1. Seismic velocity imaging and monitoring of volcanic areas using ambient seismic noise
F. Brenguier (OVPF/IPGP), D. Clarke (IPGP), N. M. Shapiro (IPGP), M. Campillo (LGIT), E. Larose (LGIT)
Institut de Physique du Globe de Paris/ Observatoire Volcanologique du Piton de la Fournaise
Laboratoire de Géophysique Interne et Tectonophysique (Grenoble)

2. Virtual seismic source reconstruction
sensor
Noise sources
Shapiro et al. (2005)

3. A case study: Piton de la Fournaise volcano, La Réunion island

4. 3D velocity imaging – Virtual sources

5. 3D velocity imaging – Results
Perturbations to a horizontally averaged model
solidified intrusive
magma bodies
effusive material
Perturbations to a vertically averaged model
Brenguier et al., GRL, 2007

6. Vp compared to Vs tomographyW E
Prôno et al., JVGR, 2009

7. Prospects Need to invert for a joint Vp/Vs tomographic model
Study of the link between seismicity and solidified intrusive magma bodies or magma reservoir using double difference relocalisation
Improve the Vs tomographic model by using broad-band seismic noise records

8. Temporal changes of seismic velocities using cross-correlations of seismic noise
Synthetic velocity decrease

9. Velocity changes on Piton de la Fournaise

10. Velocity drops preceding eruptions
Brenguier et al., Nature Geoscience, 2008

11. Velocity change regionalization

12. Seismic velocity changes as a proxy for magma forcing (1)
Magma forcing involves dilatancy of volcanic edifices
We use the term « magma forcing » do describe the mechanisms of stress induced magma pressurization and injection.
= magma forcing
Shih-Che Yuan, J. P. Harrison (2004), An empirical dilatancy index for the dilatant deformation of rock International Journal of Rock Mechanics and Mining Sciences, Volume 41, Issue 4

13. Seismic velocity changes as a proxy for magma forcing (2)
Seismic velocities are strongly sensitive to crack density.
O'Connell, R. J., and B. Budiansky (1974), Seismic Velocities in Dry and Saturated Cracked Solids, J. Geophys. Res., 79(35), 5412–5426

14. Link between temporal changes and structure
Magmatic intrusive complex
Localization of the dyke’s
injection zone ?

15. Long-term velocity changes
ALL STATIONS
Relative travel time change
coherence
VELOCITY DECREASES
SUMMIT STATIONS
Relative travel time change
coherence
year
Clarke et al., In preparation

16. Eruptive cycles Peltier et al., JVGR, 2009 Relative travel time change
coherence
year

17. Conclusions and prospects
We measure relative velocity changes with a precision of 0.02 %,
These changes are linked to dilatation induced by stress changes,
We identified precursors to the volcanic eruptions.
Prospects
We are looking to localize the velocity changes at depth (4D tomography),
Develop real-time passive monitoring procedures in Observatories,

18. UnderVolc project
20 broad-band seismometers + 16 gps + 1 infrasound sensor + photogrammetric system
Continuous recording and radio real-time data transfer and data distribution at day+1

19. Thank you for your attention !
Related presentations:
S23C-03. Temporal changes of seismic wavefield during the 2007 eruptions of Piton de la Fournaise, La Réunion island .  Y. Aoki; V. Ferrazzini
S53A-1811
V23D A real time process for detection, clustering, and relocation of volcano-tectonic events at Piton de La Fournaise volcano..  F. Massin ; V. Ferrazzini; P. Bachelery; Z. Duputel
We are gratefull to the Piton de la Fournaise volcanological Observatory staff for providing high quality data !

21. Toward real-time monitoring
Eruption of July 2006

22. Toward real-time monitoring
The last eruption of April 2007