A Study Of Seismic Ground Motion Amplification In Lushan County

Lushan County was severely damaged by the devastating "Lushan earthquake" on April 20, 2013. There are a lot of studies about the Lushan earthquake afterword. However there is still a lack of in-depth study on seismic response characteristics of near surface lateral variation in Lushan County. In this paper, the seismic amplification effect of Lushan County is studied by using the aftershock data of the "Lushan Earthquake".According to field investigation, we chose to use the reference site spectral ratio method to study the site effects of seismic ground motion in Lushan County. For the purpose, a temporary seismic array, using 9 very-broadband digital seismometers, were deployed at a rock reference site and 8 soil sites, to record aftershock data in Lushan County.The array was in operation from 11 am on April 25, 2013 to 17 pm on April 27, 2013. The seismic recordings were processed by the EDSP-IAS interactive software, and a total of 282 seismic events were identified from the observed data, of which 222 earthquake events could be found in the earthquake catalogue published by the Chinese national earthquake. The remainders, due to small size in magnitude, were not recorded in the national network earthquake catalog. After the VLF filtering and quality control, we chose 74 events with good quality for seismic amplification analysis. Signal to noise ratio for the data chosen is greater than 10. Spectral ratios of the data from soil sites to the reference site were calculated for the characterization of seismic amplification effect. In addition, ground penetrating radar exploration was also carried out. The data were used to build geological models for Sites L02 and L03. Modeling of seismic ground motion was performed on the models. Seismic amplification patterns of L02 and L03 were analyzed based on the modeling results.From the results given above, we have several conclusions on the seismic amplification in Lushan County as follows:(1) There is significant seismic amplification in the frequency range of 5 to 6Hz in Lushan County. Among all sites, the amplification at Site L07 is the largest, with an average peak amplification factor of 12.0;(2) The large difference of seismic amplification from site to site indicates that seismic site effects are closely related to local site seismic geological structure features;(3) The forward modeling of Sites L02 and L03 demonstrates that the approach we used for characterization of seismic amplification in Lushan county is effective;(4) Directional polarization is discovered to have a contribution to the seismic amplification at the sites.