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

Tomographic maps of Lg coda Q (Qc Lg) variation are now available for nearly the entire African, Eurasian, South American, and Australian continents, as well as for the United States. Qc Lg at 1 Hz (Q0) varies from less than 200 to more than 1000 and Qc Lg frequency dependence (h) varies between 0.0 and nearly 1.0. Q0 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 Q0 versus time since that activity indicates that a single Q0-time relation approximates most mean Q0 values. Those that deviate most from the trend lay in Australia, the Arabian Peninsula, and the East African rift. The increase in Q0 with time may be due to a continual increase in crustal shear wave Q (Qm) caused by the loss of crustal fluids and reduction of crustal permeability following tectonic or orogenic activity. Extrapolated values of Qc Lg at 5 Hz (using Q0 and h values measured at 1 Hz and assuming that h 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 Q0 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 Qm in the mid-crust of some regions can cause relatively large values of h, even if the frequency dependence of Qm is small.