LgCoda Q in Australia and its Relation to Crustal Structure and Evolution

We have determined Lg Coda Q (Qc Lg) from ground motion recorded at seven broadband stations in Australia, using a stacked spectral ratio method. In spite of the relatively small number of events and less than optimum station coverage, we were able to use those data to obtain a tomographic map Qc Lg and its frequency dependence, at 1 Hz for almost the entire island continent. Qc Lg at 1 Hz in Australia varies between about 330 and 600. The lowest values (330–400) characterize the Tasman Fold Belt in eastern Australia; these may be associated with fluids produced by orogenic activity that occurred during the Devonian and Carboniferous periods or by sedimentation that occurred in Jurassic and Triassic times. Smaller reductions of Qc Lg, relative to maximum values, in central and western Australia, may be associated with sedimentation that occurred over a long-time interval between late Precambrian time and the Carboniferous period or with deformation that occurred in the Central Australian Mobile Belts during the Carboniferous. Qc Lg throughout most of Australia is 30 to 60% lower than it is in most other stable continental regions. Assuming that Qc Lg varies in the same proportion as QLg, ground motion computations for one-dimensional models of the Australian crust and upper mantle indicate that up to 20% reductions in Qc Lg can be produced if a velocity gradient, rather than a sharp boundary, resides at the crust-mantle transition. The remaining portion of the Qc Lg reduction may be caused by lateral variability in the thickness, depth, and severity of the velocity gradient which causes additional Lg energy to leak into the mantle.