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Smooth Crustal Seismic Velocity Models Cause a Depletion of High-Frequency Ground Motions on Soil

Session: How Well Can We Assess Site Effects So Far? II

Type: Oral

Date: 4/20/2021

Presentation Time: 03:00 PM Pacific

Description: 

A depletion of high-frequency ground motions on soil sites has been observed in recent large earthquakes and is often attributed to the nonlinear soil response. Here we show that this behavior of soil can also be caused by a smooth crustal seismic velocity model with low shear wave velocities underneath soil sites. We calculate near-fault ground motions using both 2-D dynamic rupture simulations and point source models for both rock and soil sites. The 1-D 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 horizontal-to-vertical spectral ratios on soil, though overall higher than those on rock, exhibit a depletion of high-frequency energy. Our results reveal the significant role of shallow crustal velocity structure in the generation of high-frequency ground motions on soil sites.

Presenting Author: Yihe Huang

Student Presenter: No

Authors