Using ambient noise to characterise seismic slope response: The case of Qiaozhuang peri-urban hillslopes (Sichuan, China)

The Mw 7.9 Wenchuan earthquake of May 12th, 2008 shattered and induced failures on hillslopes surrounding the centre of the town of Qiaozhuang, located 250 Km NE of the mainshock epicentre. This motivated investigations on the possible occurrence of site amplification phenomena. The initial efforts involved a temporary accelerometer monitoring carried out between April and October 2009 on Weigan hill and Mount Dong, located SW and NE of the Qiaozhuang centre, respectively. The monitoring results revealed that the local geological setting, characterised by Silurian phyllites with sub-vertical schistosity (at Weigan) and by fractured Precambrian limestones (at Dong), exert major influence on the slopes resonance phenomena, with a secondary contribution due to the topographic setting. To extend the investigation on local slope response, a series of ambient noise recordings were conducted at several sites of the two hills, as well as at Mount Shizi (located NW of the town centre), which is topographically and geologically similar to Mount Dong. The focus was on the sites monitored by accelerometer stations, whose seismic records provided the opportunity to validate the outcomes of ambient noise analysis. Noise data were analysed using two approaches: the standard Nakamura's method and a new technique based on analysis of instantaneous polarization properties, aimed at estimating ellipticity of Rayleigh waves. Data interpretation was hampered by a contingent factor, i.e. environmental conditions characterised by a strong persistent E-W polarized noise at low frequencies (below 1 Hz), and by the complex of geologic and geomorphic conditions. The latter, particularly in the case of Weigan hill, appear responsible for a considerable amplification of vertical ground motions. The repeated recordings and the comparison of the outcomes of the two techniques of noise analysis with the accelerometer data showed that the new technique provides more stable and consistent results. Furthermore, the new technique was able to reveal site resonance properties that Nakamura's method fails to recognise because of transient overlapping of different type waves. Overall, the ambient noise analysis provided evidence of i) low frequency resonance (~1 Hz) acting at the scale of the entire relief (on Weigan Hill), ii) directional amplifications at intermediate frequencies (~4 Hz) affecting ridge crests made of fractured limestone (on Mounts Dong and Shizi), and iii) higher frequency resonances (7-20 Hz), which vary at a very local scale. This information is relevant for inferring the susceptibility of local slopes to earthquake-induced failures, and hence for the collateral seismic hazard assessment in the Qiaozhuan peri-urban areas.