Crustal velocity and anisotropy temporal variations at Etna volcano (1) Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli, Italy (2) Seismology and Computational Rock Physics Lab., School of Geological Sciences, University College Dublin, Ireland (3) Istituto Nazionale di Geofisica e Vulcanologia, sezione di Pisa, Italy Zaccarelli L..(1), Pandolfi D.(2), Bianco F.(1), Saccorotti G.(3), Bean C.J.(2), Del Pezzo E.(1)

temporal changes in the seismic wave propagation characteristics due to Stress field time variations 2 techniques: Coda Wave Interferometry (CWI)  velocity variations Shear Wave Splitting analysis (SWS)  anisotropy changes high resolution in detecting small changes in the parameter estimates How to be sure about the temporal (no spatial) effect?

Doublets or multiplets events recorded at the same station similar waveforms cross-correlation max. > 0.9 almost same locations hypocentral distance < 100 m  same source & ray path doublet changes reflect time variation of the medium elastic properties Poupinet et al., 1984 Geller and Mueller, 1980

CWI & SWS applications in volcanic environments Etna eruption NE fissure: 28 Oct 2002 – 5 Nov 2002 seismic records: 31 Oct 2002 – 4 Feb 2003 Broad Band seismic stations: high dynamics, continuous digital acquisition 2 km

d1-d7 small volume  homogeneous Data set: 1124 VT recorded  11 doublets

Coda Wave Interferometry discriminates among: source displacements scatterer movements velocity variations Cross-correlation of subsequent coda portions i  (i) = time shift = mean travel time perturbation

CWI technique least squares estimation over those points visually aligned  (  v / v)  - (  /  )  (i) = - (  v/v)  i + q

T d = time delay between the 2 qS-waves   crack system characteristics (density & geometry)  = qS1 polarization  stress field main direction Shear Wave Splitting analysis describes the crustal anisotropy field through 2 observables:

SWS analysis rotation along   – diagonalization of the covariance matrix T d – cross correlation of fast and slow components

CWI – percentage velocity variations

SWS –  and T n =T d /D   90  N = EW oriented  background value  overpressurized system

Results doubletsCWISWS t 1 – t 2 vvTnTn d131 Oct 10:25 – 3 Nov 06:02 +- d231 Oct 12:50 – 2 Nov 11:20 +- d31 Nov 02:11 – 3 Nov 04:26 ++ d41 Nov 07:49 – 4 Nov 12:44 -+ d52 Nov 11:20 – 4 Nov 11:19 -+ d64 Nov 09:52 – 4 Nov 12:58 -+ d74 Nov 10:31 – 4 Nov 10:37 +- TREND INVERSION on 1–3 November

Comparing CWI-SWS mean doublet percentage variations per day NE fissure eruption end

CONCLUSIONS 3 – 4 days before the NE fissure eruption’s ending:  v/v   Td/Td  We observe stress  fluid content  We interpret  #  cracks

Conceptual model 1.empting of the plumbing system 3.fluid attraction from the surrounding rocks 2.depressurization RELAXATION + FLUID MIGRATION CWI and SWS analysis are sensitive to even small stress field variations  indicator of crustal stress state in time v and Td temporal trends change before the start and /or the end eruptive activity  volcano monitoring and eruption forecasting