Numerical simulation of ground motion amplification in Modong area, Lushan

The April 20, 2013, Ms 7.0 Lushan Earthquake was a major earthquake that followed the Ms 8.0 Wenchuan Earthquake on May 12, 2008. Frequent earthquakes have caused heavy casualties and property loss in Western Sichuan. Earthquake disasters are often closely related to the amplification effect of ground motion. Studying the ground motion characteristics of near-surface geological structures helps to understand the distribution of potential earthquake disasters. In this study, we investigated ground motion amplification in the downtown area of Lushan using numerical simulation and aftershock data from the Lushan Earthquake. Using the Lushan earthquake aftershock data from nine seismic stations distributed in the area, the amplification effect of the sites was determined using the “reference site spectral ratio” method. The results show that the frequency of the ground motion amplification in the area was in the range 5–10 Hz, and the corresponding amplification peak was from 3 to 14. Among the study sites, the amplification (14 times) at L07 was the most prominent. To study further the amplification characteristics, shear-wave velocity models for the structures under these sites were established using passive-source Rayleigh surface-wave exploration. One-dimensional (1D) and two-dimensional (2D) seismic amplification effects were simulated using horizontally propagating shear-wave modeling. Except Site L07, the 1D simulation results of each site well reflected the variation feature of the seismic amplification on the frequency band below the observed peak frequency, although the overall simulated amplification peaks were smaller than the observed results. The 2D simulation of the remarkable amplification phenomenon at L07 was in better agreement with the observation result than was the 1D simulation, indicating that the seismic amplification in the Modong area is influenced by lateral variation of the Quaternary sediments.