Abstract

     The geometry of three-dimensional subsurface structures plays an important role in determining local seismic site effects as in the case of alpine valleys. Detailed knowledge of these structures is fundamental in seismic hazard and risk studies. In this study we investigate an area in the upper Rhone valley around Visp, in the southwestern part of Switzerland. A large dataset of geological and geophysical data, consisting of borehole logs, microtremor horizontal to vertical spectral ratios and shear-wave velocity measurements, was compiled to build a detailed 3D model of the subsurface. By combining fundamental frequency information from noise recordings and shear-wave velocity profiles, three main geophysical discontinuities were identified and their physical properties constrained through a stepwise process. First, the bedrock depth was estimated; in a second step a generic velocity model was defined and finally, combining all the available geological and geophysical information, we developed a 3D geophysical model. The model was compared with a local 3D geological model and a model derived from gravimetric data. The study area is a complex alpine valley where 2D/3D wave propagation phenomena occur. In such case a purely 1D response assumption is considered to be invalid. In order to test the 3D model, we modelled different ambient-vibration wave fields and compared observed and synthetic H/V spectral ratios. We slightly modified our 3D geophysical model in some areas based on this comparison. Finally, a good match between simulated and empirical spectral ratios corroborated the model. The results suggest that the use of ambient vibration techniques are a powerful and cost-effective tools to reconstruct three-dimensional models of the subsurface. Finally, we used the 3D model to predict amplification of earthquake ground motion in the basin.

Introduction

     The Rhone valley in the South-Western part of Switzerland (see inset in Fig. 1) is located in the most active seismic zones of the country [1]. This area was struck by a series of strong seismic events such as the 1755 Brig (Mw 5.7), the 1855 Visp (Mw 6.2) and the 1946 Sierre (Mw 5.8) earthquakes ([[2], [3]]. The shape of the valley and the high velocity contrast between sediments and bedrock make this area susceptible to 2D/3D seismic site amplification effects [[4], [5], [6], [7], [8], [9]]. Due to the ongoing urban development, the Rhone valley is an area of particular interest to study site amplification effects. For this purpose, a detailed knowledge of the geometry, thickness and velocity of the main sedimentary layers in the valley is required. Previous studies have shown that such 3D structures play an important role in seismic wave propagation and amplification (e.g. Refs. [[10], [11], [12]]. In particular, several authors have successfully modelled, with numerical simulations, the amplification in a 3D basin setting [[13], [14], [15]]. Among these for the Rhone valley area Roten et al. [6] developed a 3D geophysical model for the city of Sion in the central Swiss Rhone valley, and successfully compared simulated earthquake ground motion with observations. They quantified the effects of 2D/3D resonances and edge-generated surface waves on the ground motion amplification.

Conclusions

     We characterized the main geophysical discontinuities and their properties in the alpine basin in the Visp area based on ambient vibration data and earthquake recordings integrated with borehole information.

     The ambient vibrations recorded with small aperture seismic arrays were processed by means of the f-k technique to retrieve dispersion curves and to determine 1D VS profile at several locations, whereas HVSR were used to determine f0 related to the sediment thickness. The VS profiles allowed us to highlight the presence of three main layers for which we derived VS values. The two shallowest layers were considered homogenous and with constant velocity, whereas for the thick third layer the effect of sediment compaction was taken into account by introducing an exponential functional form. The VP values were determined based on earthquake P-wave arrival times recorded at the SVISP vertical array. For VS at the bedrock we adopted an approach similar to that proposed by Poggi et al. [55] by extracting from the empirical amplification function for station SVIO a VQWL profile subsequently inverted using a global optimization process.