SEISMIC Q AND EVOLUTION OF THE CONTINENTAL CRUST

Tomographic maps of Lg coda Q variation are now available for nearly the entire continents of Africa, Eurasia, South America, and Australia, as well as for the United States. The Q variations, at a frequency of 1 Hz (Qo), vary between less than 200 and 1000 or more, with highest values occurring in stable shields and lowest values occurring in regions of current or recent collisional tectonic activity. A plot of Qo versus time since the most recent major episode of tectonic or orogenic activity in any region indicates that a single Qo-time relation approximates most Qo values so far determined. Regions that have undergone upper mantle heating (e.g. the Arabian Peninsula and a portion of eastern Siberia) in the past exhibit somewhat higher Qo values than regions that underwent collisional tectonics at about the same time.

Depth-distributions of shear-wave Q as a function of depth, obtained from the inversion of intermediate-period surface waves, are consistent with measured Lg coda Q variation wherever both have been determined. Regional variations in Qo and shear-wave Q are both consistent with a scenario in which hydrothermal fluids are generated above subducting slabs or other upper mantle heat sources. During, and shortly following tectonic or orogenic activity, much fluid is present and seismic waves must expend energy to move that fluid through permable crust. Fluids are gradually lost, either by flow to the surface or by retrograde metamorphism, causing Q to increase gradually with time.