Coseismic and Postseismic Deformation from

the Sumatra-Andaman Earthquake

Observed by GRACE

Joint International GSTM and DFG SPP Symposium, October 15-17, 2007 at GFZ Potsdam

J.L. Chen 1, C.R. Wilson 1,2,3, B.D. Tapley 1, S.P. Grand 2,3

Center for Space Research, University of Texas at Austin, USA 1

Department of Geological Sciences, University of Texas at Austin, USA 2

Jackson School of Geosciences, University of Texas at Austin, USA 3

Motivation:Motivation:

Previous studies revealed co-seismic gravity changes due to Previous studies revealed co-seismic gravity changes due to Sumatra-Andaman earthquake (Mw=9.3) of December 26, Sumatra-Andaman earthquake (Mw=9.3) of December 26, 2004. 2004. [special processing using GRACE range rate data, Han et al. [special processing using GRACE range rate data, Han et al. 2006].2006].

Examine GRACE RL04 spherical harmonic products for Examine GRACE RL04 spherical harmonic products for evidence of co-seismic change.evidence of co-seismic change.

If RL04 SH products show co-seismic change, then time If RL04 SH products show co-seismic change, then time series may be examined for series may be examined for pre-pre- and and post-seismicpost-seismic changes changes

GRACE Data and ProcessingGRACE Data and Processing

CSR GRACE RL04 solutionsCSR GRACE RL04 solutions April 2002 through February 2007;April 2002 through February 2007;

55 monthly solutions;55 monthly solutions;

To degree, order 60To degree, order 60

Applied FiltersApplied Filters De-correlation filtering [Swenson and Wahr, 2006]De-correlation filtering [Swenson and Wahr, 2006]

300 km Gaussian smoothing300 km Gaussian smoothing

Data ProcessingData Processing

Coseismic ChangeCoseismic Change Estimate Minimize seasonal and other climate effects by computing: Minimize seasonal and other climate effects by computing:

[mean of (2005 + 2006)] [mean of (2005 + 2006)] minusminus [mean of (2003 + 2004)] ; [mean of (2003 + 2004)] ;

21 solutions in each mean;21 solutions in each mean; (2003 + 2004): Jan. 2003 - Nov. 2004;(2003 + 2004): Jan. 2003 - Nov. 2004;

(2005 + 2006): Jan. 2005 - Sept. 2006;(2005 + 2006): Jan. 2005 - Sept. 2006;

Dec. 2004 excluded;Dec. 2004 excluded;

Postseismic Change EstimatePostseismic Change Estimate Use entire 55 point time series;Use entire 55 point time series;

Examine pre-seismic and post-seismic periodsExamine pre-seismic and post-seismic periods

GRACE Mass Change GRACE Mass Change [(mean of 2005+2006) - (mean of 2003+2004)] [(mean of 2005+2006) - (mean of 2003+2004)]

cm of water equivalentcm of water equivalent4 Different Filtering Schemes4 Different Filtering Schemes

500 km Gaussian 300 km Gaussian

Decorrelation & 500 km Gaussian Decorrelation & 300 km Gaussian

Coseismic Deformation Estimate Coseismic Deformation Estimate (cm water equivalent mass change)(cm water equivalent mass change)

Epicenters of Sumatra-Andaman (Mw=9.3) and Epicenters of Sumatra-Andaman (Mw=9.3) and Nias (Mw=8.7) earthquakes marked by pink and white triangles.

Coseismic Deformation Estimates Coseismic Deformation Estimates (microgals left; cm water right)(microgals left; cm water right)

Estimate from RL04 – Estimate from RL04 – Sumatra-Andaman (Mw=9.3) and Sumatra-Andaman (Mw=9.3) and Nias (Mw=8.7)

epicenters marked by pink and white triangles.

Figure 2 from Han et al, Science, v 313, August 2006 – Gravity change from Sumatra-Andaman event using GRACE range and range-rate data. 1 microgal is about 2 cm of water equivalent mass

Coseismic Deformation in Time Series at Points A & BCoseismic Deformation in Time Series at Points A & B

Preseismic Mass Rate MapPreseismic Mass Rate Map

Postseismic Mass Rate MapPostseismic Mass Rate Map

Time Series Over Land at Points M & NTime Series Over Land at Points M & N

Mass Rate Time Series at Points A-FMass Rate Time Series at Points A-F

Postseismic Mass Rates at Points G-LPostseismic Mass Rates at Points G-L

ConclusionsConclusions

GRACE RL04 gravity data are of sufficient spatial resolution to GRACE RL04 gravity data are of sufficient spatial resolution to capture mass redistribution of Sumatra-Andaman earthquake.capture mass redistribution of Sumatra-Andaman earthquake.

After appropriate filtering, RL04 data clearly delineate effects of the After appropriate filtering, RL04 data clearly delineate effects of the rupture, extending over 1800 km, consistent with previous GRACE rupture, extending over 1800 km, consistent with previous GRACE estimates, deformation model estimates and other geodetic estimates, deformation model estimates and other geodetic measurements.measurements.

RL04 time series show steady mass increase, presumably related to RL04 time series show steady mass increase, presumably related to post-seismic uplift, post-seismic uplift, equivalent to ~ 8 cm/year of water mass in a broad region northwest of the epicenter centered on the Andaman-Sunda fault zone.

Time series of gravity change (mass rates) provide a new Time series of gravity change (mass rates) provide a new coseismic/postseismic measure of major earthquakes.coseismic/postseismic measure of major earthquakes.

Mass rate estimates (for tectonic, seismic and other non-climate Mass rate estimates (for tectonic, seismic and other non-climate studies) may be contaminated by interannual variability in regions studies) may be contaminated by interannual variability in regions with large seasonal signalswith large seasonal signals

Portions of this study have been published in

Chen, J.L., C.R. Wilson, B.D. Tapley, S. Grand, GRACE Detects Coseismic and Postseismic Deformation from the Sumatra-Andaman Earthquake, Geophys. Res. Lett., Vol. 34, No. 13, L13302 10.1029/2007GL030356, 2007.