Ambient vibration measurements in the Southern Rhine Graben close to Basle

M Ohrnberger M Ohrnberger,F Scherbaum F Scherbaum,S Stange S Stange,A Köhler A Köhler,F Kind F Kind

doi:10.4401/ag-3374

Abstract

This study presents results of ambient noise measurements from temporary single station and small-scale array deployments in the northeast of Basle. H/V spectral ratios were determined along various profiles crossing the eastern masterfault of the Rhine Rift Valley and the adjacent sedimentary rift fills. The fundamental H/V peak frequencies are decreasing along the profile towards the eastern direction being consistent with the dip of the tertiary sediments within the rift. Using existing empirical relationships between H/V frequency peaks and the depth of the dominant seismic contrast, derived on basis of the ?/4-resonance hypothesis and a power law depth dependence of the S-wave velocity, we obtain thicknesses of the rift fill from about 155 m in the west to 280 m in the east. This is in agreement with previous studies. The array analysis of the ambient noise wavefield yielded a stable dispersion relation consistent with Rayleigh wave propagation velocities. We conclude that a significant amount of surface waves is contained in the observed wavefield. The computed ellipticity for fundamental mode Rayleigh waves for the velocity depth models used for the estimation of the sediment thicknesses is in agreement with the observed H/V spectra over a large frequency band.

Highlights

The Southern Rhine Rift Valley is among the most active seismic regions within Central Europe
In order to allow a quantitative interpretation of the relation between peak frequency and sediment thickness cover, we derived a depth estimate from the measured horizontal to vertical (H/V) peak frequencies
Assuming that the origin of the H/V spectral ratio fundamental peak is related to S-wave resonances in a single sediment layer over halfspace (λ /4-hypothesis for shear waves: Nakamura, 1989), the layer thickness h can be related to the H/V fundamental peak frequency f as f nvs 4h where n are uneven integers n = 1, 3, 5, and vs is the shear wave velocity of the sediment layer

Summary

Introduction

The Southern Rhine Rift Valley is among the most active seismic regions within Central Europe. Nakamura and Ueno (1986) and Nakamura (1989, 2000) suggested that taking the H/V ratio reduces the effect of the source, and the spectral ratio represents the stationary amplification function of the soft rock due to «shear wave resonance» (λ/4-hypothesis) The condition for this interpretation is that the wavefield contains a significant portion of body waves and that there is a sufficiently high impedance contrast between sediment layer and underlying bedrock. On the basis of phase velocity analysis results from microtremor array measurements (Asten and Henstridge, 1984; Tokimatsu and Miyadera, 1992) many authors argued, in contrast to Nakamura’s interpretation of the H/V ratio, that the ambient vibration wavefield is often dominated by surface waves (Lachet and Bard, 1994; Fäh et al, 2001; Cornou and Bard, 2003; Scherbaum et al, 2003). For S-wave velocity contrasts above 3.5, the peak frequencies fell and fSH match closely

Measurement and data

FK analysis of array data

Discussion and conclusions