agu98
1
Rotation axis of pendulum Welded silica bar (period) Pyrex watertight cover Silica cone fixed in the rock Red LED Free silica bar (centering) Aluminium mask Photo- electric cell Soil SITE 3 section N40W Lava flow North Summit Lat. 7° 32' 36.6" S, Lon. 110° 27' 44.0" E, Alt. 1550 m 10 m CH429 CH427 CH380 CH379 CH376 SITE 1 SITE 3 SITE 2 MDA MDB Rain LIP Res. DELES TILT STATION LIP LM35 1995 1996 1997 -2 -1 0 1 2 3 Normalized Wall Shear Stress Variations 0 1 2 3 4 Number of Multiphase Seisms (10 4 ) 1995 1996 1997 z r w P(h) t P Dome ra, μ Fracture Medium r r , G hmax Edi Suhanto, Made Agung Volcanological Survey of Indonesia [email protected], [email protected] François Beauducel & François-Henri Cornet Institut de Physique du Globe de Paris, Dpt Seismology [email protected], [email protected] Constraint from displacement data on magma flux at Merapi volcano, Java Objectives From deformation field analysis, we solve the boundary conditions common to the solid mechanics and the magma flux problems: internal structures geometry ( plumbing system ), pressure, stress and volume variations. The study also allows a better appraisal of monitoring measurements, thus helps hazard mitigation. Methodology We focus our study on Mt. Merapi, Java, Indonesia, a Decade Volcano with a quasi continuous activity: dome growth, explosions and pyroclastic flows. Deformation field measurement come from GPS network and multi- component continuous stations (tiltmeters and extensometers). Data are always validated by compensation to determine uncertainties. For modeling, we use three-dimensional elasto-static boundary elements [Cayol & Cornet, 1997] including topography, fractures and complex geometry sources. Inversion processes are used to estimate parameter evolution and model probability from observations. Some conclusions L Combination of high-precision tilt, displacements and topography constrains the modeling L Evidence for a deep magma chamber at Merapi L Evidence for fracture existence and their implications on the summit deformation field L Compatibility of displacement observations and multi-phase seismic events: summit deformation field is controlled by magma flux L Apparent elastic behaviour, but Young s modulus less than 1 GPa (possibly linear visco-elastic reality) L Mass balance must include rock avalanches L Identify potential rock slope problems References Cayol, V., and F.H. Cornet, 3D mixed boundary elements for elastostatic deformation field analysis, Int. J. Rock Mech. Min. Sci., 34, 275 287, 1997. Beauducel, F., and F.H. Cornet, Collection and three-dimensional modeling of GPS and tilt data at Merapi volcano, Java, J. Geophys. Res., 1998, in press. Ø Photos. (a) View of Merapi from the North that reveals the strong asymetry of the edifice, and the high slopes of the volcano, reaching 57° near the summit. (b) The January 30 1992 lava dome. (c) November 22, 1994 pyroclastic flow that killed 69 people. Merapi presents an activity of quasi continuous extrusion of lava which forms a dome in a horse-shoe shaped crater. The dome is continuously and partially destroyed by avalanches and pyroclastic flows. 106° E 108° E 110° E 112° E 114° E 8° S 7° S 6° S Indian Ocean Java Sea 95° E 105° E 115° E 125° E 135° E -10° N -5° N 0° N 5° N Indian Ocean Pacific Ocean South China Sea AUSTRALIAN Plate CAROLINE Plate EURASIAN Plate 6.5 cm/year 110° E 111° E 8° S 7° S Indian Ocean Java Sea Yogyakarta G. Merapi G. Merbabu G. Lawu G. Ungaran Dieng Plateau G. Muria G. Sundoro G. Sumbing G. Telemojo G. Prahu Surakarta Semarang Central Java Java 50 km Ø Figure 2. Geological setting and deformation network at Merapi. In red, benchmarks and stations installed for this study (GPS, tilts and extensometers). In blue, other collaborations with VSI (USA, Germany, Japan). Ø Figure 4. GPS baselines measured in November 1996, with 2 single-frequency receivers SERCEL NR101. The network was measured in 1993, 1994, 1995, 1996 and 1997. Table 1. Deles tilt station sensor characteristics (range, digital resolution and RMS noise for 2-min and 1-day periods). Sensor Unit Range Resol. 2-Min σ 1-day σ Tilt tan. CH379 (site 1) μrad 155 1.15 10 5 0.0121 0.4106 Tilt rad. CH380 (site 1) μrad 195 5.40 10 5 0.0099 0.3888 Tilt rad. CH376 (site 2) μrad 248 2.14 10 5 0.0136 0.3163 Tilt tan. CH427 (site 3) μrad 218 2.00 10 5 0.0117 0.7835 Tilt rad. CH429 (site 3) μrad 578 1.00 10 5 0.0302 0.2693 Temp. LIP (site 1) °C 100 1.75 10 4 0.0050 0.0837 Rainmeter (station) mm 100 000 1 Resistor bridge (site 3) mV 10 000 1.09 10 3 0.0968 0.4471 Ø Figure 3. The Deles tilt station is located on the South-Est flank of Merapi (3 km from summit), on a compact and massive rock identified as a 5,000 years old pyroxene andesitic lava flow. The thickness reaches 200 m at some places and a few meters of young pyroclastic deposits cap it, constituting a thermal isolator, as compared with the massive rock below. (a) Scheme of the horizontal pendulum Compact Blum made from welded silica. (b) Sensor location (horizontal view) : 5 tiltmeters, 1 thermometer and 1 rainmeter. (c) Vertical section of site 3 tilt installation. Figure 5. Meteorological data measured on the field during 1997 GPS campain. They have been reduced with respect to 3000-m elevation using standard vertical profiles of temperature, pressure and relative humidity, and are presented on a single day scale in local time. These data allow to determine a local meteo model of the troposphere for each time period of the GPS measurements, and are introduced in the double difference processing to compute baselines vectors. Introduction of meteo data is essential when the network covers large height differences as is the case for Merapi (1600 m). Ø Figure 7. (a) Relative tilt signals at Deles between the 2 GPS campains (dashed zones): 2 radial and 2 tangential components. Inset plot shows the horizontal projection of the average motion of the tip of the normal to ground surface. (b) Dome volume estimation (solid line and triangles) and number of pyroclastic flows (bars) within the period. Dotted vertical line corresponds to the time of the dome explosion (January 17 th , 1997). Ø Figure 6. Cumulated displacements at the summit obtained by compensation of GPS baselines from 1993 to 1997. An important movement occured (about 40 cm) on the Northern part of the crater rim. Four « independent » zones separeted by fractures with different behavior are observed. (a) Horizontal view. (b) Vertical view from azimuth N145°E. Figure 11. 3-D deformation field modeling at Merapi on the 1996-1997 period: GPS displacements (red arrows), computed tilt field at the surface of the volcano (contour lines) and location of the tilt station (DEL). Deep magma chamber is deduced from both displacements and tilt observations. Because in elasticity, ∆P ∆V displacements, we avoid the difficult task of Young s modulus estimation, and resolve only the volume variation of the source. Superficial magma chamber has been proposed from seismic observations but is not compatible with our deformation data. The computed volume variation is 3 times larger than the observed volume at the summit (3.2 ± 0.2 10 6 m 3 ). Ø Figure 9. Monte-Carlo near-neighbor sampling inversion method (least squares best solution for 3D displacements and tilt vectors): projection of the model space in a 2-parameter plan (volume variation versus source depth). Dot size represents probability of each model (364 samples). Volume variation is well constrained but not the source shape and size. Ø Figure 12. Types of source considered for the summit modeling of displacement field: dome weight effect on the crater floor, magma pressure in the duct and wall shear stress due to flux variation of viscous fluid. Computation of the dome weight effect for 1993-1994 period showed that this effect is negligeable on displacements. Ø Figure 13. 3-D modeling of the summit deformations with 3 free fractures in the medium (horizontal view): displacement field (light arrows) and blow-up displacements on GPS points (heavy arrows). The displacement unit corresponds to 1 mm with a 1 MPa source and a Young s modulus of 30 GPa. (a) Results with wall shear stress in the duct. (b) Results with pressure source. A linear combination of the two sources effects allows to reproduce correctly the observed displacements pattern from 1993 to 1997 for all the points except for the North-West one (NTR), which probably exhibits an inelastic behavior. Ø Figure 14. We processed an inversion from the linear combination of the two forward problem solutions (Fig. 13), constrained by the 1993-1997 GPS 3-D displacements. The computed wall shear stress variations are compatible with recorded « multi- phase » seismic events variations. Because the two observations are independent, this gives further support that these seismic events are related with shear stress release at the duct wall. Ø Figure 8. Example of tilt signal correction from temperature effects by a non-stationary linear method and comparison with theoretical earth tide variations computed for this location. This result validates the sensor coupling with the ground and the signal processing method used, which did not affect the complex tide signal included. Figure 10. (a) 3-D mesh of topography around Deles tilt station. (b) Relative tilt computed along the cross-sections for the final elliposoidal source solution. The tilt varies within ±1 μrad at ±20 m around the station. Figure 1. Location and geodynamical context of Mt. Merapi (2964 m). Merapi is a young strato- volcano located in Central Java, Indonesia, in a frontal subduction zone. Population of Yogyakarta (25 km from the summit) and arround is about 3 millions people, up to 500,000 are living directly on the flank of the volcano, above 500 m of elevation. Merapi is one of the « Decade Volcano » declared by the United Nations IDNDR program.
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From deformation field analysis, we solve the boundaryconditions common to the solid mechanics and themagma flux problems: internal structures geometry(“plumbing system”), pressure, stress and volumevariations. The study also allows a better appraisal ofmonitoring measurements, thus helps hazardmitigation.
Transcript of agu98
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Rotation axis of pendulum
Weldedsilica bar(period)
Pyrex watertightcover
Silica conefixed in the rock
Red LED
Free silica bar(centering)
Aluminiummask
Photo-electric cell
Soil
SITE 3 section N40W
Lava flow
North
Summit
Lat. 7 32' 36.6" S, Lon. 110 27' 44.0" E, Alt. 1550 m
10 m
CH429CH427
CH380
CH379
CH376
SITE 1
SITE 3
SITE 2
MDA MDBRain
LIP
Res.
DELES TILT STATION
LIP LM35
1995 1996 1997-2
-1
0
1
2
3NormalizedWall Shear Stress
Variations
0
1
2
3
4
Number ofMultiphase
Seisms (104)
1995 1996 1997
z
r
w
P(h)
P
Dome a,
Fracture
Medium r, G
hmax
Edi Suhanto, Made Agung
Volcanological Survey of [email protected], [email protected]
Franois Beauducel & Franois-Henri Cornet
Institut de Physique du Globe de Paris, Dpt [email protected], [email protected]
Constraint from displacement data on magma flux at Merapi volcano, Java
ObjectivesFrom deformation field analysis, we solve the boundaryconditions common to the solid mechanics and themagma flux problems: internal structures geometry(plumbing system), pressure, stress and volumevariations. The study also allows a better appraisal ofmonitoring measurements, thus helps hazardmitigation.
MethodologyWe focus our study on Mt. Merapi, Java, Indonesia, aDecade Volcano with a quasi continuous activity: domegrowth, explosions and pyroclastic flows. Deformationfield measurement come from GPS network and multi-component continuous stations (tiltmeters andextensometers). Data are always validated bycompensation to determine uncertainties. For modeling,we use three-dimensional elasto-static boundaryelements [Cayol & Cornet, 1997] including topography,fractures and complex geometry sources. Inversionprocesses are used to estimate parameter evolution andmodel probability from observations.
Some conclusions Combination of high-precision tilt, displacements
and topography constrains the modeling
Evidence for a deep magma chamber at Merapi
Evidence for fracture existence and theirimplications on the summit deformation field
Compatibility of displacement observations andmulti-phase seismic events: summit deformationfield is controlled by magma flux
Apparent elastic behaviour, but Youngs modulusless than 1 GPa (possibly linear visco-elastic reality)
Mass balance must include rock avalanches
Identify potential rock slope problems
ReferencesCayol, V., and F.H. Cornet, 3D mixed boundary elements for elastostatic
deformation field analysis, Int. J. Rock Mech. Min. Sci., 34, 275287,1997.
Beauducel, F., and F.H. Cornet, Collection and three-dimensional modelingof GPS and tilt data at Merapi volcano, Java, J. Geophys. Res., 1998,in press.
Photos. (a) View of Merapi from the North that reveals the strong asymetry of the edifice, andthe high slopes of the volcano, reaching 57 near the summit. (b) The January 30 1992 lava dome.(c) November 22, 1994 pyroclastic flow that killed 69 people. Merapi presents an activity of quasicontinuous extrusion of lava which forms a dome in a horse-shoe shaped crater. The dome iscontinuously and partially destroyed by avalanches and pyroclastic flows.
106 E 108 E 110 E 112 E 114 E
8 S
7 S
6 S
Indian Ocean
Java Sea
Jakarta
BALI
M A D U R A
S U M A T R A
95 E 105 E 115 E 125 E 135 E
-10 N
-5 N
0 N
5 N
Indian Ocean
Pacific OceanSouth China Sea
AUSTRALIAN Plate
CAROLINE Plate
EURASIAN Plate
6.5 cm/year
110 E 111 E
8 S
7 S
Indian Ocean
Java Sea
Yogyakarta
G. Merapi
G. Merbabu
G. Lawu
G. Ungaran
Dieng Plateau
G. Muria
G. Sundoro
G. Sumbing
G. TelemojoG. Prahu
Surakarta
Semarang
Central Java
Java
50 km
Figure 2. Geological setting and deformationnetwork at Merapi. In red, benchmarks and stationsinstalled for this study (GPS, tilts andextensometers). In blue, other collaborations withVSI (USA, Germany, Japan).
Figure 4. GPS baselines measured inNovember 1996, with 2 single-frequencyreceivers SERCEL NR101. The network wasmeasured in 1993, 1994, 1995, 1996 and 1997.
Table 1. Deles tilt station sensor characteristics (range, digital resolution and RMSnoise for 2-min and 1-day periods).
Sensor Unit Range Resol. 2-Min 1-day Tilt tan. CH379 (site 1) rad 155 1.15 105 0.0121 0.4106 Tilt rad. CH380 (site 1) rad 195 5.40 105 0.0099 0.3888 Tilt rad. CH376 (site 2) rad 248 2.14 105 0.0136 0.3163 Tilt tan. CH427 (site 3) rad 218 2.00 105 0.0117 0.7835 Tilt rad. CH429 (site 3) rad 578 1.00 105 0.0302 0.2693 Temp. LIP (site 1) C 100 1.75 104 0.0050 0.0837 Rainmeter (station) mm 100 000 1 Resistor bridge (site 3) mV 10 000 1.09 103 0.0968 0.4471
Figure 3. The Deles tilt station is located on the South-Est flank of Merapi (3 km fromsummit), on a compact and massive rock identified as a 5,000 years old pyroxene andesiticlava flow. The thickness reaches 200 m at some places and a few meters of young pyroclasticdeposits cap it, constituting a thermal isolator, as compared with the massive rock below.(a) Scheme of the horizontal pendulum Compact Blum made from welded silica.(b) Sensor location (horizontal view) : 5 tiltmeters, 1 thermometer and 1 rainmeter.(c) Vertical section of site 3 tilt installation.
Figure 5. Meteorological datameasured on the field during 1997GPS campain. They have beenreduced with respect to 3000-melevation using standard verticalprofiles of temperature, pressure andrelative humidity, and are presentedon a single day scale in local time.These data allow to determine a localmeteo model of the troposphere foreach time period of the GPSmeasurements, and are introduced inthe double difference processing tocompute baselines vectors.Introduction of meteo data isessential when the network coverslarge height differences as is the casefor Merapi (1600 m).
Figure 7. (a) Relative tilt signals at Delesbetween the 2 GPS campains (dashed zones): 2radial and 2 tangential components. Inset plotshows the horizontal projection of the averagemotion of the tip of the normal to groundsurface. (b) Dome volume estimation (solidline and triangles) and number of pyroclasticflows (bars) within the period. Dotted verticalline corresponds to the time of the domeexplosion (January 17th, 1997).
Figure 6. Cumulateddisplacements at the summit obtainedby compensation of GPS baselinesfrom 1993 to 1997. An importantmovement occured (about 40 cm) onthe Northern part of the crater rim.Four independent zones separetedby fractures with different behaviorare observed. (a) Horizontal view. (b)Vertical view from azimuth N145E.
Figure 11. 3-D deformation field modeling at Merapi on the 1996-1997 period: GPSdisplacements (red arrows), computed tilt field at the surface of the volcano (contourlines) and location of the tilt station (DEL). Deep magma chamber is deduced from bothdisplacements and tilt observations. Because in elasticity, P V displacements,we avoid the difficult task of Youngs modulus estimation, and resolve only the volumevariation of the source. Superficial magma chamber has been proposed from seismicobservations but is not compatible with our deformation data. The computed volumevariation is 3 times larger than the observed volume at the summit (3.2 0.2 106 m3).
Figure 9. Monte-Carlo near-neighborsampling inversion method (least squares bestsolution for 3D displacements and tilt vectors):projection of the model space in a 2-parameterplan (volume variation versus source depth).Dot size represents probability of each model(364 samples). Volume variation is wellconstrained but not the source shape and size.
Figure 12. Types of source considered for thesummit modeling of displacement field: domeweight effect on the crater floor, magma pressurein the duct and wall shear stress due to fluxvariation of viscous fluid. Computation of thedome weight effect for 1993-1994 period showedthat this effect is negligeable on displacements.
Figure 13. 3-D modeling of the summit deformations with 3 free fractures in the medium (horizontalview): displacement field (light arrows) and blow-up displacements on GPS points (heavy arrows). Thedisplacement unit corresponds to 1 mm with a 1 MPa source and a Youngs modulus of 30 GPa. (a)Results with wall shear stress in the duct. (b) Results with pressure source. A linear combination of thetwo sources effects allows to reproduce correctly the observed displacements pattern from 1993 to 1997for all the points except for the North-West one (NTR), which probably exhibits an inelastic behavior.
Figure 14. We processed an inversion from thelinear combination of the two forward problemsolutions (Fig. 13), constrained by the 1993-1997 GPS3-D displacements. The computed wall shear stressvariations are compatible with recorded multi-phase seismic events variations. Because the twoobservations are independent, this gives furthersupport that these seismic events are related withshear stress release at the duct wall.
Figure 8. Example of tilt signal correctionfrom temperature effects by a non-stationarylinear method and comparison with theoreticalearth tide variations computed for this location.This result validates the sensor coupling withthe ground and the signal processing methodused, which did not affect the complex tidesignal included.
Figure 10. (a) 3-D mesh of topographyaround Deles tilt station. (b) Relative tiltcomputed along the cross-sections for thefinal elliposoidal source solution. The tiltvaries within 1 rad at 20 m around thestation.
Figure 1. Location and geodynamical context of Mt. Merapi (2964 m). Merapi is a young strato-volcano located in Central Java, Indonesia, in a frontal subduction zone. Population of Yogyakarta(25 km from the summit) and arround is about 3 millions people, up to 500,000 are living directlyon the flank of the volcano, above 500 m of elevation. Merapi is one of the Decade Volcano declared by the United Nations IDNDR program.
