A depletion of high‐frequency ground motions on soil sites has been observed in recent large earthquakes and is often attributed to a nonlinear soil response. Here, I show that the reduced amplitudes of high‐frequency horizontal‐to‐vertical spectral ratios (HVSRs) on soil can also be caused by a smooth crustal velocity model with low shear‐wave velocities underneath soil sites. I calculate near‐fault ground motions using both 2D dynamic rupture simulations and point‐source models for both rock and soil sites. The 1D velocity models used in the simulations are derived from empirical relationships between seismic wave velocities and depths in northern California. The simulations for soil sites feature lower shear‐wave velocities and thus larger Poisson’s ratios at shallow depths than those for rock sites. The lower shear‐wave velocities cause slower shallow rupture and smaller shallow slip, but both soil and rock simulations have similar rupture speeds and slip for the rest of the fault. However, the simulated near‐fault ground motions on soil and rock sites have distinct features. Compared to ground motions on rock, horizontal ground acceleration on soil is only amplified at low frequencies, whereas vertical ground acceleration is deamplified for the whole frequency range. Thus, the HVSRs on soil exhibit a depletion of high‐frequency energy. The comparison between smooth and layered velocity models demonstrates that the smoothness of the velocity model plays a critical role in the contrasting behaviors of HVSRs on soil and rock for different rupture styles and velocity profiles. The results reveal the significant role of shallow crustal velocity structure in the generation of high‐frequency ground motions on soil sites.

中文翻译：

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平滑地壳速度模型导致 2D 动态破裂模拟中土壤的高频地面运动损耗

在最近的大地震中已经观察到土壤场地高频地面运动的消耗，这通常归因于非线性土壤响应。在这里，我表明，土壤上高频水平垂直谱比（HVSR）的幅度减小也可能是由土壤部位下方具有低剪切波速度的平滑地壳速度模型引起的。我使用二维动态破裂模拟和岩石和土壤场地的点源模型计算近断层地震动。模拟中使用的一维速度模型源自加利福尼亚北部地震波速度和深度之间的经验关系。与岩石场地相比，土壤场地的模拟具有较低的剪切波速度，因此在较浅的深度具有更大的泊松比。较低的剪切波速度导致较慢的浅层破裂和较小的浅层滑动，但土壤和岩石模拟对于断层的其余部分具有相似的破裂速度和滑动。然而，模拟的土壤和岩石场地的近断层地震动具有明显的特征。与岩石上的地面运动相比，土壤上的水平地面加速度仅在低频时被放大，而垂直地面加速度在整个频率范围内被去放大。因此，土壤上的 HVSR 表现出高频能量的消耗。平滑和分层速度模型之间的比较表明，速度模型的平滑度在 HVSR 对不同破裂类型和速度剖面的土壤和岩石上的对比行为中起着关键作用。