ANELASTIC STRUCTURE AND EVOLUTION OF THE CONTINENTAL-CRUST AND UPPER-MANTLE FROM SEISMIC SURFACE-WAVE ATTENUATION

Regional variations of the intrinsic shear wave quality factor Q(mu) i n both the upper crust and upper mantle of continents are large, with values in old, stable cratons exceeding those in tectonically active r egions in both depth ranges by as much as an order of magnitude or mor e. Q(mu) depends upon frequency, at least near 1 Hz, and that frequenc y dependence also varies regionally in the upper crust. It is typicall y low in tectonically active regions and higher in stable regions. Bec ause of the large variations in Q(mu) from region to region, it is eas y to map regional variations of both upper crustal Q(mu) and Q estimat ed from the coda of Lg waves (Q(Lg)(c)) even though both measurements may be marked by large uncertainties. Although coda Q of direct body w aves may be strongly affected by scattering, Q(Lg)(c), appears to be p rimarily governed by intrinsic Q(mu) in the upper crust. Both upper cr ustal Q(mu) and Q(Lg)(c) values correlate with the time that has elaps ed since the most recent tectonic activity in continental regions. A t omographic image of the variation of Q(Lg)(c) values across Africa sho ws reduced Q values which correspond to recent tectonic activity in th e East African rift system and other regions of Mesozoic or younger ag e. Reductions of Q(Lg)(c) that correlate with tectonic activity that o ccurred in the early Paleozoic during the coalescence of the cratons w hich formed that continent can also be detected. Q(mu) increases rapid ly at midcrustal depths, in a range which appears to coincide with the transition to the plastic lower crust. In the lower crust and upper m antle, Q(mu) decreases with increasing depth, possibly by progressive unpinning of dislocations with increasing temperature. Observed region al variations in upper mantle Q(mu) at depths of about 150 km can be e xplained by differences in temperature alone, but those at crustal dep ths cannot. Regional variations of Q(mu) in the upper crust are most e asily explained by differences in the density of fluid-filled fracture s in which fluids can move during the propagation of seismic waves. St udies of the regional variation of Q(mu) and Q(Lg)(c) indicate that cr ack density is greatest during and immediately following tectonic acti vity in a region and that it decreases with time. Permeability determi nations in deep wells show that fluid movements in those cracks may be largely restricted to zones of crustal fracturing. That situation wil l produce widely differing values of Q in local studies, depending on the location of the study relative to the fractures. The fluid volume in cracks appears to decrease with time by loss to the surface or by r etrograde metamorphism, causing a reduction in the number of open crac ks and a concomitant increase in Q(mu).