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Shear wave splitting - Anisotropy:

Seismic anisotropy - Theory:

Shear wave splitting occurs when a linear polarized shear wave traverse a seismically anisotropic medium and are split into orthogonally polarized fast and slow shear waves separated by characteristic delay time, dt (see figure 3 below]. In the upper mantle, splitting is due to the lattice preferred orientation of olivine resulting from deformation of olivine-containing aggregates. The fast polarisation direction, F, expressed as an azimuth, is parallel to the principal axis of extensional finite strain in the deformed, anisotropic medium. The delay time between fast and slow shear wave arrivals is closely related to the thickness of the constant anisotropy layers traversed, and to the strength of fabric development within these layers. Thus, measurements of shear wave splitting can be used to deduce the orientation and approximate thickness of the anisotropic mantle strain field beneath regions of interest, like the Dronning Maud Land in Antarctica, yielding important insight into tectonic processes occuring at depth.

Experiment and area of investigation - Dronning Maud Land (DML)/Antarctica:

During several polar expeditions within the VISA project seismological stations were temporarily deployed at several sites of the mountain ranges such as Kottas, Wohlthatmassif, across the Heimefrontfjella shear zone or at the Jutul-Penck-Graben, an old rift system beside the South African overwintering station Sanae IV (see figure 1). These registrations as well as registrations of a permanent deployed seismograph at Sanae station and a seven month lasting registration of the Russian overwintering station Novolazarevskaja were investigated regarding anisotropic effects. Individual splitting measurements of generated SK(K)S phases as well as S(cS) phases generated of deep earthquakes were only taken into account when

  1. there was a clear correlation between the waveforms of fast and slow component,
  2. a clear elliptical partical motion of the uncorrected waveform were observed,
  3. the particalmotion after correction were linear,
  4. the energy of the transverse component was removed after correction,
  5. the SNR between the wavelets were significant high.

 

Geological and tectonic settings of DML:

Several geological and tectonic phases during the earth history formed the presentday Antarctic

continent. Within the DML outcrops were formed by several orogenies:

 

  1. the Grenville 1.1 Ga years ago by building the supercontinent Rodinia,
  2. the Ross-/Panafrican 500 Ma ago by building the supercontinent Gondwana because of collision between West and East Gondwana.

 

The break-up of Gondwanaland 180 Ma ago started at the Weddell-Sea (oceanic area off DML) and was accompanied by large volcanic activity, magmatism and major outpourings of continental flood basalts.

 

Observations:

So far we can say that most of the observations are well explained by models that includes the upper mantle, processes therein and the crustal orogenies. Two layer modelling were carried out for stations with wide backazimuthal event coverage, such as Sanae and Novolazarevskaja. With the help of smallscaled magnetic anomalies EMAGE grid the results are well unterstood for most of the investigated stations (publication in preparation). For example a two layer model of anisotropic medium to explain the observations for registrations of Neumayer data (VNA3) were suggested by [Müller 2000, see figure 2].

 

 

Figures:

 

Animated gif that demonstrates the shear wave splitting process.

Figure 3: If a linear polarized shear wave encounters an anisotropic medium it is split into two waves with different velocities and perpendicular polarisation directions. Two onsets could be observed on the seismogram, one on the radial, one on the transversal registration. The vertical component should be free from energy. That is for shear waves, that traverses directly to the core, such as SK(K)S), PK(K)S. [src: animated picture from B. Bayer, 2006].

A two layer model for permanent station Neumayer

Figure 2: A two layer model for permanent station Neumayer (VNA3) that describes the observed shear wave splitting. Splitting parameter for the upper layer were determined to 60 degrees and one second. Lower layer has got a fast polarisations axis of 110 degrees and a time delay of half a second. [src: Müller, 2000].

Map shows temporarily deployed stations

Figure 1: Map shows temporarily deployed stations at Kottas mountains (KOT), Wohlthatmassif (WM), Novolazarevskaja (NOVO), Jutul-Penck-Graben (JUT) and across the Heimefront shear zone (KOH) as well as permanent station Neumayer (VNA) and Sanae (SNAA).


 
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