Lg Coda Q and its Relation to the Structure and Evolution of Continents: A Global Perspective

Abstract

Tomographic maps of Lg coda Q (Q ^c_Lg ) variation are now available for nearly the entire African, Eurasian, South American, and Australian continents, as well as for the United States. Q ^c_Lg at 1 Hz (Q ₀) varies from less than 200 to more than 1000 and Q ^c_Lg frequency dependence (η) varies between 0.0 and nearly 1.0. Q ₀ appears to increase in proportion to the length of time that has elapsed since the most recent major episode of tectonic or orogenic activity in any region. A plot of Q ₀ versus time since that activity indicates that a single Q ₀-time relation approximates most mean Q ₀ values. Those that deviate most from the trend lay in Australia, the Arabian Peninsula, and the East African rift. The increase in Q ₀ with time may be due to a continual increase in crustal shear wave Q (Q _μ) caused by the loss of crustal fluids and reduction of crustal permeability following tectonic or orogenic activity. Extrapolated values of Q ^c_Lg at 5 Hz (using Q ₀ and η values measured at 1 Hz and assuming that η is constant in all regions between 1 and 5 Hz) show a similar percentage-wise increase with times that has elapsed since the most recent activity. Other factors that can reduce Q ₀ in continental regions include thick accumulations of sediment (especially sandstone and shale of Mesozoic age and younger), severe velocity gradients at the crust-mantle transition and, possibly, lateral variations in the depth, thickness, and severity of those gradients. Severe and large increases of Q _μ in the mid-crust of some regions can cause relatively large values of η, even if the frequency dependence of Q _μ is small.