Temporal changes in velocity and scattering properties in the Sichuan region, China

The 2008 M7.9 Wenchuan Earthquake is the most devastating event in the last two decades in China. Here, we analyze 8 years (2007–2014) of seismic records to track the normal background level of the media properties, as well as the transient changes associated with tectonic activities (e.g., earthquakes). Understanding the long-term background pattern contributes to identifying transient changes. Temporal velocity variations of the surface waves show clear seasonal fluctuations and a co-seismic velocity drop after the Wenchuan mainshock in the 2-10 s period band. A comparison with meteoric data allows us to conclude that the main mechanism of the seasonal variation is the loading due to precipitation. The seasonal velocity changes exhibit spatial characteristics, where the amplitudes of the seasonal velocity changes are larger in the Tibet Plateau and smaller in the Sichuan basin. The spatial pattern is consistent with that of the tectonic deformation. The deformation is strong in the Tibet Plateau, while the Sichuan basin is relatively stable. Furthermore, there is a higher density of cracks in the Tibet Plateau than that in the basin.

Moreover, we increased the time resolution of the noise cross-correlations to twenty days to investigate changes in media associated with the 2008 mainshock by analyzing 1.5 years of data from mid 2007 to 2008 for the surface wave at 2-10 s period band. Compared with the waveforms from seismically quiet time, we observe that the waveform similarity of surface waves decreased significantly about 10 days prior to the mainshock and persisted low until the end of September 2008. To exclude that our observation is related to a changing source pattern, we analyzed the seismic activity in the Sichuan region and the frequency spectrum of the ambient noise field in the corresponding time. Our results suggest that the decorrelation may indeed indicate a regional change in the scattering properties starting 10 days prior to the Wenchuan earthquake.