-surface seismic structure in the SCEC community velocity model version 4
Calibration of the near-surface seismic structure in the SCEC community velocity model version 4
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SCEC社区速度模型版本4中近地表地震结构的校准
DOI：
10.1093/gji/ggac175
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发表时间：
2022
影响因子：
2.8
通讯作者：
Day, Steven M.
中科院分区：
文献类型：
--
作者：
Hu, Zhifeng;Olsen, Kim B.;Day, Steven M.
近地表地震结构(埋深约1000米)，特别是横波速度(VS)对地震波的传播有很大影响，因此必须对其进行精确标定，以便进行地面运动模拟和地震危险性评估。顶部(300微米)地壳的VSS通常通过钻孔研究、岩土测量以及水井和油井来很好地描述，而大约1000微米以下的物质的速度通常由层析成像研究确定。然而，在缺乏浅层岩性分层信息的深度范围内，通常是沉积盆地外的岩石遗址，由于地震层析成像的分辨率限制，这两个区域之间的材料参数特征通常很差。当隐含的地质约束不可用时，模型，如南加州地震中心(SCEC)社区速度模型(CVM)，默认使用不能解决最高VS值的区域层析估计，因此提供不切实际的高浅层VS值。SCEC统一社区速度模型(UCVM)软件包括一种合并近地表地球结构的方法，方法是基于时间平均VSin top 30 m(VS30)的测量应用通用覆盖，将模型的上部逐渐变细，与350米深的层析成像合并，这可以应用于通过UCVM访问的任何速度模型。然而，我们使用CVM-S4.26.M01模型对洛杉矶地区2014-5.1拉哈布拉地震进行的3-D模拟大大低估了顶部350米材料特性被通用覆盖显著修改的场地的低频(<；1赫兹)地面运动。另一方面，将基于VS30的浅层速度锥度向下延伸到约1000米的深度，改善了我们的合成数据与这些场地的地震数据之间的拟合，而不会影响良好约束场地的拟合。我们探索了不同的锥化深度，表现出随着锥化深度的增加而增加的放大作用，1000米锥化深度的模型产生了总体上有利的结果。与速度锥化的影响相比，变化的滞弹性衰减的影响很小，并且不会显著地偏离估计的锥化深度。虽然在模型中采用了统一的锥化深度，但我们观察到了一些空间变化，这可能会进一步改进我们的方法。
The near-surface seismic structure (to a depth of about 1000 m), particularly the shear wave velocity (VS), can strongly affect the propagation of seismic waves and, therefore, must be accurately calibrated for ground motion simulations and seismic hazard assessment. TheVSof the top (<300 m) crust is often well characterized from borehole studies, geotechnical measurements, and water and oil wells, while the velocities of the material deeper than about 1000 m are typically determined by tomography studies. However, in depth ranges lacking information on shallow lithological stratification, typically rock sites outside the sedimentary basins, the material parameters between these two regions are typically poorly characterized due to resolution limits of seismic tomography. When the alluded geological constraints are not available, models, such as the Southern California Earthquake Center (SCEC) Community Velocity Models (CVMs), default to regional tomographic estimates that do not resolve the uppermostVSvalues, and therefore deliver unrealistically high shallowVSestimates. The SCEC Unified Community Velocity Model (UCVM) software includes a method to incorporate the near-surface earth structure by applying a generic overlay based on measurements of time-averagedVSin top 30 m (VS30) to taper the upper part of the model to merge with tomography at a depth of 350 m, which can be applied to any of the velocity models accessible through UCVM. However, our 3-D simulations of the 2014Mw5.1 La Habra earthquake in the Los Angeles area using the CVM-S4.26.M01 model significantly underpredict low-frequency (<1 Hz) ground motions at sites where the material properties in the top 350 m are significantly modified by the generic overlay (‘taper’). On the other hand, extending theVS30-based taper of the shallow velocities down to a depth of about 1000 m improves the fit between our synthetics and seismic data at those sites, without compromising the fit at well-constrained sites. We explore various tapering depths, demonstrating increasing amplification as the tapering depth increases, and the model with 1000 m tapering depth yields overall favourable results. Effects of varying anelastic attenuation are small compared to effects of velocity tapering and do not significantly bias the estimated tapering depth. Although a uniform tapering depth is adopted in the models, we observe some spatial variabilities that may further improve our method.
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