1. 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

2. Motivation:  Previous studies revealed co-seismic gravity changes due to Sumatra-Andaman earthquake (Mw=9.3) of December 26, 2004. [special processing using GRACE range rate data, Han et al. 2006].  Examine GRACE RL04 spherical harmonic products for evidence of co-seismic change.  If RL04 SH products show co-seismic change, then time series may be examined for pre- and post-seismic changes

3. GRACE Data and Processing  CSR GRACE RL04 solutions  April 2002 through February 2007;  55 monthly solutions;  To degree, order 60  Applied Filters  De-correlation filtering [Swenson and Wahr, 2006]  300 km Gaussian smoothing

4. Data Processing  Coseismic Change  Coseismic Change Estimate  Minimize seasonal and other climate effects by computing: [mean of (2005 + 2006)] minus [mean of (2003 + 2004)] ;  21 solutions in each mean;  (2003 + 2004): Jan. 2003 - Nov. 2004;  (2005 + 2006): Jan. 2005 - Sept. 2006;  Dec. 2004 excluded;  Postseismic Change Estimate  Use entire 55 point time series;  Examine pre-seismic and post-seismic periods

5. GRACE Mass Change [(mean of 2005+2006) - (mean of 2003+2004)] cm of water equivalent cm of water equivalent 4 Different Filtering Schemes 500 km Gaussian300 km Gaussian Decorrelation & 500 km GaussianDecorrelation & 300 km Gaussian

6. Coseismic Deformation Estimate (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.

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

8. Coseismic Deformation in Time Series at Points A & B

9. Preseismic Mass Rate Map

10. Postseismic Mass Rate Map

11. Time Series Over Land at Points M & N

12. Mass Rate Time Series at Points A-F

13. Postseismic Mass Rates at Points G-L

14. Conclusions  GRACE RL04 gravity data are of sufficient spatial resolution to capture mass redistribution of Sumatra-Andaman earthquake.  After appropriate filtering, RL04 data clearly delineate effects of the rupture, extending over 1800 km, consistent with previous GRACE estimates, deformation model estimates and other geodetic measurements.  RL04 time series show steady mass increase, presumably related to post-seismic uplift,  RL04 time series show steady mass increase, presumably related to 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 coseismic/postseismic measure of major earthquakes.  Mass rate estimates (for tectonic, seismic and other non-climate studies) may be contaminated by interannual variability in regions with large seasonal signals

15. 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.