Introduction to Shear Wave Splitting Introduction to Shear Wave Splitting http://en.wikipedia.org/wiki/Shear_wave_splitting Outline

 Concept  Definitions  History  Applications  Common Misunderstandings  Conclusions

What is Shear Wave Splitting?

 When a polarized shear- wave enters an anisotropic medium, it splits into two quasi shear-waves (fast and slow).

Shear Wave Splitting (Seismic )

http://garnero.asu.edu/research_images/anis/garnero_splitting.gif History

Measurements of P-wave azimuthal velocity variations - Hess, 1964.

S-wave identified in upper mantle - Ando, 1980

Azimuthally-aligned shear wave splitting- Crampin, 1981 Definitions

 Microcrack  Azimuthally Aligned  Diagrams

Microcracks

Crack density Azimuthally Aligned

 the angles that the arrivals or waves make with a reference point are all equal.  The waves are approximately parallel:

http://en.wikipedia.org/wiki/Azimuth Polarization Diagrams (Hodograms)

 Displays a 2-D projection of the trajectory of a point moving in space.  a cross plot of two components of motion over a time window.

http://www.crewes.org/ResearchLinks/ExplorerPrograms/Hodogram/Hodogram.html Applications

 Possible prediction  for M 1.7 to M 7.7

 CO2 Injection mapping

 in fractured carbonate reservoirs

Common Misunderstandings

 Polarizations of split shear waves are orthogonal  Crack anisotropy always decreases with depth as fluid filled cracks are closed by lithostatic pressure.  Signal-to-noise ratios of shear-wave splitting above small earthquakes can be improved by stacking.

Conclusion

 Stress aligned fluid-saturated microcracks are the predominant cause of the nearly universal observations of azimuthally aligned shear wave splitting both in the earth’s crust and in the upper mantle. Questions?