deformation and stress-change modeling of sierra negra...
TRANSCRIPT
Deformation and Stress-Change Modeling ofDeformation and Stress-Change Modeling of
Sierra Negra Volcano, Galápagos, fromSierra Negra Volcano, Galápagos, from
Envisat InSAR and GPS ObservationsEnvisat InSAR and GPS Observations
Sigurjón Jónsson, Institute of Geophysics, ETH Zürich, Switzerland
William W. Chadwick Jr., Oregon State University and NOAA, USA
Dennis Geist, Geological Sciences, University of Idaho, USA
Michael Poland, Hawaii Volcano Observatory, USGS, USA
Fringe ‘07, 28 November, 2007
Radar data provided by ESA through Category-1 project #3493
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
PurposePurpose
§ To explain the sequence of events leading to theOct. 2005 eruption at Sierra Negra and theinterplay between caldera inflation and faulting
Outline:
§ Background
§ Measurements of the inflation and modeling
§ Observations of the intra-caldera faulting
§ Stress change calculations
§ Conclusions
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
GalGaláápagos Islandspagos Islands
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
GalGaláápagos Volcanoespagos Volcanoes
§ Basaltic shield volcanoes withlarge summit calderas, 1-2000 min elevation,
§ Erupt frequently, e.g. Fernandinain 1995 and 2005, Cerro Azul in1998, and Sierra Negra 1979,2005.
§ Deformation map from InSAR1992-1998 showing that all thevolcanoes are actively deforming
§ Sierra Negra uplifted by over 2 m,and subsidence found on recentlava flows
Amelung, Jónsson,
Zebker, & Segall, Nature,
2000
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Sierra Negra Caldera UpliftSierra Negra Caldera Uplift
Uplift 1992 – Oct. 2005
- Almost 5 m !!
InSAR - 1
GV04
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Uplift 12 Feb. 2004 - 27 Jan. 2005Uplift 12 Feb. 2004 - 27 Jan. 2005
Ø Over 50 cm of uplift in one year
Ø Mogi or finite ellipsoid do not work
Ø Inflating sill at 2.2 km depth can explain thedeformation
Ø Still differences, which may be due to 3-dimensionality of the source
10 cm
fringes
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Sierra Negra Caldera UpliftSierra Negra Caldera Uplift
Uplift 1992 – Oct. 2005
- Almost 5 m !!
InSAR - 1
GV04
GV06
InSAR - 2
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Deformation due to FaultingDeformation due to Faulting
10 cm fringes
• From GPS we know much deformation is due to uplift
• Assume uplift pattern is the same as in 2004, then scale and subtract it from the interferogram
• Only left with deformation due to the faulting
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Trapdoor Faulting on 16 Apr 2005Trapdoor Faulting on 16 Apr 2005
§ Only InSAR recorded the trapdoor faulting in1998 and then the interpretation was a south-dipping fault !!
§ Resolving this dip-inconsistency is importantto understand the volcano dynamics
§ How well can we constrain the fault dip fromthe deformation data?
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Estimating the Fault-Dip UncertaintyEstimating the Fault-Dip Uncertainty
§ We analyzed the structure andpower of a few non-deformingareas north of the caldera
§ Determined the average empiricalcovariance structure, which weused to form the data covariancematrix.
§ Created multiple synthetic data setsby adding several thousandsdifferent random realizations to theoriginal data
§ Estimated model parameters fromeach synthetic data set and inspectthe distribution of parameters
§ Approximate the marginal PDF witha Gaussian distribution
§ Fault dip 67-74° to the north, at a95% confidence level.
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Sierra Negra Stress-ChangesSierra Negra Stress-Changes
Mean stresschange: / 3
kk!"
Bar thickness reflectsdifferential stress change:
1 3| | 2! !" #"
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Coulomb Failure Stress Change due to the Inflating SillCoulomb Failure Stress Change due to the Inflating Sill
• Pure dip-slip (north sideup), and calculate !CFSat 1.5 km depth
• Positive !CFS = closerto failure
Assume the following:
Coeff. friction: !f = 0.75
Shear modulus: ! = 10 GPaPoisson’s ratio: " = 0.25
Skempton’s coeff.: B = 0.5
CFS3
s f n kk
B! µ " "
# $% = % + % & %' (
) *
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
!!CFS Calculation ResultsCFS Calculation Results
Ø Inflating sill promotesthrust faulting on northdipping faults
Ø Vertical or south-dippingnormal faults unlikely
Ø The maximum CoulombFailure Stress Change isfound on a fault dipping72 degrees to the north
Ø Cool !
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Sierra Negra Sequence of EventsSierra Negra Sequence of Events
Did the faulting reduce the magma pressure?
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Mean Stress Change due to FaultingMean Stress Change due to Faulting
Ø The trapdoor faulting
causes more than 3
MPa mean stress
change, relieving the
magma pressure
Ø Negative mean stress
change to the south,
preventing sill growth to
the south
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
ConclusionsConclusions
§ Inflating magma sill triggers repeatingtrapdoor faulting on Sierra Negra
§ Coulomb failure stress calculationsconfirm the steeply dipping thrust faultestimated from the geodetic data
§ The trapdoor faulting relieves the magmapressure, tends to thicken the sill,prevents southward magma propagationand somewhat postpones eruptions
§ More than 5 m of co-eruption subsidenceoccurred on Sierra Negra in October2005 and after the eruption the calderafloor has already uplifted by 2-3 meters.
Photo by M. Hall, GVP
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
!!CFS on Optimally Oriented FaultsCFS on Optimally Oriented Faults
Ø Now the !CFS ispositive everywhere.
Ø Large !CFS occur nearthe peripheries of the sill
Ø The maximum occurs inthe south, exactly on theestimated fault plane
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
GPS Data improve the Fault-Dip EstimateGPS Data improve the Fault-Dip Estimate
Does a 70o dipping thrust fault make sense?
Jónsson et al.: Sierra NegraETHEidgenössische Technische Hochschule Zürich
Swiss Federal Institute of Technology Zurich
Fault model parameter marginal pdfsFault model parameter marginal pdfs