Jon Kirby

Anomalously large gradients in Version 1 of the “GEODATA 9 SECOND” Digital Elevation Model of Australia, and their effects on gravimetric terrain corrections

Large gradients, when calculated by a first-difference method, have been detected in Version 1 of the 9 arc-second National Digital Elevation Model (DEMv1) of Australia released by the Australian Surveying and Land Information Group. Gradient values implied by the mean elevations in the DEMv1 between adjacent grid cells of up to 74° have been observed, most notably in Australias more mountainous regions in the east. Comparisons with topographic maps indicate that these are anomalous gradients in the DEMv1 that are not present in the mapped topography. It is recommended that the first-difference method is used to test DEMs before they are used to compute terrain corrections.

A Comparison of Techniques for the Integration of Satellite Altimeter and Surface Gravity Data for Geoid Determination

Two methods are tested whereby satellite altimeter measurements of the geoid height are combined with surface measurements of the free-air gravity anomaly. The study area comprises the oceans around the Australian continent. The first method involves draping a grid of the free-air anomaly from satellite data onto a grid of the ship and land data. The second method utilises grids of the altimeter-derived geoid height, combining these with the surface data in an iterative superposition. Preliminary results show that the draping method yields a fit of 5.4 mgal between the satellite and marine data, while the iterative procedure returns 8.1 mgal. Further work can be done, however, to improve these results. The impact of the combined marine gravity datasets is illustrated by comparing the effects on an Australia-wide spherical-FFT geoid solution.

The long-wavelength admittance and effective elastic thickness of the Canadian Shield

The strength of the cratonic lithosphere has been controversial. On the one hand, many estimates of effective elastic thickness (Te) greatly exceed the crustal thickness, but on the other the great majority of cratonic earthquakes occur in the upper crust. This implies that the seismogenic thickness of cratons is much smaller than Te, whereas in the ocean basins they are approximately the same, leading to suspicions about the large Te estimates. One region where such estimates have been questioned is the Canadian Shield, where glacial isostatic adjustment (GIA) and mantle convection are thought to contribute to the long-wavelength undulations of the topography and gravity. To date these have not been included in models used to estimate Te from topography and gravity which conventionally are based only on loading and flexure. Here we devise a theoretical expression for the free-air (gravity/topography) admittance that includes the effects of GIA and convection as well as flexure and use it to estimate Te over the Canadian Shield. We use wavelet transforms for estimating the observed admittances, after showing that multitaper estimates, which have hitherto been popular for Te studies, have poor resolution at the long wavelengths where GIA and convection predominate, compared to wavelets. Our results suggest that Te over most of the shield exceeds 80 km, with a higher-Te core near the southwest shore of Hudson Bay. This means that the lack of mantle earthquakes in this craton is simply due to its high strength compared to the applied stresses.

Correction to “An accuracy assessment of the fan wavelet coherence method for elastic thickness estimation”

Kirby, J. F., and C. J. Swain (2008), An accuracy assessment of the fan wavelet coherence method for elastic thickness estimation, Geochem. Geophys. Geosyst., 9, Q03022, doi:10.1029/ 2007GC001773. Simons, F. J. (2002), Structure and evolution of the Australian continent: Insights from seismic and mechanical heterogeneity and anisotropy, Ph.D. thesis, 261 pp., Mass. Inst. of Technol., Boston, Mass. Simons, F. J., and R. D. van der Hilst (2003), Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere, Earth Planet. Sci. Lett., 211, 271–286. G Geochemistry Geophysics Geosystems