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, LAquila, 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

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 Faults on Mt. Etna 10 cm The study area: the Pernicana Fault Rigano et al. (2008) observe a strong wavefield polarization in fault zones

Ambient noise Earthquakes

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

Summit crater, Mt. Etna Falsaperla et al. 2010, JGR At 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

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

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

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

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

Ratio 150/10 m Frequency (Hz) Energy propagation Frequency (Hz) Q50m Q150m E (f, R) = 1/R Eo(f) exp (-2πfR/VsQ) Body wave model E (f, R) = 1/R Eo(f) exp (-2πfR/VsQ) Vs 500 m/s Q 20 E (f, R) = 1/R 0.5 Eo(f) exp [-2πfR/V(f)Q] Rayleigh wave model E (f, R) = 1/R 0.5 Eo(f) exp [-2πfR/V(f)Q]

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

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.

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