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A controlled-source experiment to investigate the origin of wavefield polarization in fault zones

A controlled-source experiment to investigate the origin of wavefield polarization in fault zones. Giuseppe Di Giulio 1 , Antonio Rovelli 2 , Fabrizio Cara 2 , Pier Paolo Bruno 3 , Michele Punzo 4 , and Francesco Varriale 5 1 Istituto Nazionale di Geofisica e Vulcanologia, L’Aquila, Italy

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A controlled-source experiment to investigate the origin of wavefield polarization in fault zones

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  1. A controlled-source experiment to investigate the origin of wavefield polarization in fault zones Giuseppe Di Giulio1, Antonio Rovelli2, Fabrizio Cara2, Pier Paolo Bruno3, Michele Punzo4, and Francesco Varriale5 1 Istituto Nazionale di Geofisica e Vulcanologia, L’Aquila, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Seismology and Tectonophysics , Roma, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy 4 Consiglio Nazionale delle Ricerche, IAMC, Napoli, Italy 5 AMRA, Napoli, Italy

  2. The study area: the Pernicana Fault Faults on Mt. Etna Seismotectonic setting (after Monaco et al., 2005) White arrows represent the horizontal displacement vectors as measured by Bonforte et al. (2008) between July 2005 and June 2006. Rigano et al. (2008) observe a strong wavefield polarization in fault zones 10 cm 10 cm

  3. Ambient noise Earthquakes

  4. A more precise azimuth estimate is made in the time domain (Jurkevics, 1988) What is the origin of the strong horizontal polarization at 1 Hz in this fault zone?

  5. Summit crater, Mt. EtnaFalsaperla et al. 2010, JGRAt the crater stations, horizontal site polarization is independent of the source position and seismic signal nature (earthquakes, exposive volcanic episodes, tremor) and tends to be transversal to the radial fracture field

  6. Orthogonal relation between wavefield polarization and fracure orientation (Pischiutta et al. 2013, GJI) Software FRAP (Salvini et al., 1999)

  7. A controlled-source experiment at Piano Pernicana Vertical excitation, sweep modality (4.5-Hz geophones) Experiment site 30 s 2 s Vs30 ≈400 m/s 1D models cannot reproduce a 1 Hz resonance

  8. Vertical comp. Distance 10 m Horizontal comp. Distance 10 m Vertical comp. Distance 300 m Horizontal comp. Distance 300 m Array layout N240, Source Polarization N330 ''Natural Site Polarization'' Horizontal excitation, constant-frequency modality (5-s seismometers) 5-Hz band-pass filter Ambient noise site polarization

  9. Array layout N330, Source Polarization N240 Noise 1 Hz Noise 5 Hz Vibroseis 5 Hz Array layout N240, Source Polarization N330 ''Natural Site Polarization'' Noise 1 Hz Noise 5 Hz Vibroseis 5 Hz -63 m 10 m 10 m 50 m 50 m 100 m 100 m 150 m 150 m

  10. E (f, R) = 1/R Eo(f) exp (-2πfR/VsQ) E (f, R) = 1/R0.5 Eo(f) exp [-2πfR/V(f)Q] E (f, R) = 1/R0.5 Eo(f) exp [-2πfR/V(f)Q] E (f, R) = 1/R0.5 Eo(f) exp [-2πfR/V(f)Q] E (f, R) = 1/R0.5 Eo(f) exp [-2πfR/V(f)Q] E (f, R) = 1/R0.5 Eo(f) exp [-2πfR/V(f)Q] E (f, R) = 1/R Eo(f) exp (-2πfR/VsQ) E (f, R) = 1/R Eo(f) exp (-2πfR/VsQ) Ratio 150/10 m Frequency (Hz)‏ Q150m Q50m Frequency (Hz)‏ Energy propagation Body wave model E (f, R) = 1/R Eo(f) exp (-2πfR/VsQ) Q≈20 Vs≈ 500 m/s Rayleigh wave model E (f, R) = 1/R0.5 Eo(f) exp [-2πfR/V(f)Q]

  11. Abrupt change of polarization (from source to natural polarization) Array layout N330, Source Polarization N240 Array layout N240, Source Polarization N330 Noise bp 5 Hz Noise bp 5 Hz radial distance from source distance from source transverse Horizontal energy attenuation Vibroseis 5 Hz Vibroseis 5 Hz radial transverse

  12. Speculations and possible models • Controlled-source experiments (horizontal excitation) can provide valuable indications on site effects. • This experiment was successful in confirming that wavefield polarization is a site property. • A more refined (smaller distance between receivers) instrumentation would have been necessary to understand the details of seismic wave propagation. • Ground motion is controlled by (anisotropic) fracture compliance (see also Moore et al., 2011, and implications for topographic effects). • In the fault zone, seismic energy propagates through (viscoelastic?) scattering and mode conversion; out of the fault zone, ground motion tends to be chaotic. • Shallow fractures are responsible for the observed wavefield polarization, at least at the site of this experiment. • The resonant frequency is likely related to the size of the fractured volume and H/V amplitude is proportional to the crack density.

  13. Thank you!

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