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)

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

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

3D velocity imaging – Virtual sources

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

Vp compared to Vs tomography W E Prôno et al., JVGR, 2009

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

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

Velocity changes on Piton de la Fournaise

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

Velocity change regionalization

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

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

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

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

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

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,

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

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-2107. 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 !

Toward real-time monitoring Eruption of July 2006

Toward real-time monitoring The last eruption of April 2007 http://www.fournaise.info/eruption2avril07.php