Plate tectonic theory offers little direct insight into large continental intraplate earthquakes beyond that they may be consequences of slow deformation within plates, and hence relatively rare. To address the nature of these events, space geodesy is used to quantify deformation, and assess possible causes.
GPS data show that plate interiors are deforming very slowly in comparison with their boundaries, with one important exception. For eastern North America, by far the strongest signal is vertical motion due to ice mass unloading following the last glacial period; its study provides powerful new constraints on the history of the ice loads and the rheology of the Earth’s crust and upper mantle.
Space geodetic observations of crustal motion have been playing an increasingly important role in the study of Glacial Isostatic Adjustment (GIA). These observations can be obtained in any land area and record the horizontal as well as the vertical signals associated with GIA. GPS velocity fields which are now used to constrain GIA model parameters with, in turn, will result in a better understanding of global sea-level change. Global warming has led to a great deal of interest in the GIA fields of Antarctica and Greenland, both of which are still poorly constrained by observations.
While variations of glacial loads associated with GIA trigger the long-term (thousands of years) viscoelastic response of Earth’s mantle, shorter period (dominantly annual) cycles dues to environmental loading processes including hydrology and atmospheric pressure are now widely recognized in geodetic time series and are the subject of much research. At the decadal time scale, GPS is now being used to “weigh” modern changes in ice loss by measuring the elastic response of Earth based on geodetic time series. has led to a strong need to quantify the contemporary rate of ice loss, and to detect any accelerations in this mass loss. However, we still have very little idea as to the total loss of ice mass of these ice sheets. The Gravity Recovery and Climate Experiment (GRACE) satellite mission is providing key data, but cannot distinguish between ice mass changes and GIA effects, which are of similar magnitude. The need to improve our knowledge of the GIA field in Antarctica and Greenland is one of the major motivations of the international Polar Earth Observing Network (POLENET) project, in which UNAVCO community is playing a leading role.
Figure 1 - Left: Vertical GPS site motions with respect to IGb00. Note large uplift rates around Hudson Bay, and subsidence to the south. Green line shows interpolated 0 mm/yr vertical "hinge line" separating uplift from subsidence. Right: Horizontal motion site residuals after subtracting best fit rigid plate rotation model defined by sites shown with black arrows. Red vectors represent sites primarily affected by GIA. Purple vectors represent sites that include effects of tectonics. Giovanni F. Sella, Seth Stein, Timothy H. Dixon, Michael Craymer, Thomas S. James,Stephane Mazzotti, and Roy K. Dokka, 2007, Observation of glacial isostatic adjustment in "stable" North America with GPS, Geophysical Research Letters, Vol. 34, L02306, doi:10.1029/2006GL027081. More information is available here.
Figure 2 - Rate of mass change over Antarctica, as inferred from GRACE Release 4 fields from CSR (U of Texas) during Jan, 2003-Sept, 2006. The top panel shows the raw GRACE results. The bottom panel shows the same data after accounting for GIA effects (using a model based on ICE-5G). The significant differences with the top panel illustrate the importance of using an accurate GIA correction when estimating ice mass changes from GRACE data (courtesy of J. Wahr and I. Velicogna).
Last modified Friday, 28-Jan-2011 21:10:24 UTC