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B.2.1 to B.2.5 - Solid Earth
(Convener: )
GIA effects on GRACE and GPS observations caused by ice loss in southern Greenland since the Little Ice Age
(A. Geruo, J. Wahr, S. A. Khan, K. H. Kjaer, K. K. Kjeldsen)
Evaluating glacial isostatic adjustment corrections using GRACE, altimetry, and regional atmospheric climate model outputs
(T. Sutterley, I. Velicogna)
Patagonian and Antarctic Peninsula mass balance over that past 150 years and constraints from GRACE data 2003-2013
(E. Ivins, D. Weiss, M. Watkins, D. Yuan, F. Landerer, C. Boening)
Topographic effect of crustal layers on coseismic gravity changes: a case study of the 2011 Tohoku-Oki earthquake
(J. Li, J.L. Chen)
Gravity changes associated with recent great earthquakes from a decade-long observation of GRACE gravity fields
(S-H. Han, R. Riva, J. Sauber, E. Okal, F.Pollitz)
Title: GIA effects on GRACE and GPS observations caused by ice loss in southern Greenland since the Little Ice Age
Presenter: A, Geruo
Co-Authors: J. Wahr; S. A. Khan; K. H. Kjaer; K. K. Kjeldsen
Abstract: The on-going deformation of the solid Earth in southern Greenland is determined by the elastic response to present-day ice mass changes and the continuing viscous relaxation of the Earth's mantle in response to past ice mass changes. To estimate the present-day ice mass variability, the viscous response of the Earth has to be computed using glacial isostatic adjustment (GIA) model, and removed from GRACE mass change observations, and from GPS surface deformation observations. The accuracy of the GIA model mainly depends on the uncertainties in the ice loading history and in the Earth's mantle viscosity structure. In this study, mass balance estimates of the southern Greenland ice sheet (sGrIS) since the Little Ice Age (LIA) have been obtained using high quality aerial stereo photogrammetric imagery, combined with contemporary ice surface differences derived using NASA's laser altimeter measurements. Linear trends for the mass loss of sGrIS are derived for three time intervals, 1900 - 1981, 1981 - 2002, and 2002 - 2010, and are used to build a post-LIA ice loading history. We compute the GIA effects on GRACE and GPS observables using the post-LIA loading history along with different mantle viscosity structures, and we find that (1) the GIA effect on GRACE present-day mass loss estimates is small; and (2) the GIA effect on GPS present-day surface displacement measurements can be significant, if the upper mantle viscosity of the Earth is small.
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Title: Evaluating glacial isostatic adjustment corrections using GRACE, altimetry, and regional atmospheric climate model outputs
Presenter: Sutterley, Tyler
Co-Authors: T. Sutterley; I. Velicogna
Abstract: We evaluate Greenland and Antarctic GIA corrections by comparing the spatial patterns of GRACE-derived ice mass trends corrected for glacial isostatic adjustment with volume changes from ICESat (Ice, Cloud, and Land Elevation Satellite), OIB (Operation IceBridge) and ENVISAT altimetry missions, and surface mass balance (SMB) products from the Regional Atmospheric Climate Model (RACMO). We show that using the spatial and temporal characteristics of the different contributions to the ice mass balance estimates that it is possible to evaluate different GIA corrections. The GRACE ice mass changes in Greenland obtained using the Simpson et al. (2009) and A et al. (2013) GIA corrections show good agreement in their spatial patterns and amplitude. The GRACE estimate corrected using the Wu et al. (2010) GIA shows similar spatial patterns to the other two, but produces an average ice mass loss for the entire ice sheet that is approximately 70 Gt/yr smaller. We analyze regional estimates of ice mass balance calculated with each correction, and check for consistency between the different techniques. With the Simpson and A corrections, we find a strong correlation between the total mass balance from GRACE and the surface mass balance from RACMO in our regional estimates. In contrast, we do not find this relationship with the Wu correction the Northeast of Greenland. In Antarctica, the total magnitude and spatial structure of the GRACE-estimated ice mass change is highly dependent on the GIA correction. In key basins of East Antarctica, the interpretation of regional ice mass changes can reflect the GIA model selection, as the ice mass to GIA signal ratio is smaller. We apply the same methodology used for the Greenland ice sheet in Antarctica to evaluate the different GIA corrections and check for consistency between the different techniques at a regional scale.
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Title: Patagonian and Antarctic Peninsula mass balance over that past 150 years and constraints from GRACE data 2003-2013
Presenter: Ivins, Erik
Co-Authors: D. Weiss; M. Watkins; D. Yuan; F. Landerer; C. Boening
Abstract: Since the late 1980's many of the outlet glaciers of the Antarctic Peninsula (AP) have experienced the loss of ice shelf buttressing forces that retard the outlet flow velocity. The Patagonian Ice Fields (PIF) hav experienced an increase in loss rate over the past 70 years. The loss of these buttressing forces in the AP has caused outlet glaciers to speed up. At the same time, it is clear from ice core records and in situ measurements that the AP snow accumulation has been increasing. For the AP both the increased precipitation and the catastrophic break-up of ice shelves are related to well-documented ocean and atmospheric warming. The 100-year-long warming trends occur at some of the largest rates any where on Earth. While there is growing confidence that all space borne data are consistent with a net mass loss of the Antarctic Peninsula since 1990, there is relatively little convergence on the total mass loss, and its temporal variability, other than that measured by the Gravity Recovery and Climate Experiment (GRACE).
Employing JPL mascon analyses, Ivins, Watkins, Yuan, Dietrich, Casassa and Rülke, (2011, JGR-B) employed 6.25 years of GRACE data, glacial isostatic adjustment (GIA) modeling with GPS data, to determine the mass trend of Graham Land (north of 67° S) at -32 ± 6 Gt/yr and -9.5 ± 3 Gt/yr for the remainder of the AP (74° S - 67° S). Neither region exhibited any significant non-secular signatures during 2003-2009.25. However, some outlet glaciers have shown both height and velocity changes over the period 2002-2012 that indicate increasing rates of loss - via laser altimetry and InSAR measurements (Anya Wendt, personal communication, 2012). In a combined examination of AP mass balance (with a slightly larger area for AP definition), recent work extended to the end of 2012, indicates that the region of the Antarctic Peninsula has a loss rate of about -38.5 ± 13 Gt/yr, with distinct and significant speed-up after 2007. Mass loss of the collective PIF cryospheric hydrological system has a subsbantial slowdown in 2010, but the average rate of loss over the nearly 11 year GRACE record is consitently at about 27 ± 7 Gt/yr. Here we reexamine Release 05 GRACE data to better define the region experiencing this speed-up and possibly its origin. Improvements to the GIA estimations are also discussed in light of more reliable GPS uplift measurements that are now available.
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Title: Topographic effect of crustal layers on coseismic gravity changes: a case study of the 2011 Tohoku-Oki earthquake
Presenter: Li, Jin
Co-Authors: J. Li; J.L. Chen
Abstract: Due to the topography of Earth's surface as well as its underlying crustal interfaces, horizontal coseismic deformations of large earthquakes would cause mass redistribution and corresponding gravitational change. In this study, we estimate the topographic effect of crustal layers on coseismic gravity changes of the 2011 Tohoku-Oki earthquake. We propose an approach to calculate equivalent vertical mass changes caused by the horizontal coseismic displacements based on a crust model. We predict coseismic deformations of the 2011 Tohoku-Oki earthquake on the Earth's surface and inner (crustal) interfaces, with a half-space layered dislocation model. The topography of the crustal layers in the earthquake region is obtained from the ETOPO1 and CRUST1.0 models. The results indicate that coseismic effect of the topography is not negligible, and is at the same order of magnitude as the seawater correction, which has been extensively investigated in series of recent studies. We also compare the coseismic gravity changes from model prediction with observations by GRACE satellite gravimetry, and find that topographic effect on coseismic gravity change is observable according to the uncertainty level of GRACE, and might be partly responsible for the evident discrepancies between model predictions and GRACE observations.
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Title: Gravity changes associated with recent great earthquakes from a decade-long observation of GRACE gravity fields
Presenter: Han, Shin-Chan
Co-Authors: R. Riva, J. Sauber, E. Okal, F.Pollitz
Abstract: We quantify gravity changes after great earthquakes present within the 11-year-long time-series of monthly global GRACE gravity fields. Using the normal-mode formulation, we present our estimates of the source parameters of moment tensor and double-couple for the events of the 2004 Sumatra-Andaman, 2007 Bengkulu, 2010 Maule, 2011 Tohoku-Oki, 2012 Indian Ocean strike-slip earthquakes. For the 2012 Indian Ocean earthquake (the first strike-slip event detected by GRACE), the GRACE gravity data delineate a composite moment of 1.9×1022 N-m regardless of centroid depth, comparing favorably with the total seismic moment of the main ruptures and aftershocks. The smallest event we successfully analyzed with GRACE was the 2007 Bengkulu earthquake with M0 ~ 5.0×1021 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. In addition, the large-scale postseismic gravity changes following the 2004 Sumatra-Andaman, 2010 Maule, and 2011 Tohoku-Oki earthquakes were evident in the GRACE time-series of the moment tensor components. Our preferred interpretation of the long-wavelength postseismic gravity change is biviscous viscoelastic flow. We present our estimates of the Earth viscoelastic structures by delineating a range of transient and steady-state viscosities. Finally, we discuss how these solutions could be used to correct the GRACE observations for the studies on climate change.
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