Abstract
This work presents the first comprehensive 3-D model of the crust beneath The Netherlands. To obtain this model, we designed the NARS-Netherlands project, a dense deployment of broadband stations in the area. Rayleigh and Love wave group velocity dispersion was measured from ambient noise cross-correlations. Azimuthally anisotropic group velocity maps were then constructed and the isotropic part was used to determine a shear wave speed model that includes the effects of radial anisotropy. Employing the Neighbourhood Algorithm for the depth inversion, we obtained probabilistic estimates of the radially anisotropic model parameters. We found that the variations in the thickness of the top layer largely match the transition from sediments of Permian age to those of Carboniferous age. Regions of high faulting density such as the West Netherlands Basin and Roer Valley Graben are recognized in our model by their negative radial anisotropy (VSH−VSV < 0). The model has a mid-crustal discontinuity at a depth of around 13km and the average Moho depth is 33km, with most of its depth variations within 2km. Specifically, a localized Moho uplift to a depth of 29km is found within Roer Valley Graben, in the Campine region in Belgium. Furthermore, our Rayleigh and Love wave group velocity data at periods of around 20s show evidence for azimuthal anisotropy with a NW-SE fast direction. This anisotropy is likely related to NW-SE rock fabric in the lower crust thought to originate from the Caledonian deformation.
Highlights
The sedimentary structure of The Netherlands, down to about 5 km depth, is well mapped based on ample seismic reflection and borehole data (TNO-NITG, 2004; van Dalfsen et al, 2006; Duin et al, 2006; van Dalfsen et al, 2007)
It has been suggested that the present dominant trend of NW-SE faults in The Netherlands dates back to the suturing of Avalonia and Baltica, and that these faults were reactivated during later tectonic events (Ziegler, 1990; de Jager, 2007)
We used ambient noise to measure interstation group velocity dispersion curves within the crust beneath The Netherlands. This technique relies on seismic interferometry which shows that the cross-correlation of ambient seismic noise recorded at two receivers produces an empirical Green's function (EGF), effectively turning one receiver into a virtual source
Summary
The sedimentary structure of The Netherlands, down to about 5 km depth, is well mapped based on ample seismic reflection and borehole data (TNO-NITG, 2004; van Dalfsen et al, 2006; Duin et al, 2006; van Dalfsen et al, 2007). It is unclear whether faults that are recognized in the sedimentary sequence continue down into the basement, and, if they do, to what depth Such knowledge is essential for seismic hazard assessment. We used the technique of ambient noise tomography to obtain a 3-D shear wave speed model of the crust. We will explain how the 3-D shear wave speed model was obtained from the group velocity measurements using the ambient noise data, the construction of the group velocity maps and the probabilistic depth inversion.
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