Using Lg Waves to Study Attenuation Characteristics of Crustal Media and Station Site Responses before The 2008 Wenchuan M8.0 Earthquake

Abstract

On May 12, 2008, a catastrophic earthquake occurred in Sichuan, China. Prior to the event, the Sichuan Seismic Network (SSN) recorded a large amount of waveform data of small and medium earthquakes in this area. In this study, we collect seismic wave data recorded by the SSN between 2006 and 2008. We use the Fast Fourier Transform (FFT) method to calculate the amplitude spectrum of the Lg wave, and then use the amplitude spectrum of the Lg wave to calculate the seismic attenuation parameters and site response parameters. We analyze a total of 3,300 waveform records of 291 earthquakes. These earthquakes have magnitudes between ML 2.0 and ML 5.7 and are recorded at distances from 100 to 700 km. A joint inversion method, which use seismic spectral ratios to estimate attenuation and site responses, is used in this study; Model residuals were analyzed using a delete-j jackknife resampling technique. To evaluate the temporal variation of seismic wave attenuation, the data is regrouped in chronological order, and inversion is repeated for all regrouped data sets. The Wenchuan earthquake occurred on Longmen Mountains Fault (LMF), the LMF is the western boundary of the Sichuan Basin and the eastern boundary of the Bayan Har Block., so we focus on the two regions. All waveforms are processed at 0.2 Hz intervals in the frequency range of 1-7 Hz. In the Bayan Har Block area, the seismic wave attenuation relationship we obtained is Q(f)=250.2±13.7f0.52±0.03, while in the Sichuan Basin area, the seismic wave attenuation relationship is Q(f)=193±23f0.0.81±0.05 on the vertical component. The attenuation curves show that the spectral amplitudes decay faster in the Sichuan Basin than that in the Bayan Har Block. We calculated site responses for 48 stations, the site response between these stations changes less than 10 times in the frequency range of interest. The results from the regrouping of data in chronological order show that when the event is approaching, the changes in attenuation occur significantly, but the changes in site responses do not occur.