Separation of coseismic and postseismic gravity changes for the 2004 Sumatra–Andaman earthquake from 4.6 yr of GRACE observations and modelling of the coseismic change by normal-modes summation

eodSpatiale (GRGS). For comparison, the Release-04 solutions of the Center for Space Research (CSR) are also inves- tigated after a spectral windowing or a Gaussian spatial smoothing. Results are shown both in terms of geoid height changes and gravity variations. Coseismic and postseismic gravitational changes estimated from the different gravity solutions are globally similar, although their spa- tial extent and amplitude depend on the type of filter used in the processing of GRACE fields. The highest signal-to-noise ratio is found with the GRGS solutions. The postseismic signature has a spectral content closer to the GRACE bandwidth than the coseismic signature and is therefore better detected by GRACE. The coseismic signature consists mainly of a strong gravity decrease east of the Sunda trench, in the Andaman Sea. A gravity increase is also detected at a smaller scale, west of the trench. The model for the coseismic gravity changes agrees well with the coseismic signature estimated from GRACE, regarding the overall shape and orientation, location with respect to the trench and order of magnitude. Coseismic gravity changes are followed by a postseismic relaxation that are well fitted by an increasing expo- nential function with a mean relaxation time of 0.7 yr. The total postseismic gravity change consists of a large-scale positive anomaly centred above the trench and extending over 15 ◦ of latitude along the subduction. After 26 months, the coseismic gravity decrease has been partly compensated by the postseismic relaxation, but a negative anomaly still remains south of Phuket. A dominant gravity increase extends over 15 ◦ of latitude west of the trench, being maximal south of the epicentre area. By investigating analyses of two global hydrology models and one ocean general circulation model, we show that our GRACE estimates of the coseismic and postseismic gravitational changes are almost not biased by interannual variations originat- ing from continental hydrology and ocean circulation in the subduction area and in the central part of the Andaman Sea, while they are biased by several μGal in the Malay Peninsula.

[1]  Wenke Sun,et al.  Coseismic deformations detectable by satellite gravity missions: A case study of Alaska (1964, 2002) and Hokkaido (2003) earthquakes in the spectral domain , 2004 .

[2]  F. Pollitz,et al.  Post-seismic relaxation following the great 2004 Sumatra-Andaman earthquake on a compressible self-gravitating Earth , 2006 .

[3]  Scott B. Luthcke,et al.  FAST TRACK PAPER: Tide model errors and GRACE gravimetry: towards a more realistic assessment , 2006 .

[4]  Olafur Gudmundsson,et al.  A regionalized upper mantle (RUM) seismic model , 1998 .

[5]  Byron D. Tapley,et al.  GRACE detects coseismic and postseismic deformation from the Sumatra‐Andaman earthquake , 2007 .

[6]  Victor Zlotnicki,et al.  Time‐variable gravity from GRACE: First results , 2004 .

[7]  H. Kayanne,et al.  Coseismic and postseismic creep in the Andaman Islands associated with the 2004 Sumatra‐Andaman earthquake , 2007 .

[8]  Jean-Charles Marty,et al.  Temporal gravity field models inferred from GRACE data , 2007 .

[9]  D. L. Anderson,et al.  Preliminary reference earth model , 1981 .

[10]  K. Heki,et al.  Slow postseismic recovery of geoid depression formed by the 2004 Sumatra‐Andaman Earthquake by mantle water diffusion , 2007 .

[11]  Albert Tarantola,et al.  Inverse problem theory - and methods for model parameter estimation , 2004 .

[12]  Chalermchon Satirapod,et al.  Insight into the 2004 Sumatra–Andaman earthquake from GPS measurements in southeast Asia , 2005, Nature.

[13]  Isabelle Panet,et al.  Can tectonic processes be recovered from new gravity satellite data , 2004 .

[14]  Matthias Holschneider,et al.  Coseismic and post-seismic signatures of the Sumatra 2004 December and 2005 March earthquakes in GRACE satellite gravity , 2007 .

[15]  B. Chao,et al.  The rotational and gravitational signature of the December 26, 2004 Sumatran earthquake , 2006 .

[16]  O. Francis,et al.  Modelling the global ocean tides: modern insights from FES2004 , 2006 .

[17]  Jeffrey P. Walker,et al.  THE GLOBAL LAND DATA ASSIMILATION SYSTEM , 2004 .

[18]  Wenke Sun,et al.  Truncated co-seismic geoid and gravity changes in the domain of spherical harmonic degree , 2004 .

[19]  L. Montési.  Controls of shear zone rheology and tectonic loading on postseismic creep , 2004 .

[20]  S. Swenson,et al.  Accuracy of GRACE mass estimates , 2006 .

[21]  U. Meyer,et al.  An Earth gravity field model complete to degree and order 150 from GRACE: EIGEN-GRACE02S , 2005 .

[22]  Petra Döll,et al.  GRACE observations of changes in continental water storage , 2006 .

[23]  Frederik J. Simons,et al.  Spatiospectral localization of global geopotential fields from the Gravity Recovery and Climate Experiment (GRACE) reveals the coseismic gravity change owing to the 2004 Sumatra‐Andaman earthquake , 2008 .

[24]  Pedro Viterbo,et al.  An Improved Land Surface Parameterization Scheme in the ECMWF Model and Its Validation. , 1995 .

[25]  Wenke Sun,et al.  Surface potential and gravity changes due to internal dislocations in a spherical earth—I. Theory for a point dislocation , 1993 .

[26]  Chen Ji,et al.  Crustal Dilatation Observed by GRACE After the 2004 Sumatra-Andaman Earthquake , 2006, Science.

[27]  Kenneth W. Hudnut,et al.  Poroelastic rebound along the Landers 1992 earthquake surface rupture , 1998 .

[28]  M. Cheng,et al.  Variations in the Earth's oblateness during the past 28 years , 2004 .

[29]  Chen Ji,et al.  Rupture Process of the 2004 Sumatra-Andaman Earthquake , 2005, Science.

[30]  V. Gahalaut,et al.  Constraints on 2004 Sumatra Andaman earthquake rupture from GPS measurements in Andaman Nicobar Islands , 2006 .

[31]  Emile A. Okal,et al.  Seismology: Speed and size of the Sumatra earthquake , 2005, Nature.

[32]  R. Bilham,et al.  Postseismic deformation of the Andaman Islands following the 26 December, 2004 Great Sumatra–Andaman earthquake , 2007 .

[33]  Carl Wunsch,et al.  State estimation improves prospects for ocean research , 2002 .

[34]  M. Watkins,et al.  GRACE Measurements of Mass Variability in the Earth System , 2004, Science.

[35]  Frédéric Frappart,et al.  WATER VOLUME CHANGE IN THE LOWER MEKONG FROM SATELLITE ALTIMETRY AND IMAGERY DATA , 2006 .

[36]  J. Freymueller,et al.  Spatial variations in present‐day deformation, Kenai Peninsula, Alaska, and their implications , 2000 .

[37]  Chen Ji,et al.  Coseismic Slip and Afterslip of the Great Mw 9.15 Sumatra–Andaman Earthquake of 2004 , 2007 .

[38]  M. Saito.  Excitation of free oscillations and surface waves by a point source in a vertically heterogeneous Earth , 1967 .

[39]  F. Gilbert.  Excitation of the Normal Modes of the Earth by Earthquake Sources , 1971 .

[40]  J. Okuno,et al.  A new method for the computation of global viscoelastic post-seismic deformation in a realistic earth model (I)—vertical displacement and gravity variation , 2006 .