GRACE Science Team Meeting

B.2 - Solid Earth
(Convener: TBD)

Assessment and Review of GIA Models for Antarctica and Greenland

Modeling Earth Deformation from Monsoonal Flooding in Bangladesh using Hydrographic, GPS and GRACE Data
(Steckler, Nooner, Akhter, Choudhury, Bettadpur, Seeber)

Inversion of satellite orbital motion changes after the great 2011 Tohoku-Oki (Japan) earthquake to constrain the earthquake's magnitude, depth, and focal mechanism
(Han, Sauber, Riva, Okal)

Maule MW 8.8 Co-seismic Signature in GRACE Range Data: What do we need for better science?
(Tanaka, Ivins, Watkins, Byun, Yuan, Klemann)

Session: B.2.a - Theme: Understanding GIA
Title: Assessment and Review of GIA Models for Antarctica and Greenland
First Author: Erik Ivins
Presenter: Erik Ivins

Abstract: One of the major obstacles to reducing the uncertainty of GRACE-based ice mass balance estimates for the ice sheets during 2002-2011 is our poor control on the ongoing glacial isostatic adjustment of bedrock (GIA). The later adjustments cause vertical motions of rock at the crustal surface and at great depths within the mantle. These are sources of positive mass trend when measured in space gravimtery data. The poorly understood signal in Antarctica may be large enough to manifest uncertainties that approach 190 Gt/yr (Velicogna and Wahr, 2006), dominating the background error in trend for Antarctica, and potentially corrupting solutions for mass balance for ice drainage basins in the north and central parts of Greenland. This source of error is independent of method, and corrupts the interpretable trend in both spherical harmonic field or mascon releases.

Two of the most recent GRACE mass balance assessments for GIA in Antarctica employ combinations of ICE5G and IJ05 models, both of which are now more than half a decade old. In part, because of the urgency to report to the IPCC on the mass balance of Antarctica with greater certainty, GIA modeling has been the focus of intense research in the past 6-7 years. Significant progress lies in three areas of research: i.) Constraint on paleo-ice sheet reconstruction coming from dated sedimentary coring (ëbathtub ringsí) and moraine and nunatuk rock nuclide exposures (ëdip sticks of the pastí). This data is now rich enough that, in fact, for some areas of Antarctica we now know much more about ice mass evolution since Last Glacial Maximum (21 thousand years ago), that we do for the great Laurentide ice sheet of North America; ii.) Integration of simple ice dynamics models that are specifically constrained by these data (Whitehouse et al., 2011, ISAES XI Edinburgh); iii.) A more robust GPS data set for vertical motion trends of longer legacy (almost two decades in some cases) involving 40 individual station records on bedrock (Thomas et al., 2011, ISAES XI Edinburgh). One upshot of these more recent Antarctic studies is that they reveal relatively older exposure and sediment dates. A great deal of ice (roughly half) had been lost to the ocean by 11.5 ka, (relatively more than in the IJ05 and ICE5G models). Furthermore, more precise lower elevation limits for ice fee nunataks and mountain outcrops throughout Last Glacial Maximum (e.g., Bentley et al., 2011) are retrieved. These reveal that the ice sheet was likely to be considerably less thick than in the ice domes of ICE5G in west Antarctica, and less thick in the western Weddell Sea, southernmost Antarctic Peninsula and coastal east Antarctica than in the IJ05 model. These two new features must be integrated into new predictions of the GRACE GIA correction for ice mass balance. The most immediate implications are that the past corrections for Antarctica are s imply too large and that GRACE-based inference of mass loss are too large by substantial amounts. ICE5G corrections are likely too large by more than 50-60% and IJ05 too large by 20%, or more. New models for both Greenland and Antarctica are discussed.

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Session: B.2.b - Solid Earth
Title: Modeling Earth Deformation from Monsoonal Flooding in Bangladesh using Hydrographic, GPS and GRACE Data
First Author: Michael Steckler
Presenter: Michael Steckler
Co-Authors: S.L. Nooner, S.H. Akhter, S. Chowdhury, S. Bettadpur, L. Seeber

Abstract: The Ganges, Brahmaputra and Meghna Rivers converge in Bangladesh with annual discharge second only to the Amazon. Most of the flow occurs during the summer monsoon causing widespread flooding. The impounded water represents a large surface load that is the second largest seasonal anomaly in the GRACE gravity field. Surface water monitoring and GRACE show that ~100GT of water are stored in Bangladesh (7.5% of annual discharge), but can reach 150GT during extreme events. Continuous GPS stations in Bangladesh record seasonal vertical motions up to 6 cm due to the water load. We have used GRACE water mass estimates and surface water monitoring to calculate the seasonal load together with GPS observations of seasonal deformation due to this load in order to invert for lithospheric properties.

Here, we present an improved estimate for the calculation of Young's modulus beneath Bangladesh. To estimate the water load in Bangladesh, we use >300 daily river gage measurements of water level and >1200 weekly groundwater level measurements from wells. The time series has now been extended through 2010. The total impounded water mass is partitioned between surface water and groundwater by using the full resolution SRTM DEM. The seasonal water loads calculated from the surface data and from GRACE are in excellent agreement. These water loads cause elastic deformation with a large lateral extent. This means that the deformation in Bangladesh is affected, not only by flooding and ground water loads in Bangladesh, but also by water loads beyond Bangladesh and our surface water data. To model these loads we have defined irregular blocks that represent the major areas of flooding and groundwater storage in the surrounding region (West Bengal, Bihar, Assam, Myanmar). We project GRACE solutions to estimate the water mass stored in these areas and then make preliminary calculations of their contribution to the deformation at our GPS sites. By extending our time series to 2010, we are now able to incorporate all 18 of our installed GPS stations to provide an improved areal coverage of the surface deformation. The net result of these improvements is a significant decrease in the estimated value for Young's modulus (E) and the strength of the lithosphere beneath the Ganges-Brahmaputra Delta (GBD).

Having completed these improvements to the calculations, we have begun modeling the depth-dependence of E and thus the lithospheric structure beneath the Ganges-Brahmaputra Delta. We will present initial calculations using a simple 3-layer parameterization of E for the sediments, crust and mantle. The main goals are to resolve the sediment thickness and Moho depth.

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Session: B.2.b - Solid Earth
Title: Inversion of satellite orbital motion changes after the great 2011 Tohoku-Oki (Japan) earthquake to constrain the earthquake's magnitude, depth, and focal mechanism
First Author: Shin-Chan Han
Presenter: Shin-Chan Han
Co-Authors: J. Sauber, R. Riva, E.A. Okal

Abstract: The great Tohoku-Oki earthquake on 11th March 2011 shook not only the Honshu island in Japan creating devastating tsunami waves, but it also perturbed the motions of the satellites above the island. Earthquakes redistribute mass within the Earth, change the gravitational field, and thus modify the satellite orbits. We looked into changes in inter-satellite distance between two GRACE low Earth orbiters (at altitude 480 km) one month before and after the earthquake. Such orbital perturbation data (aka L1B data) could be directly used to estimate the magnitude and focal mechanism of the earthquake, just like a centroid moment tensor analysis of long-period seismic waves. Here we report on a novel approach to invert the GRACE L1B data to estimate earthquake source parameters. Due to the sensitivity of the gravimetric (orbital) data to surficial displacement as well as internal Earth deformation, we were able to delineate a group of earthquake moment tensor solutions that yield increasing dipping angles (7 - 16deg +/- 2deg) and, simultaneously, decreasing moment magnitudes (Mw 9.17 - 9.02 +/- 0.04) with increasing source depths from 15 to 24 km within the lower crust. Our 'slip' centroid moment tensor solution indicates the averaged earthquake moment over approximately a month and includes postseismic effects of afterslip and viscoelastic deformation that may be as large as 10% of coseimic moment release. The GRACE data give a unique view of this great earthquake because it includes the long-wavelength gravimetric response to all mass change processes associated with the dynamic rupture plus the short-term pre- and post-seismic mechanisms.

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Session: B.2.b - Solid Earth
Title: Maule MW 8.8 Co-seismic Signature in GRACE Range Data: What do we need for better science?
First Author: Yoshi Tanaka
Presenter: Erik Ivins
Co-Authors: Erik R. Ivins, Michael M. Watkins, Sung Byun, Dah-Ning Yuan, Volker Klemann

Abstract: Shortly after the Feb. 27, 2010 Maule, Chile subduction zone mega-thrust earthquake of energy scale magnitude 8.8, we began to use a data stacking method to examine the effects of the change in gravity field on the GRACE A-B intersatellite range accelerations. The initial discovery of the robust influence of the in co-seismic + afterslip deformational changes could be seen very clearly in the monthly JPL global mascon solution. Detection was, quite independently, reported by Han, Sauber and Luthcke. They also employed the raw ranging data (GRL, Dec., 9, 2010). The detection is important to advancing concepts for both the science and technical capabilities for potential earthquake studies in future space gravimetry experiments. The slip distribution and surface vertical displacements for the Maule quake are fairly well-constrained using terrestrial GPS, InSAR and broadband seismic wave analyses. Pollitz et al. (GRL, May 6, 2011) used a spherical self-gravitating layered earth model to provide a comprehensive simulation of the co-seismic deformation character of the event, noting that the slip distribution was roughly 8 meters over an area of about 1.2 x 105 km2. Much of the slip is projected to areas in the crust/lithosphere that are submarine. GPS recorded as much as 14.2 cm of negative vertical motion at the northern coastline of the rupture field (Delouis et al., GRL, Sept, 10, 2010), while other measurements suggest positive motions on off shore island which emerged by as much as 240 cm. Here we reexamine some of the detection and resolution issues of the gravitational signature, modeling both solid earth and ocean responses and sea-level equation using a layered spherical self-gravitating model as described by Tanaka et al. (2009). Such research helps determine the necessary spatio-temporal density of space and terrestrial gravity observations that are required for improving our understanding of the deformation in the off-shore subduction-zone environment where GPS and ! InSAR ob servations cannot be made.

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