1.5

2

2.5

Ele

vatio

n (k

m)

15981596

1594

North (km)

15921590

766 7687641588

East (km)762760758756

4.InSARModelingoftheJanuary– March2014event4.1MixedBoundaryElementMethod(MBEM)[5]• The shape, depth and overpressure of potential sources of the observed

deformation (Fig. 7) were obtained with a 3D-MBEM model combining the Directand Displacement Discontinuity Methods [5]

• Noisy areas were masked out (Fig. 7)• These bodies as well as topography were modeled with triangular elements [7]• Initial models include a single dike, breaching the surface along a linear fissure

connecting the 2014 eruptive vents. [1]

Assumptions: The medium is linearly elastic, homogeneous, isotropic. A Young’sModulus of 50 GPa and a Poisson’s ratio of 0.25 are used for all depths [5]

4.2Monte-CarloInversion[6]The Monte-Carlo inversion calculates the fit between the observed InSAR dataand the LOS displacement models produced through MBEM and constrains thebest fitting model that minimizes the following misfit function:

Χ" = (𝑢& − 𝑢()*𝐶,-.(𝑢& − 𝑢()

where 𝑢& and 𝑢( are vectors of observed and modeled LOS displacements and CDis the covariance matrix [6]

Fig. 7: Wrapped RSAT2 interferogram spanning January 8th 2014 - March 21st 2014, with mask, left, andwithout, right. The surface expression of the dike is traced in green.

755 756 757 758 759 760 761East (km)

1587

1588

1589

1590

1591

1592

No

rth

(km

)

4.3PreliminaryInversionResultsA joint NA+MBEM approach [7] was used to invert for the dip, bottomelevation, bottom length and distance to top of a preliminary single dike,providing a 60% misfit. Fig. 8 shows the best-fitting preliminary dike result. Fig.9 depicts the model residuals.

Fig. 9: LOS modeled deformation and residual signal not explained by this model (60% misfit)

Fig. 8: Best-fitting dike with dip = 105, bottom elevation = 1628 m above sea-level, bottom length = 0.065and distance to top = 741 m . The topographic mesh for Pacaya is overlain in grey.

OnedikealonedoesnotfittheInSARdatasatisfactorily– moresourcesshouldbeinvestigated

755 756 757 758 759 760 761East (km)

1587

1588

1589

1590

1591

1592

Nort

h (

km)

755 756 757 758 759 760 761East (km)

1587

1588

1589

1590

1591

1592

Nor

th (

km)

InvestigatingDeformationBehavioratPacayaVolcano,Guatemala,throughInSARTime-seriesAnalysisand3DMixedBoundaryElementModeling

JuditGonzalezSantana1,ChristelleWauthier1,2jmg6885@psu.edu

1DepartmentofGeosciencesand2 InstituteforCyberScience,ThePennsylvaniaStateUniversity

KeyQuestions:• IssurfacedeformationatPacayacontinuousorepisodic?• Aretheretemporallinksbetweendeformationandmagmaticevents?• Whatarethepotentialsourcesofsurfacedeformation?

Fig. 1: Location and geological setting of Pacaya Volcano (redtriangle). Pacaya lies at the intersection of the extensionalGCG (Guatemala City Graben) (8 mm/yr E-W extension) andthe right-lateral JFZ (Jalpatagua Fault Zone) (10-14 mm/yrright-lateral motion) and South of the MFZ (Motagua FaultZone) and PFZ (Polochic Fault Zone)

1.GeologicalSettingandMotivation• Pacaya is an active basaltic

stratovolcano with anunstable SW flank [1],located in Guatemala(Fig.1)

• Overall stress regime is ENE- WSW directed tension. [2]

• 9000 people live <5kmaway from the active cone[2]

• SW flank collapse produceda 0.65 km3 debrisavalanche between 0.6 -1.6 ka [2]

• An eruption in 2010 wasaccompanied by 4m offlank displacement [4]

• RSAT2 radar satellite datafrom January-March 2014show flank displacement.

92˚W 91˚W 90˚W 89˚W 88˚W13˚N

14˚N

15˚N

16˚N

17˚N

18˚N

Pacaya

Guatemala

Mexico Belize

Honduras

El Salvador

PFZ

MFZ

JFZ

GCG

Carribean Plate

Cocos Plate

North American Plate

Middle America Trench

3.InSARTime-seriesAnalysis

Interferometric Synthetic Aperture Radar (InSAR) uses the difference in phase betweentwo radar images to determine cm scale surface deformation. InSAR images show thechange in distance of the Earth’s surface in the Line Of Sight (LOS) of the satelliterecorded in between the two image acquisitions (Fig.2)

Jul 14 Feb 15 Aug 15 Mar 16 Sep 16 Apr 17 Nov 17Date

-250

-200

-150

-100

-50

0

50

100

150

200

250

Per

pend

icul

ar B

asel

ine

(m)

Baseline plot - RSAT2 (Asc)

Sep 10 Apr 11 Oct 11 May 12 Nov 12 Jun 13 Jan 14 Jul 14 Feb 15 Aug 15Date

-200

-150

-100

-50

0

50

100

150

200

250

Per

pend

icul

ar B

asel

ine

(m)

Baseline plot - RSAT2 (Des)

Fig. 2: How InSAR works (by Conway)

Fig. 3: Plot of temporal and spatial baselines < 200 days and < 300 m. Fig. 4: Plot of temporal and spatial baselines < 180 days and < 150 m.

Datasets:• 35descendingscenes(September2010– July2015)(Fig.3)• 52ascendingscenes(June2014– November2017)(Fig.4)

AcknowledgementsandReferences:This work was supported by NASA grant NNX16AK87G issued through the Science Mission Directorate’s Earth Science Division. RSAT-2 SAR data was provided by The Canadian Space Agency, theEuropean Space Agency (Third Party Mission Projects #16819 and #28777) and the Committee on Earth Observation Satellites (CEOS) Volcano Pilot Program. The 3D-MBEM code was developedby Cayol and Cornet (1997), the NA inversion by Sambridge (2001) and coupled into a NA+MBEM approach by Fukushima et al. (2005). SBAS scripts were developed and provided by Dr.Susanna Ebmeier.

1. Wnuk and Wauthier (2017) Surface deformation induced by magmatic processes at Pacaya Volcano, Guatemala revealed by InSAR. J. Volcanol. Geotherm. Res., 344, 197-2112. Schaeffer et al. (2013) An integrated field numerical approach to assess slope stability hazards at volcanoes: the example of Pacaya, Guatemala. Bull. Volcanol., 75:7203. Wunderman and Rose (1984) Amatitlan, an actively resurging cauldron 10km south of Guatemala City. J. Geophys. Res., 89:8525-85394. Schaeffer et al. (2017) Three-dimensional displacements of a large volcano flank movement during the May 2010 eruptions at Pacaya Volcano, Guatemala. Geophys. Res. Lett., 44, 135-1425. Cayol and Cornet (1997) 3D mixed boundary elements for elastostatic deformation field analysis. Int. J. Rock Mech. Min. Sci., 34(2):275-2876. Sambridge (1999) Geophysical inversion with a neighbourhood algorithm – I. Searching a parameter space. Geophys. J. Int., 138, 479-4947. Fukushima and Cayol (2005) Finding realistic dike models from interferometric synthetic aperture radar: The February 2000 eruption at Piton de la Fournaise. J. Geophys. Res., 110, B032068. Hooper (2008) A multi-temporal InSAR method incorporating both persistent scatterer and small-baseline approaches, Geophys. Res. Lett., 35, L16, 302

5.CurrentandFutureWork:• SBAS time-series analysis of the ascending data – implementing atmospheric corrections to obtain a clearer

signal:

• Addition of further potential sources of deformation with more complex geometries to the MBEM model – eg.detachment fault on the SW flank.

• InSAR modelling of 2010 deformation events using ALOS-1 and UAVSAR data• Examining more recent surface deformation with Sentinel-1• Pixel offset tracking for the observed deformation events in 2010 and 2014

2.WhatisInSAR?

PreliminarySBASprocessingwasperformedforthedescendingdataset.Fig.5showsthecumulativegrounddisplacementbetween26th September2010and9th April2015relativetothereferencepoint(redcircle).Fig.6showsthecumulativeLOSdisplacementattheyellowsquarelabelledinFig.5relativetothereferencepoint.

Flankmotionbetween2010and2015appearstofollowasemi-continuousLOSsubsidencetrend,beforeandafterthe2014eruptiveevent

Fig. 6: Cumulative LOS displacement plot at the yellow square relative to the red circlein Fig. 5, showing ~4.23 cm/yr subsidence between 2010 and 2015. The red box showsthe dates over which the 2014 deformation event modeled in section 3 was recorded.

Fig. 5: Map of the cumulative ground displacement between September2010 and April 2015 relative to the reference point (red circle). Theorange square is the location chosen for the time-series plot in Fig. 6

Cumulative LOS displacement

Jan-10 Jan-11 Jan-12 Jan-13 Jan-14 Jan-15 Jan-16Year

-25

-20

-15

-10

-5

0

Cum

ulat

ive

LOS

dis

plac

emen

t (cm

)

LOS