GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 2.2 261 INVESTIGATION OF THE ROCK AVALANCHE DEPOSIT OF YANG JIA GOU (SICHUAN-CHINA) THROUGH THE INTEGRATION OF PASSIVE SEISMIC TECHNIQUES P. Capone 1 , V. Del Gaudio 1 , J. Wasowski 2 , N. Venisti 1 , H. Wei 3 1 Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari “Aldo Moro”- Bari, Italy 2 Consiglio Nazionale delle Ricerche - Istituto di Ricerca per la Protezione Idrogeologica - Bari, Italy 3 State Key Laboratory of Geo-Hazard Prevention and Geo-Environment Protection, Chengdu University of Technology, P.R. China Introduction. One of the over 200,000 landslides triggered in the mountainous region of Longmen Shan (Sichuan – China) by the Mw 7.9 Wenchuan earthquake of May 12 2008 (cf. Xu et al., 2014) was a large rock avalanche that dammed a stream in a gully named Yang Jia Gou. We investigated structure and physical properties of the rock avalanche deposit as a contribution to define the stability conditions of the landslide dam (e.g., Li et al. , 2021), whose partial failures have already caused dangerous downstream flooding. For this purpose, we used single-station recordings of ambient noise, acquired along two profiles passing across the rock avalanche deposit (Fig. 1), analyzed using the traditional HVNR method (Nakamura 1989) and a recently proposed technique, named HVIP (Del Gaudio, 2017). Fig. 1 - Google Earth image of the breached landslide dam in the gully named Yang Jia Gou with the positions of the ambient noise measurement stations. The blue point represents the reference station (YJG0) and the yellow triangles are measurement stations (YJG1-YJG8; YJG100-YJG103). Red lines are two faults reported from Wasowski et al. (2021): the continuous and dashed lines represent, respectively, the observed and inferred faults. The black dashed line, in the eastern part of the dam, represents the contact between the 2008 co-seismic rock avalanche deposit and the older (pre-historic) rock avalanche; the blue line indicates the contact between the base of the prehistoric rock avalanche and the slate bedrock (the continuous line is observed, the dashed line is presumed). Bedrock dips to the west at an angle of about 20°. Both techniques are based on the calculation of curves of the H/V ratios between horizontal and vertical components of the ground motion as function of frequency. In presence of strong velocity contrast between a surface soft deposit and a stiffer substratum, these curves can present one or more peaks at site resonance frequencies which are related to peaks of Rayleigh wave ellipticity depending on the vertical layering of seismic wave velocity under the measurement station. These passive seismic techniques are cheaper and quicker than other seismic techniques and are particularly suitable for data acquisition on topographically complex sites. A limitation is the interpretative ambiguity because of the large number of unknowns to consider which requires the introduction of additional constraints in the parameterization to