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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 multipletsevents recorded at the same station
similar waveforms cross-correlation max. > 0.9 almost same locations hypocentral distance < 100 m
same source & ray pathdoublet changes reflect time variation of the medium elastic properties
Poupinet et al., 1984Geller and Mueller, 1980
CWI & SWS applications in volcanic environments
2002 - 2003 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
Td = time delay between
the 2 qS-waves
crack system characteristics
(density & geometry)
= qS1 polarization
stress field main direction
Shear Wave Splitting analysisdescribes the crustal anisotropy field through 2 observables:
SWS analysis
rotation along
– diagonalization of the covariance matrix
Td – cross correlation of fast and slow components
CWI – percentage velocity variations
SWS – and Tn=Td/D
90 N = EW oriented background value overpressurized system
Resultsdoublets CWI SWS
t1 – t2 v Tn
d1 31 Oct 10:25 – 3 Nov 06:02 + -
d2 31 Oct 12:50 – 2 Nov 11:20 + -
d3 1 Nov 02:11 – 3 Nov 04:26 + +
d4 1 Nov 07:49 – 4 Nov 12:44 - +
d5 2 Nov 11:20 – 4 Nov 11:19 - +
d6 4 Nov 09:52 – 4 Nov 12:58 - +
d7 4 Nov 10:31 – 4 Nov 10:37 + -
TREND INVERSION on 1–3 November
Comparing CWI-SWSmean doublet percentage variations per day
NE fissureeruption 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