The April 2007 eruption at Piton de la Fournaise, Réunion Island, imaged with ENVISAT-ASAR and ALOS-PALSAR data

Transcription

1 The April 27 eruption at Piton de la Fournaise, Réunion Island, imaged with ENVISAT-ASAR and A-PALSAR data A. Augier (1), J. L. Froger (1), V. Cayol (1) (1) Laboratoire Magmas et Volcans, Clermont Ferrand

2 I Introduction Réunion island, geological context La Réunion, a volcanic island Topography of the Piton de la Fournaise A very active volcano : 29 eruptions since

3 I Introduction InSAR database ( For the April 27 eruption and post eruptive period: 35 ASAR interferograms in various ascending and descending swaths (ESA AO-ENVISAT 746) 25 ascending PALSAR interferograms (ESA JAXA A-ADEN 3622)

4 I Introduction April 27 eruption 3 2 April March : new : small dyke dyke intrusion intrusion and and high 6 April 1 May : Dolomieu lava : end flow of the crater eruption collapse Photo OVPF The Dolomieu The Lava Dolomieu before flow, the april collapse 6 27

5 II Contribution of the InSAR data InSAR view of the April 27 eruption -.5 Central Cone Grandes Pentes 4.47cm/month Complex signal combining various displacements Displacement (m) Signal at the summit -.25 Signal in the Grandes Pentes cm/month.55 cm/month Signals amplitude decrease 1/5/27 1/9/27 1/1/28 1/5/28 1/9/28

6 II Contribution of the InSAR data Characterization of the post-eruptive displacements ENVISAT interferograms Ascending interferograms Descending interferograms A interferograms

7 II Contribution of the InSAR data Characterization of the post-eruptive displacements Post-eruptive 3cm/y

8 II Contribution of the InSAR data Characterization of the post-eruptive displacements N GPS data : OVPF April 6 S

9 III Model of the post-eruptive summital displacements Displacements Tomography : principes s D f P, R mod T GG 1 G Discretisation of the source Subsampling of the volume under the topography T d obs d sp i j s D f P,1 with : P P mod i i i i G Displacement of the j point under the i source effect (McTigue model,radius = 1, pressure = 1) d d 1 sp s s p 1 1 n 1 sp d d s p m n m S Pressure (McTigue model ) Gs d obs

10 III Model of the post-eruptive summital displacements Displacements Tomography : roughness control 2D equivalent situation of i-1,j-1 i-1,j i-1,j+1 i-1,j+2 i,j-1 i,j i,j+1 i,j+2 s... 4s s s s s...s 1,1 i, j i1, j i, j1 i, j1 i1, j I, J i+1,j-1 i+1,j i+1,j+1 i+1,j+2 Ls i+2,j-1 i+2,j i+2,j+1 i+2,j+2 G d obs s L

11 III Model of the post-eruptive summital displacements Displacement Tomography : model calculation ORIGINAL DATA log1(misfit) high low roughness Best High Low roughness??? log1(roughness) Compromise between low roughness and low misft low high roughness MODEL.5 RESIDUE

12 III Model of the post-eruptive summital displacements Tomography : cross-validation sum of square (CVSS) log1(misfit) Best model log1(roughness) Unpredictability log1(roughness) CVSS : Wahba, 199

13 III Model of the post-eruptive summital displacements Displacements Tomography : results 5 ascending interferograms in swath 7 OBSERVED DATA.5.5 OBSERVED DATA.5 OBSERVED DATA.5 OBSERVED DATA.5 OBSERVED DATA MODEL RESIDUE MODEL RESIDUE MODEL RESIDUE Same position with time MODEL A unique source RESIDUE RESIDUE.5 MODEL Decreasing pressure

14 III Model of the post-eruptive summital displacements Model of the post-eruptive summital signal Method : combination of a 3D boundary element modelling with a near-neighbourhood inversion (Fukushima et al., JGR, 25) Altitude (km) original data model residue

15 III Model of the post-eruptive summital displacements Pressure evolution Altitude (km) North (km) model East (km) d mod p d obs 188 interferograms Time serie calculation on pressure

16 III Model of the post-eruptive summital displacements Model interpretations Tomography Boundary element model Altitude (km) Both models give the same result : Too shallow for a magma chamber shallow source Draining decreasing of the hydrothermal pressure system???? Mechanical readjustment of the structure???? Pressure = f(time)

17 III Results of model Hypothetical origin of the summital subsidence Gravimetry results Model results Différence de gravité (mgal) relevés Bory Dolomieu 3 Altitude (km) Bory Dolomieu P( h) ( ) g(altitude h) e c max Altitude.5.5 interférogramme original.5.5 interférogramme modélisé.5.5 résidus (modélisé - original) Longitude (UTM,m) Gailler et al JVGR, 29

18 III Results of model Hypothetical origin of the summital subsidence Evidences of a shallow hydrothermal system 9 avril 27 4 mai juin novembre avril 28 Fumaroles Lava flow Temperature anomalies ASTER FTIR data (temperature anomalies) Urai et al. 27 Lenat et al 2 Photo Contarin 6avril 27

19 Conclusion Possible chronology of events 1 April to 6 April 27 April six to may 27 May 27 to October 28 Dolomieu Emptying of collapse the magma chamber by the low elevation eruption. Opening of the hydrothermal system Deflation of the entire Enclos Fouquet Draining of the hydrothermal system Mechanical readjustment? The draining of the hydrothermal system continued until October 28

20 ENVISAT DATA : (ESA AO-ENVISAT 746) A DATA :(ESA JAXA A-ADEN 3622)

21 III Effects of the summital cone readjustment Altitude (m) Altitude (km) Est(km) Ph ( ) g(altitude h) max interférogramme original interférogramme modélisé résidus (modélisé - original)

22 II Contribution of the InSAR data Characterization of the syn-eruptive displacements ENVISAT interferograms Ascending interferograms Descending interferograms A interferograms

23 II Contribution of the InSAR data Characterization of the syn-eruptive displacements Grande Pentes signals : consequence of a dyke injection? C Syn-éruptif A B A A C C B B 45cm 8cm

24 II Contribution of the InSAR data Characterization of the syn-eruptive displacements Grande Pentes signals : consequence of a dyke injection? 45cm 8cm Données GPS : OVPF

25 II Contribution of the InSAR data Characterization of the syn-eruptive displacements North oriented lobes : a dyke injection? A B A A B B

26 II Contribution of the InSAR data Characterization of the syn-eruptive displacements Central cone signal : centripetal subsidence after the collapse Syn-éruptif Données GPS : OVPF

27 III Numerical modeling Modeling : method and sources Method : combination of a 3D boundary element modelling with a nearneighbourhood inversion (Fukushima et al., JGR, 25) Effect of edifice readjustement ellipsoid Altitude (m) Est(km) ellipse sphere Incoherent column