We report the detection of an earthquake by a space-based measurement. The Gravity Recovery and Climate Experiment (GRACE) satellites observed a +/-15-microgalileo gravity change induced by the great December 2004 Sumatra-Andaman earthquake. Coseismic deformation produces sudden changes in the gravity field by vertical displacement of Earth's layered density structure and by changing the densities of the crust and mantle. GRACE's sensitivity to the long spatial wavelength of gravity changes resulted in roughly equal contributions of vertical displacement and dilatation effects in the gravity measurements. The GRACE observations provide evidence of crustal dilatation resulting from an undersea earthquake.

[1] We quantify gravity changes after great earthquakes present within the 10 year long time series of monthly Gravity Recovery and Climate Experiment (GRACE) gravity fields. Using spherical harmonic normal-mode formulation, the respective source parameters of moment tensor and double-couple were estimated. For the 2004 Sumatra-Andaman earthquake, the gravity data indicate a composite moment of 1.2 Â 10 23 N m with a dip of 10 , in agreement with the estimate obtained at ultralong seismic periods. For the 2010 Maule earthquake, the GRACE solutions range from 2.0 to 2.7 Â 10 22 N m for dips of 12 -24 and centroid depths within the lower crust. For the 2011 Tohoku-Oki earthquake, the estimated scalar moments range from 4.1 to 6.1 Â 10 22 N m, with dips of 9 -19 and centroid depths within the lower crust. For the 2012 Indian Ocean strike-slip earthquakes, the gravity data delineate a composite moment of 1.9 Â 10 22 N m regardless of the centroid depth, comparing favorably with the total moment of the main ruptures and aftershocks. The smallest event we successfully analyzed with GRACE was the 2007 Bengkulu earthquake with M 0~5 .0 Â 10 21 N m. We found that the gravity data constrain the focal mechanism with the centroid only within the upper and lower crustal layers for thrust events. Deeper sources (i.e., in the upper mantle) could not reproduce the gravity observation as the larger rigidity and bulk modulus at mantle depths inhibit the interior from changing its volume, thus reducing the negative gravity component. Focal mechanisms and seismic moments obtained in this study represent the behavior of the sources on temporal and spatial scales exceeding the seismic and geodetic spectrum.Citation: Han, S.-C., R. Riva, J. Sauber, and E. Okal (2013), Source parameter inversion for recent great earthquakes from a decade-long observation of global gravity fields,

S U M M A R YMonthly mass variations within the Earth system produce temporal gravity changes, which are observable by the NASA/GFZ Gravity Recovery and Climate Experiment (GRACE) twinsatellite system. Mass load changes with spatial scales larger than 1000 km have been observed using conventional filters based on a Gaussian smoother, which applies a weight to GRACE spherical harmonic (SH) coefficients depending only on SH degree. This practice is consistent with a degree-dependent error model for GRACE monthly geopotential solutions. The Gaussian filters effectively dampen all power of ill-determined higher-degree components in the estimates. However, the spatial sampling provided by GRACE yields errors that vary with both SH degree and order. The consequence is that maps of spatial loads shall not be smoothed with an isotropic (degree-only) filter, but shall be constructed using anisotropic smoothing thus also yielding better spatial resolution in latitude. We have developed a non-isotropic filter to optimize the smoothing of GRACE temporal gravity observations by considering the degreeand order-dependent quality of GRACE estimates, the latter analysed from the correlation with the predicted signals of hydrologic and ocean models. In order to retain GRACE coefficients in the filtering process that show reasonable correlation with the geophysical (hydrology and ocean) models, we applied Gaussian-type smoothing but with averaging radius depending on the order of the geopotential coefficient estimates. Applied to 2 yr of GRACE data, we showed that the resulting non-isotropic filter yields enhanced GRACE signals with significantly higher resolution in latitude and the same resolution in longitude without reducing the accuracy as compared to the conventional Gaussian smoother.Climate-related mass redistribution on the Earth has been observed in the time-varying gravity components derived from the NASA/GFZ Gravity Recovery and Climate Experiment (GRACE) satellite mission at monthly temporal scales and spatial scales of 1000 km or greater (Tapley et al. 2004a;Wahr et al. 2004). The GRACE twin co-orbiting satellites were launched in 2002 and are in near-circular orbits of 89 • mean inclination and 500 km mean altitude. Monthly time-series of geopotential spherical harmonic (SH) coefficients constitute the GRACE Level-2 (L2) science data product (Tapley et al. 2004b). To obtain reasonable estimates of time-varying gravity signals and related surface load changes due to air and water, L2 coefficients should be filtered because high spatial-frequency (high SH degree) components are poorly determined. The current approach uses degree-dependent filters such as Gaussian filters (Tapley et al. 2004a;Wahr et al. 2004). These produce spatially isotropic smoothing of surface load maps as a function of time. The spatial radius of the smoothing filter is empirically selected by considering the magnitude of the a priori temporal gravity signal (e.g. from hydrological or ocean circulation models) and the GRACE error power spectrum. H...

[1] We explore the mechanisms whereby groundwater influences terrestrial water storage (TWS) in the Amazon using GRACE observations and two contrasting versions of the LEAF-Hydro-Flood hydrological model: one with and the other without an interactive groundwater. We find that, first, where the water table is shallow as in northwestern Amazonia and floodplains elsewhere, subsurface stores (vadose zone and groundwater) are nearly saturated year-round, hence river and flooding dominate TWS variation; where the water table is deep as in southeastern Amazonia, the large subsurface storage capacity holds the infiltrated water longer before releasing it to streams, hence the subsurface storage dominates TWS variation. Second, over the whole Amazon, the subsurface water contribution far exceeds surface water contribution to total TWS variations. Based on LEAF-Hydro-Flood simulations, 71% of TWS change is from subsurface water, 24% from flood water, and 5% from water in river channels. Third, the subsurface store includes two competing terms, soil water in the vadose zone and groundwater below the water table. As the water table rises, the length of vadose zone is shortened and hence the change in groundwater store is accompanied by an opposite change in soil water store resulting in their opposite phase and contributions to total TWS. We conclude that the inclusion of a prognostic groundwater store and its interactions with the vadose zone, rivers, and floodplains in hydrological simulations enhances seasonal amplitudes and delays seasonal peaks of TWS anomaly, leading to an improved agreement with GRACE observations.

This article provides a survey on modern methods of regional gravity field modeling on the sphere. Starting with the classical theory of spherical harmonics, we outline the transition towards space-localizing methods such as spherical splines and wavelets. Special emphasis is given to the relations among these methods, which all involve radial base functions. Moreover, we provide extensive applications of these methods and numerical results from real space-borne data of recent satellite gravity missions, namely the Challenging Minisatellite Payload (CHAMP) and the Gravity Recovery and Climate Experiment (GRACE). We also derive high-resolution gravity field models by effectively combining space-borne and surface measurements using a new weighted level-combination concept. In addition, we outline and apply a strategy for constructing spatiotemporal fields from regional data sets spanning different observation periods.

[1] The Gravity Recovery and Climate Experiment (GRACE) satellite mission will provide new measurements of Earth's static and time-variable gravity fields with monthly resolution. The temporal effects due to ocean tides and atmospheric mass redistribution are assumed known and could be removed using current models. In this study we quantify the aliasing effects on monthly mean GRACE gravity estimates due to errors in models for ocean tides and atmosphere and due to ground surface water mass variation. Our results are based on simulations of GRACE recovery of monthly gravity solution complete to degree and order 120 in the presence of the respective model errors and temporal aliasing effects. For ocean tides we find that a model error in S 2 causes errors 3 times larger than the measurement noise at n < 15 in the monthly gravity solution. Errors in K 1 , O 1 , and M 2 can be reduced to below the measurement noise level by monthly averaging. For the atmosphere, model errors alias the solution at the measurement noise level. The errors corrupt recovered coefficients and introduce 30% more error in the global monthly geoid estimates up to maximum degree 120. Assuming daily CDAS-1 data for continental surface water mass redistribution, the analysis indicates that the daily soil moisture and snow depth variations with respect to their monthly mean produce a systematic error as large as the measurement noise over the continental regions.

[1] We report Gravity Recovery and Climate Experiment (GRACE) satellite observations of coseismic displacements and postseismic transients from the great Sumatra-Andaman Islands (thrust event; M w $9.2) earthquake in December 2004. Instead of using global spherical harmonic solutions of monthly gravity fields, we estimated the gravity changes directly using intersatellite range-rate data with regionally concentrated spherical Slepian basis functions every 15-day interval. We found significant step-like (coseismic) and exponential-like (postseismic) behavior in the time series of estimated coefficients (from May 2003 to April 2007) for the spherical Slepian functions. After deriving coseismic slip estimates from seismic and geodetic data that spanned different time intervals, we estimated and evaluated postseismic relaxation mechanisms with alternate asthenosphere viscosity models. The large spatial coverage and uniform accuracy of our GRACE solution enabled us to clearly delineate a postseismic transient signal in the first 2 years of postearthquake GRACE data. Our preferred interpretation of the long-wavelength components of the postseismic gravity change is biviscous viscoelastic flow. We estimated a transient viscosity of 5 Â 10 17 Pa s and a steady state viscosity of 5 Â 10 18 -10 19 Pa s. Additional years of the GRACE observations should provide improved steady state viscosity estimates. In contrast to our interpretation of coseismic gravity change, the prominent postearthquake positive gravity change around the Nicobar Islands is accounted for by seafloor uplift with less postseismic perturbation in intrinsic density in the region surrounding the earthquake.

The 2011 great Tohoku‐Oki earthquake, apart from shaking the ground, perturbed the motions of satellites orbiting some hundreds km away above the ground, such as GRACE, due to coseismic change in the gravity field. Significant changes in inter‐satellite distance were observed after the earthquake. These unconventional satellite measurements were inverted to examine the earthquake source processes from a radically different perspective that complements the analyses of seismic and geodetic ground recordings. We found the ‘average’ slip located up‐dip of the hypocenter but within the lower crust, as characterized by a limited range of bulk and shear moduli. The GRACE data constrained a group of earthquake source parameters that yield increasing dip (7–16° ± 2°) and, simultaneously, decreasing moment magnitude (9.17–9.02 ± 0.04) with increasing source depth (15–24 km). The GRACE solution includes the cumulative moment released over a month and demonstrates a unique view of the long‐wavelength gravimetric response to all mass redistribution processes associated with the dynamic rupture and short‐term postseismic mechanisms to improve our understanding of the physics of megathrusts.