L. Stehly, B. Fry, M. Campillo, N.M Shapiro, J. Guilbert, L. Boschi and D.Giardini
We use correlations of the ambient seismic noise to study the crust in western
Europe (Shapiro et al, 2005). Cross correlation of one year of noise recorded at 150 3-component
broadband stations yields more than 3000 Rayleigh and Love wave group velocity
measurements. These measurements are used to construct Rayleigh and Love group
velocity maps of the Alpine region and surrounding area in the 5-80s period
band. Finally, we invert the resulting Rayleigh wave group velocity maps to
determine the Moho depth.
We used one year of continuous records from October 2004 to October 2005
from 150 3-component broadband European stations. Our aim is
to focus on the Alps, where we have a particularly high density of stations
All the records are processed day by day. First the data
are decimated to 1 Hz and corrected for the instrumental
response. North and East horizontal components are rotated to get radial and
transverse components with respect to the inter-station azimuth. The records are
then band-pass filtered and their spectrum whitened between 5 and 150 s. We
correlated signals recorded on the components that correspond to Rayleigh and
Love waves (ZZ, ZR, RZ, RR, and TT).
Correlations of one-day records are stacked.
Rayleigh and Love wave dispersion curves are evaluated from the emerging
Green's function using frequency-time analysis (Levshing et al, 1989, Ritzwoller and Levshin, 1998)
for the 11,000 inter-station paths. For each path, we get eight evaluations of
the Rayleigh-wave dispersion curves by considering four components
of the correlation tensor (ZZ, RR, RZ and ZR) and both the positive and the
negative part of the NCF. Similarly, we get two estimates
of the Love-wave dispersion curves from positive and negative parts
of TT correlations.
We reject waveforms 1) with S/N (ratio between
Rayleigh wave's amplitude and noise variance after it) lower than
seven; 2) with group velocities measured on the positive and
negative correlation time differing by more than 5 percents, and 3)
with paths shorter than two wavelengths at the selected period for the
group velocity map. This results in about 3,500 paths over
the initial 11,000 inter-station paths at 16 s.
We then apply a tomographic inversion following
(Barmin et al, 2001) to this data set to obtain group velocity maps on
100100 = 10,000 cells of 2525 km across Europe
(Fig. 2.34). Several geological features can be seen on those maps.
At 16s, low velocity anomalies are associated with sedimentary basins, such as the Po
basin (Northern Italy), the North Sea basin and the Pannonian
basin (Slovakia and Hungary). Both Rayleigh and Love waves exhibit
smaller values below the molassic sediments (Southern Germany and
Austria) than in the surrounding area.
Rayeigh group velocity maps at 16s (left) and 35s (right).
At each cell of our model, we extracted Rayleigh wave dispersion curves from
our group velocities map, and inverted them using a Monte Carlo algorithm
in order to determine the depth of the Moho in the Western Alps (Switzerland,
Austria, southern Germany). Our results clearly show thickening of the crust
below the Alps (Fig2.35). Our map of Moho depth shares striking similarities with the
compilation of (Waldhauser et al, 1998) in the region where we have a high density of
paths. This comparison confirms that seismic noise can be efficiently used to
obtain high resolution Love and Rayleigh wave group velocity maps at periods up
to 80s and 3D images of the crust and the upper mantle. This method provides
spatially continuous seismic velocity distributions on large areas. The
resolution of the obtained model depends mostly on the density of stations and
is not limited by the uneven distribution of earthquakes. At period less than 10s,
the resolution length is not isotropic as the noise is strongly directional.
3D view of the Moho depth.
M. P Barmin, M. H. Ritzwoller, and A. L. Levshin,
A fast and reliable method for surface wave tomography,
Pure and Applied Geophysics, 158:1351-1375, 2001.
A.L Levshin, T. B. Yanocskaya, A. V. Lander, B. G. Bukchin, M. P. Barmin, L. I.
Ratnikova, and E. N. Its,
Seismic surface waves in a laterally inhomogeneous Earth,
Kluwer Academic Publishers, 1989.
M. Ritzwoller and A. L. Levshin,
Eurasian surface wave tomography: group velocities,
Journal of Geophysical Research, 103(4839-4878):4839, 1998.
N. M. Shapiro, M. Campillo, L. Stehly, and M. H. Ritzwoller,
High-resolution surface wave tomography from ambient seismic noise,
Science, 307:1615-1618, march 2005.
F. Waldhauser, E. Kissling, J. Ansorge, and St. Mueller,
Three-dimensional interface modelling with two-dimensional seismic
data: the Alpine crust mantle boundary,
Geophysical Journal International, 135:264-278, 1998.
Berkeley Seismological Laboratory
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