Seismic response of steep slopes inferred from ambient noise and accelerometer recordings: the case of Dadu River valley, China

Seismic site effects (local topographic / lithologic amplification, directivity phenomena) can have substantial impact on slope dynamic response and susceptibility to earthquake-induced failures. However, the instrumental data needed to quantify the site effects on slopes are still scarce. Here we investigate dynamic response of steep slopes in the Dadu River valley (Sichuan Province), one of the China’s regions most struck by large magnitude earthquakes. We analyse ambient noise recordings acquired at the sites of a local accelerometer network established few years after the 2008 Wenchuan earthquake. The analysis benefits from the application of a new technique that determines the ellipticity of Rayleigh waves present in noise wavefield from the instantaneous polarization analysis of the recorded signals. The technique is adapted for application to steep surface by rotating the noise recordings according to a reference having as axes the slope down-dip direction, the slope surface strike and the normal to the slope surface. This allows obtaining the curve of Rayleigh wave ellipticity as function of frequency, whose maxima provide information on site response characteristics, and leads to a more reliable identification of resonance phenomena on steep slopes (40° inclination). The results of the ambient noise analysis are consistent with the available accelerometer data and provide clear indication of significant resonance effects, with regard to their potential impact on local seismic slope stability. In particular, the slopes around the confluence of a local torrent with the Dadu River near the Lengzhuguan village exhibit strong site amplifications related to the presence of surficial deposits overlying intensely fractured granite bedrock. The frequency, amplitude and direction of resonance phenomena derived from the analysis of instrumental data indicate that site amplification is likely to have lesser impact on the stability of the lower part of the Mt. Dagang, which forms a long steep sub-planar slope facing the Dadu River. Our findings demonstrate that expeditious ambient noise recordings can provide useful information on relative susceptibility of slopes to seismically induced failures.