Investigation of the deep subsurface of the Swiss Molasse basin using ambient vibrations and earthquake recordings
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Date
2020
Publication Type
Doctoral Thesis
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Abstract
Passive seismic methods represent a low-cost and non-invasive alternative to
the use of active sources (e.g. explosive) and invasive drilling activities. Passive
seismic methods aim at estimating the soil parameters as a function of
depth, in particular the S-wave velocities. The data analysed by passive seismic
methods include ambient seismic vibrations and earthquakes, recorded
using single stations or arrays. Site-effect surveys and microzonation generally
investigate the shallow subsurface using ambient seismic vibrations and
earthquake recordings in the frequency range from 0.5 to 20 Hz. The depth
of the estimated velocity profiles is between 200 and 300 meters depending on
the site and the methods used. Tomography studies, using arrays deployed at
regional and global scales, investigate the deep structure of the Earth down
to 100 or more kilometres, but the velocity profiles have low or no resolution
in the first kilometres. In this thesis, we aimed at filling the investigation
gap between the velocity profiles for site-effect analysis and the ones for tomography
by estimating the velocity profiles from the free surface down to
several kilometres using ambient seismic vibrations and earthquake recordings
collected at several sites in Switzerland. Ambient seismic vibrations were
recorded at Herdern (Canton Thurgau) and Schafisheim (Canton Aargau), two
sites within the Swiss Molasse basin, the Alpine foreland basin located north
of the Alps. The Swiss Molasse basin extends from Lake Geneva (south-west)
to Lake Constance (north-east). The collected seismic data was analysed using
techniques developed for site investigation studies of shallow structures:
single-station methods (horizontal-to-vertical spectral ratio and estimation of
Rayleigh wave ellipticity) and array processing techniques (three-component
high-resolution f-k, Modified SPatial AutoCorrelation and Wavefield Decomposition).
The measured Rayleigh and Love wave dispersion curves were jointly
inverted with the Rayleigh wave ellipticity angles to determine the S-wave velocity
profiles down to large depths of several kilometers. The deployment in
Herdern was performed using two arrays of increasing size with maximum interstation
distances of 900 m and 29.6 km, respectively. At the second site in
Schafisheim, three arrays were deployed with maximum interstation distances
of 346 m, 2.4 km and 26.8 km, respectively. In Schafisheim, in addition to
the array measurements, a dense single station survey was performed to map
lateral variations of the structure in the investigated area. While site Herdern
provided clear dispersion curves that could be used to invert the velocity structure,
site Schafisheim showed different branches of the same mode due to the
lateral variability of the structure. The survey of single-station measurements
helped to separate the structure into two main parts, explaining the different
branches of dispersion curves of the same mode. To verify the potential of earthquake recordings in investigating the subsurface, we analysed the seismic
events recorded at 23 stations (short-period, broadband and strong motion
sensors) of the Swiss Seismic Network. The data consist of local and regional
earthquakes with magnitudes between 2.5 and 4.6. Using the signal after
the shear-wave arrival, we computed the Horizontal-to-Vertical spectral ratio
curves and used the Hybrid Heuristic Search algorithm (HHS - Nagashima
et al., 2014) to invert the S-wave velocity profile. HHS is a combination of
Simulated Annealing and Genetic algorithm. Constraints for the shallow layers
are needed to obtain stable inversion results. The constraints are velocity
profiles at the site of the seismic stations obtained from site-characterization
measurements, performed by the Swiss Seismological Service (SED) in the past
decade. We tested the inversion method by allowing more or less liberty to
the initial near-surface velocity profile. Recurrent features of the performance
of the inversion were found that we classified in three classes. The inverted
velocity profiles from HHS, using different initial constraints and liberties, investigate
the subsurface down to depths between 245 and 5930 m, depending
on the site. For some sites, the velocity profiles of the shallow layers can be
improved. For 16 out of 23 sites, we can conclude that the HHS inversion
for H/V curve with a-priori constraints for shallow structure works well to
determine S-wave velocity structure down to the seismological bedrock.
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Contributors
Examiner : Fäh, Donat
Examiner : Hobiger, Manuel
Examiner : Robertsson, Johan O.A.
Examiner : Ohrnberger, Matthias
Examiner : Fichtner, Andreas
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ETH Zurich
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02818 - Schweiz. Erdbebendienst (SED) / Swiss Seismological Service (SED)
03953 - Robertsson, Johan / Robertsson, Johan