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 birefringence)
http://garnero.asu.edu/research_images/anis/garnero_splitting.gif History
Measurements of P-wave azimuthal velocity variations - Hess, 1964.
S-wave anisotropy identified in upper mantle - Ando, 1980
Azimuthally-aligned shear wave splitting- Crampin, 1981 Definitions
Microcrack Azimuthally Aligned Polarization 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 earthquake prediction for earthquakes 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?