Shear Wave Splitting Introduction to Shear Wave Splitting Outline

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Shear Wave Splitting Introduction to Shear Wave Splitting Outline

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? .

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Shear Wave Splitting Time Variation by Stress-Induced Magma Uprising at Mount Etna Volcano
Shear wave splitting time variation by stress-induced magma uprising at Mount Etna volcano Francesca Bianco1, Luciano Scarfì2, Edoardo Del Pezzo1 & Domenico Patanè2 1Istituto Nazionale Geofisica e Vulcanologia - Osservatorio Vesuviano, Napoli (Italy) 2 Istituto Nazionale Geofisica e Vulcanologia Sez. Catania (Italy) Abstract Shear wave splitting exhibits clear time variations before the July 17th – August 9th, 2001 flank eruption at Mount Etna. The normalized time delays, Tn, detected through an orthogonal transformation of singular value decomposition, exhibit a clear increase starting 20 days before the occurrence of the eruption (July 17th); the qS1 polarization direction, obtained using a 3D covariance matrix decomposition, shows a 90°-flip several times during the analyzed period: the last flip 5 days before the occurrence of the eruption. Both splitting parameters also exhibit a relaxation phase shortly before the starting of the eruption. Our observations seem in agreement with Anisotropic Poro Elasticity (APE) modelling, suggesting a tool for the temporal monitoring of the build up of the stress leading to the occurrence of the 2001 eruption at Mt. Etna. Introduction Shear wave splitting is the elastic analogue of the well known optical birefringence phenomenon. A shear wave entering into an anisotropic volume, is split into two quasi shear waves, qS1 and qS2, propagating with different velocities and approximately orthogonal polarizations. The splitting parameters are the time delay between qS1 and qS2 wave (hereafter referred to as Td); and the polarization direction of the leading split (qS1) wave (hereafter referred to as Lspd, Linear split polarization direction).. The Extensive Dilatancy Anisotropy (EDA) model[Crampin et al, 1984], suggests that the principal source of seismic anisotropy in the crust is stress aligned fluid filled microcracks.
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