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