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Ancona, 23-04-2013

STAR : l’anisotropia sismica come indicatore delle variazione delle propriet à elastiche della crosta. Pastori M., Piccinini D., Guerri M., Margheriti L ., Barchi M. Ancona, 23-04-2013. Shear Wave Splitting.

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Ancona, 23-04-2013

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  1. STAR: l’anisotropia sismica come indicatore delle variazione delle proprietàelastiche della crosta Pastori M., Piccinini D., Guerri M., Margheriti L., Barchi M. Ancona, 23-04-2013

  2. Shear Wave Splitting The main technique to study Seismic Anisotropy is based on the Shear Wave Splitting (SWS) phenomena Shmin Equivalent to the birefringence in optics Anisotropic parameters Fast Polarization Direction(Φ),typically polarized parallel to the direction of the cracks-fractures (Crampin & Lovell, 1991) Delay Time (dt),the lag of the slower wave, it is a measure of intensity and/or thickness of the anisotropic medium Vertical Stress

  3. Possible crustal anisotropic causes • Inherited fracture field • Stress open crack EDA • Time dependent APE (Extensive Dilatancy Anisotropy Model; Crampin,1993) (Anisotropic Poro Elasticity Model; (Zinke and Zoback, 2000) Zatsepin & Crampin, 1995) Fluid-filled micro-cracks aligned or ‘opened’ by active stress field

  4. Anisomat+is a set of MatLab scripts that use the cross-correlation method to compute anisotropic parameters: fast direction and delay time • Three-component seismogram in SAC format (having the same length) • P & S picking • Event and Station Coordinates • Test incidence angle • Test signal quality Piccinini et al. Computer and Geoscience 2013 • OUTPUTS • Fast direction and delay times • Errors on the parameters • Cross correlation

  5. Comparison between the average values of the anisotropy for the 2010 seismicity (Guerri et al 2012 and the 2000-2001 CDC experiment (Pastori et al., 2012). the comparison show a robust agreement both for the average fast directions and for the average delay time values. Green and purple bars represent fats direction and their length is proportional to the delay time. Crustal Seismic Anisotropy in the region

  6. Fast Null Guerri et al. 2012 Fast directions vs Shmin

  7. Anisotropy in space and time Pietralunga Seismic sequence 2010 Waveforms picking and locatio by AN Why are ATPI and ATPC different? Lithology? Local stress? 90°flip due to fluid overpressure?

  8. Manual pickers vs automatic pickers>> >>Anisotropic parameters Manual pickings ATPC ATPI 1-10hz 0.3s Automatic pickings 1-10hz - 0.35s ATPC ATPI NULLE NULLE

  9. Manual pickers vs automatic pickers>> >>Anisotropic parameters Manual pickings- 1-10hz 0.3s ATPC ATPI Automatic pickings 1-10hz - 0.35s ATPC ATPI NULLE NULLE

  10. Andameno della sismicità 2010 3.8

  11. Manual pickings- 1-10hz 0.3s window ATPC ATPI Temporal variation of anisotropic parameters

  12. ATPI ATPC Automatic pickings- 1-10hz 0.35s window Temporal variation of anisotropic parameters

  13. Final remarks The dense seismic network allow to locate many small earthquakes in the “S window” of each station, giving results all the back-azimuths and well sampling the rock volume Manual pickings seems to give more stable results but on averages the completely automatic procedure give consistent results We should test selection of the results on the base of picking weight or…? At Taboo thanks to the geological-and structural knowledge of the area we should be able to understand why we observe different anisotropic parameters in space and time! Work in progress … Real Time monitoring of anisotropic parameters? 90°flip for high luid pressure; can we see them? Yes go ahead, Let’s try it!!!

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