ANISOTROPIC SEISMOLOGY IN ANTARCTICA

Jean-Paul Montagner,Genevieve Roult and Eric Beucler
Seismological Laboratory, Institut de Physique du Globe, Paris, France

Clear evidence has accumulated during the last 40 years, demonstrating that seismic anisotropy 
is not a second order effect but that it is present at all scales, though not in all depth ranges of the 
Earth. Conversely, in spite of the inherent theoretical complexity, seismologists must realize that 
they must incorporate anisotropy in their modelling in order to avoid wrong interpretations. 
Different physical processes or geometrical configurations an give rise to observable seismic 
anisotropy, crystal anisotropy, fine layering, cracks or fluid inclusions, and several conditions 
must be fulfilled to detect it at large scale (strong intrinsic anisotropy, or fine layering, efficient 
mechanisms of orientation of minerals and aggregates, large scale coherent strain field).

We review the seismic datasets, which provide insight into the location at depth of large scale 
anisotropy, with emphasis on anisotropy below the Antarcticplate. Part of crustal and shallow 
uppermost mantle anisotropy can be related to the past strain field (frozen-in anisotropy), but the 
deep anisotropy is probably due to the present strain field.    

In addition to the well-documented seismic anisotropy in lithosphere and asthenosphere, there 
are some new evidences of seismic anisotropy in the upper (400-660km) and lower (660-900km) 
transition zones and in the D"-layer in many places around the world except Antarctica. Whereas 
the directions of anisotropy derived from surface waves, below the ridges surrounding the 
Antarctic plates, are clearly related to the extension directions, the anisotropy below the 
Antarctica continent is is smaller and much more complex. SKS splitting measurements do not 
provide any tectonic information. It is now possible to relate observed anisotropy to global flow 
model, but the deep mantle dynamics below Antarctic is still out of reach.
  
The scientific potential of seismic anisotropy is enormous and it makes it possible to gain more 
insight into a large variety of geophysical issues such as the depth extent of continental roots, the 
mapping of deformation and geodynamics processes in play in the deep mantle. In the upper 
mantle, seismic anisotropy is primarily reflecting plate tectonics and predominant deformation 
processes. However, Antarctica is almost a Terra Incognita from the anisotropic point of view. 
New observations of seismic anisotropy are necessary below the Antarctic plate to understand 
the coupling between Antarctic continent and underlying convection.