GRACE Science Team Meeting 2008

B.1 - Solid Earth Sciences

Part - 1, Friday, 14:30 - 15:20

(Recent variation in the Earth Dynamic Oblateness, J2, from SLR and GRACE data)
Minkang Cheng

(Modeling Earth Deformation from Monsoonal Flooding in Bangladesh using Hydrographic, GPS and GRACE Data)
Michael Steckler

(Global Simultaneous Estimation of Present-Day Surface Mass Trend and GIA from Geodetic Data Combination)
Xiaoping Wu

(Global Glacial Isostasy and late Holocene Ice Mass Balance: the GRACE Contribution)
Erik Ivins

THEME: Deformations, Friday, 15:40 - 17:10

(A Hydrological Modeling Primer)
Matthew Rodell

(Using GRACE for land uplift investigations - significance, problems and validation of results)
Holger Steffen

(Constraints on GIA estimates from geodetic data assimilation)
M. E. Tamisiea

(Improved GIA estimates from GRACE and InSAR)
Isabella Velicogna

Part - 3, Friday, 17:10 - 18:00

(Insights into the Sumatra December 2004 and March 2005 post-seismic signals from GRACE gravity variations)
Isabelle Panet

(Co-seismic and Post-seismic Gravity Changes caused by the 2004 Sumatra- Andaman earthquake ñ comparison of GRACE data with SNREI Model)
Hasegawa Takashi

(Implications of postseismic gravity change following the great 2004 Sumatra-Andaman earthquake from the regional harmonic analysis of GRACE inter-satellite tracking data)
Shin-Chan Han

(GRACE-observed gravity changes in areas of large earthquakes)
Virendra Tiwari

Title: Recent variation in the Earth Dynamic Oblateness, J2, from SLR and GRACE data
Session: B.1 - Solid Earth Sciences
First Author: Minkang Cheng
Presenter: Minkang Cheng
Co-Authors: Byron D. Tapley

Abstract: Satellite Laser Ranging (SLR) data tracked by the ILRS network have recorded the global nature of long-wavelength mass redistribution occurring within the Earth system for more than three decades. Studying the variations in J2 has provided a clear vision of the large-scale mass redistribution with a long-term signature within the Earth system from analysis of SLR data. Early analysis of 28-year time series of monthly SLR estimates of J2 [Cheng and Tapley, 2004] has indicated that in addition to the secular, 18.6 year tidal and seasonal variations, the J2 has undergone significant interannual variations with time scales of ~4-6 years related to the strong El Nino-Southern Oscillation (ENSO) events during the periods of 1987-1991 and 1996-2002. Recent analysis of 33-year SLR data spanning from 1976 to October 2008 suggests that those ENSO related variations superimposed on a significant fluctuation with a period of ~9.4 years, 2 cycles have occurred since later 1987. It appears that a new cycle may have started from ~2006. The secular variation of J2 as determined from the linear trend of J2 variation is significant affected by the recent decadal variation. This paper presents detail analysis for the variations in J2 from analysis of multiple geodetic satellites over the period from 1976 to 2008, and a comparison with the monthly solutions from GRACE measurements.

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Title: Modeling Earth Deformation from Monsoonal Flooding in Bangladesh using Hydrographic, GPS and GRACE Data
Session: B.1 - Solid Earth Sciences
First Author: Michael Steckler
Presenter: Michael Steckler
Co-Authors: S.L. Nooner, S.H. Akhter, S. Chowdhury, S. Bettadpur

Abstract: The Ganges, Brahmaputra and Meghna Rivers, which combine in Bangladesh, together have a mean annual discharge second only to the Amazon. The vast majority of the flow occurs during the summer monsoon. The immense monsoon river discharge and rainfall causes widespread flooding in the Ganges-Brahmaputra Delta (GBD). In an average year, 1/3 of Bangladesh is submerged; it can reach 2/3 during an extreme flood. The mass of this impounded water represents a large load on the surface of the earth that can be readily observed in the GRACE gravity field and by GPS geodesy. It is the second largest seasonal anomaly in the GRACE gravity field, reflecting terrestrial water storage in SE Asia over the GBD. The 18 continuous GPS stations we have installed Bangladesh record a vertical signal of up to 6 cm that are inversely correlated to river level and discharge, in addition to rapid subsidence of up to 1.5 cm/yr from tectonics and sediment loading. To calculate the water load throughout Bangladesh, use >300 river gages of the Bangladesh Water Development Board (BWDB). In addition, the GRACE gravity field provides estimates of the integrated water storage. We model the water loading as elastic deformation at this timescale using both the analytic code of Becker and Bevis (2004) and the finite-element code PyLith. We vary elastic properties beneath the flooded region to fit the observed GPS motions. We will present results of the modeling including estimates of the elastic properties of the earth beneath the GBD.

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Title: Global Simultaneous Estimation of Present-Day Surface Mass Trend and GIA from Geodetic Data Combination
Session: B.1 - Solid Earth Sciences

First Author: Xiaoping Wu

Presenter: Xiaoping Wu

Co-Authors: Michael Heflin, Hugo Schotman, and Bert Vermeersen

Abstract: GRACE gravity and surface geodetic data contain both present-day surface mass trend and Glacial Isostatic Adjustment (GIA) signatures. Separating and resolving them over satisfactory spatiotemporal scales is both a challenge and an opportunity for modern global geodesy to learn about climate change and its history. We take a kinematic approach to this problem by simultaneously solving for 3 complete sets (from n=1 to 60) of global spherical harmonic coefficients of the present-day surface mass trend, vertical and horizontal GIA induced surface velocity fields. The estimation is done through a least squares procedure with reduced a priori information, using a combination of GRACE geoid trend, 3-dimensional velocities measured at 216 SLR/VLBI/GPS sites, and the data-assimilated JPL ECCO ocean model. Rotation vectors of 15 major tectonic plates are also estimated. The ICE-5G/IJ2005 (VM2) predictions are used as a priori GIA mean model. Their differences from those of ICE-3G (with a simpler mantle rheological profile) are used to construct a priori GIA uncertainties. No significant a priori information is used for present-day surface mass trend, or for plate motions. Estimated present-day mass trend and GIA will be presented globally. Preliminary results show that significant present-day trend and rather large deviations from the a priori GIA model are detected. Geocenter velocity due to present-day mass trend is very well resolved. This solution does not depend critically on direct geodetic determination of geocenter velocity, which is currently considered to be not reliable due to ITRF origin instability. However, such direct determination when improved in the future, can significantly contribute to the estimation of GIA induced geocenter velocity.

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Title: Global Glacial Isostasy and late Holocene Ice Mass Balance: the GRACE Contribution
Session: B.1 - Solid Earth Sciences
First Author: Erik Ivins
Presenter: Erik Ivins
Co-Authors: Xiaoping Wu, Thomas S. James

Abstract: A strong glacial isostatic adjustment (GIA) signal has been identified in GRACE time-series. Although discrepant interpretations and estimated error budgets remain in debate, there is now emerging a consensus regarding the unique contribution that this landmark satellite mission has brought to GIA investigations. This presentation reviews some of these discoveries, now that the consensus is taking real shape, and discusses the large number of error sources that could lead to false, or questionable, interpretations. Possible discoveries are: (i) A mantle rebound signature from a multi-domed Laurentide ice sheet, similar to the ice reconstruction prescribed in the moraine-based model of Dyke (2004) and co-workers. (ii) When highly Gauss-filtered, the GRACE-derived secular change in the gravity field is fairly consistent with predictions by global models ICE-5G: VM-X (U. Toronto) and ANU-series (Australian National University) for both the Fennoscandian and Laurentide ice sheets. (iii) Consistency with current model scenarios for other rebounding regions, such as in Greenland, Antarctica and northwestern Russia; (iv) evidence that the Kara-Barents Sea rebound is smaller than predicted by typical models, implying either thinner regional ice sheet or a smaller regional upper mantle viscosity, or a combination of both. Problems that remain are numerous, and near the top of the list are recently recognized, and yet uncalibrated, trends in hydrological models. Uncalibrated sampling errors in the global solutions of non-geophysical origin are also a significant concern. An additional question is the accuracy of mass-bearing ocean models, and even of ocean tide models, in shallow marginal seas where the subtle GIA peripheral lithospheric flexure and mantle deformation have a long-wavelength, but much smaller gravitational signal, than near the rebounding centers. Future GOCE mapping of the peripheral static field might assist in discerning subtle secular changes in these peripheral regions. A clean extraction of GIA signal from Antarctica has yet to be realized, but the increasing accuracy of flux and altimetry-based estimates of the cryospheric balance, if coincident in spatial and temporal observation with GRACE observations, offer the potential for a breakthrough in this area of GIA studies.

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Title: A Hydrological Modeling Primer
Session: B.1 - Solid Earth Sciences
First Author: Matthew Rodell
Presenter: Matthew Rodell
Co-Authors: H.K. Beaudoing

Abstract: Hydrological models have been used for comparison, validation, and even generation of certain GRACE products. I first will describe the basic aspects of hydrological models and different types of models. Then I will discuss their deficiencies, particularly those that would affect studies which rely upon model output, and how recent innovations are addressing those deficiencies.

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Title: Using GRACE for land uplift investigations - significance, problems and validation of results
Session: B.1 - Solid Earth Sciences
First Author: Holger Steffen
Presenter: Holger Steffen
Co-Authors: J. Müller; H. Denker

Abstract: The Earth's gravity field observed by GRACE is provided by several institutions in form of monthly, 10-days or weekly solutions, which differ slightly due to the application of different reduction models and center-specific processing schemes. We use such different GRACE data to investigate the long-term mass variations in Fennoscandia and North America. These areas are strongly influenced by glacial isostatic adjustment (GIA) resulting in a long-term trend of about 1 µGal/yr. As the GRACE solutions always show the integral effect of all mass variations, various reduction models are used to separate single processes, like the GIA-related mass increase in Fennoscandia. On the one hand this requires, that the reduction models (e.g. from hydrology) have the same quality and resolution as the processes to be addressed. On the other hand, the restricted time span of the measurements may limit the separation of long-periodic and secular signals. We discuss different filters, time spans and reduction models for an adequate processing of the solutions and separation of the GIA signal. Furthermore, the resulting GRACE-based mass variations are compared to global hydrology models (WGHM, LaDWorld, GLDAS) to (a) separate possible hydrological signals and (b) briefly validate the hydrology models for long-term investigations.

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Title: Constraints on GIA estimates from geodetic data assimilation
Session: B.1 - Solid Earth Sciences
First Author: M. E. Tamisiea
Presenter: M. E. Tamisiea
Co-Authors: E. M. Hill; J. L. Davis

Abstract: With continued improvements in the spatial distribution of geodetic data in Polar Regions, there is the possibility of improving regional estimates of the ongoing effects of glacial isostatic adjustment (GIA). To explore the constraints that these data place on GIA estimates, we use an assimilation technique that allows us to incorporate these observations into an a priori GIA model without having to refer the variations back to specific changes in the ice sheet loading history or earth model parameterization. This technique allows us to self-consistently compare inferences from different data types, such as GPS-derived crustal velocities, tide-gauge-derived relative sea-level rates, and GRACE-derived gravity rates, while taking into account the inherent uncertainties within each set of observations, e.g., reference frame parameterization for GPS, or GRACE processing parameters. The method also allows us to identify sampling biases caused by the spatial distributions of the measurements. In this talk, we focus on the impact of including GRACE data on the GIA estimates over Canada and Scandinavia. In each case, including the GRACE data increases the inferred uplift over the region compared to the uplift estimated using only GPS or tide-gauge solutions. We have also expanded the technique to incorporate tide-gauge rates. This allows us to explore the robustness of an estimate of non-GIA related sea-level rise inferred from the technique for the Baltic Sea area. The inferred relative sea-level rate is particularly sensitive to the inclusion of GRACE-derived estimates.

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Title: Improved GIA estimates from GRACE and InSAR
Session: B.1 - Solid Earth Sciences
First Author: Isabella Velicogna
Presenter: Isabella Velicogna
Co-Authors: Erik Ivins, Eric Rignot

Abstract: Comparison of spatial pattern and amplitude of the Ice Mass changes estimated from GRACE with the one based on InSAR and regional climate modeling allows improved constrains on GIA and surface mass balance model output. The two estimates are completely independent and characterized by different error sources. While largest source of error for the GRACE balance estimates is the uncertainty in the glacial isostatic adjustment (GIA) signal, FOR the mass budget method a large source of uncertainty can be traced to the surface mass balance component from the regional climate modeling output. The comparison allows improved constrains on GIA and surface mass balance model output. We find that a thinner ice sheet at the Last Glacial Maximum may be required in the East Antarctic Ice sheet as well as in the Ross Sea region. In the Bellingshausen Sea area comparisons indicate that both a larger signal from the long-term accumulation and a larger GIA signal are likely. The adjusted forward model explanation for the Bellinghausen Sea time-dependent gravity is attributed to a combination of lower viscosity structure and geologically more recent ice mass loss.

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Title: Insights into the Sumatra December 2004 and March 2005 post-seismic signals from GRACE gravity variations
Session: B.1 - Solid Earth Sciences
First Author: Isabelle Panet
Presenter: Isabelle Panet
Co-Authors: Valentin Mikhailov; Fred Pollitz; Michel Diament

Abstract: From an analysis of the GRACE-derived decadal geoids computed by the CNES/GRGS team (Biancale et al., 2007), we investigate the post-seismic gravity variations associated with the Sumatra December 2004 and March 2005 earthquakes, over a period from early 2005 to the end of 2007. In order to extract the relevant geodynamic signal, we apply a continuous wavelet analysis to the GRACE-derived geoid models stacked over varying periods. We show a clear post-seismic relaxation signal over the area affected by the earthquakes. We interpret the observed gravity variations and discuss their geodynamic implications by comparing the observations with the signal predicted from a visco-elastic relaxation model.

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Title: Co-seismic and Post-seismic Gravity Changes caused by the 2004 Sumatra- Andaman earthquake ñ comparison of GRACE data with SNREI Model
Session: B.1 - Solid Earth Sciences
First Author: Hasegawa Takashi
Presenter: Hasegawa Takashi
Co-Authors: Fukuda Y., Sun W., Fu G., Okuno J. and Yamamoto K.

Abstract: We compared gravity changes associated with the 2004 Sumatra-Andaman earthquake observed by GRACE and predicted by the dislocation theory in spherically symmetric, non-rotating, perfect elastic and isotropic (SNREI) earth covered with ocean.

Our dislocation theory enables us to make an accurate interpretation of observed gravity signals because we took into account the curvature, stratification and ocean layer of the Earth (which all previous studies neglected). We found great differences between the observed and predicted co-seismic gravity changes, which mean GRACE data include significant post-seismic gravity changes.

We constructed afterslip model to interpret the observed gravity changes. Our afterslip model also showed good consistent with the post-seismic ground displacements observed by GPS, not only the gravity trends observed by GRACE. Our results demonstrated the impact of our dislocation theory to the large-scale gravity observation associated with the earthquake and new application of GRACE gravity data to the after slip observation.

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Title: Implications of postseismic gravity change following the great 2004 Sumatra-Andaman earthquake from the regional harmonic analysis of GRACE inter-satellite tracking data
Session: B.1 - Solid Earth Sciences
First Author: Shin-Chan Han
Presenter: Shin-Chan Han
Co-Authors: J. Sauber; S. Luthcke; C. Ji; F. Pollitz

Abstract: We report GRACE satellite observations of coseismic displacements and postseismic transients from the great Sumatra-Andaman Islands (thrust event; Mw ~9.2) earthquake in December 2004. Instead of using global spherical harmonic solutions of monthly gravity fields, we estimated the gravity changes directly using inter-satellite 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 two years of post-earthquake 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 5×1017 Pa s and steady-state viscosity of 5×1018 – 1019 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.

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Title: GRACE-observed gravity changes in areas of large earthquakes
Session: B.1 - Solid Earth Sciences
First Author: Virendra Tiwari
Presenter: Virendra Tiwari
Co-Authors: J.M. Wahr, R. Gross, S. Swenson

Abstract: Appreciable gravity changes were detected by GRACE during the great Sumatra earthquake of 2004. Subsequently, several studies have documented GRACE gravity changes during and after the Sumatra earthquake and have discussed the possibilities of detecting large earthquakes of M > 7.5. To investigate further, we have processed GRACE data over regions of large earthquakes M > 8, and of some earthquakes of M > 7 in continental regions, that occurred between 2003 and mid-2007. Although the observed changes do not show a unique spatial or temporal pattern, we have noticed offsets in the GRACE results at the times of most of these earthquakes. Also, time series from the regions surrounding some of the earthquakes, including the Sumatra earthquake, show distinct changes in the gravity field before the earthquake, in addition to changes during and after the earthquake. We investigate the question of whether these apparent changes in gravity that occur before large earthquakes could be produced by the tectonic process leading to the earthquake itself. If this were the case, the GRACE gravity signal in tectonically active regions could hold great promise for earthquake studies.

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