The Ground Truth of Crustal Anisotropy from Receiver Functions

Abstract:

As one of the rare observational tools for studying deformation and stress within the Earth, seismic anisotropy has been one of the focuses of geophysical studies over the last decade. Recently, in order to unravel the anisotropic properties of the crust, the teleseismic receiver functions (RFs) methodology has been largely applied. Effects of anisotropy on the RFs dataset were illustrated in more than one theoretical study, showing the strong backazimuthal dependence of RFs on the 3D characteristics of the traversed media. The use of teleseismic RFs has the advantage of not being affected by heterogeneous depth distribution of local earthquakes, since teleseismic rays sample the entire crust beneath the stations. The application of this technique anyway, needs to be critically assessed using a suitable field test.

To test this technique, we need a crustal block where the underground structure is reasonably well-known, e.g., where there is extensive knowledge from local seismic experiments and drilling. Therefore a field test around the KTB (Kontinental Tiefbohrung) site in the Oberpfalz in Southeastern Germany, has been carried out to test the technique, and to compare with previous results from deep drilling, and high-frequency seismic experiments around the drill site.

The investigated region has been studied extensively by local geophysical experiments. The deep borehole was placed into gneiss rocks of the Zone Erbendorf-Vohenstrauss. Drilling activity lasted from 1987 to 1994, and descended down to a depth of 9101 meters, sampling an alternating sequence of paragneiss and amphibolite, with metamorphism of upper amphibolite facies conditions, and ductile deformation produced a strong foliation of the rocks.

The application of the RFs reveals strong seismic anisotropy in the upper crust related to the so-called Erbendorf body.