Lg coda Q and its relation to the structure and evolution of continents: Aglobal perspective

Tomographic maps of Lg coda Q (Q(Lg)(c)) variation are now available for ne arly the entire African, Eurasian, South American, and Australian continent s, as well as for the United States. Q(Lg)(c) at I Hz ce,, varies from less than 200 to more than 1000 and Q(Lg)(c) frequency dependence (eta) varies between 0.0 and nearly 1.0. Q(0) appears to increase in proportion to the l ength of time that has elapsed since the most recent major episode of tecto nic or orogenic activity in any region. A plot of Q(0) versus time since th at activity indicates that a single Q(0)-time relation approximates most me an Q(0) values. Those that deviate most from the trend lay in Australia, th e Arabian Peninsula, and the East African rift. The increase in Q(0) with t ime may be due to a continual increase in crustal shear wave Q (Q(mu)) caus ed by the loss of crustal fluids and reduction of crustal permeability foll owing tectonic or orogenic activity. Extrapolated values of Q(Lg)(c) at 5 H z (using Q(0) and eta values measured at 1 Hz and assuming that eta is cons tant in all regions between 1 and 5 Hz) show a similar percentage-wise incr ease with times that has elapsed since the most recent activity. Other fact ors that can reduce Q in continental regions include thick accumulations of sediment (especially sandstone and shale of Mesozoic age and younger), sev ere velocity gradients at the crust-mantle transition and, possibly, latera l variations in the depth, thickness, and severity of those gradients. Seve re and large increases of Q(mu) in the mid-crust of some regions can cause relatively large values of eta, even if the frequency dependence of Q(mu) i s small.