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