Anelastic structure and evolution of the continental crust and upper mantle from seismic surface wave attenuation

Abstract

Regional variations of the intrinsic shear wave quality factor Qµ in both the upper crust and upper mantle of continents are large, with values in old, stable cratons exceeding those in tectonically active regions in both depth ranges by as much as an order of magnitude or more. Qµ depends upon frequency, at least near 1 Hz, and that frequency dependence also varies regionally in the upper crust. It is typically low in tectonically active regions and higher in stable regions. Because of the large variations in Qµ from region to region, it is easy to map regional variations of both upper crustal Qµ and Q estimated from the coda of Lg waves (QLgc), even though both measurements may be marked by large uncertainties. Although coda Q of direct body waves may be strongly affected by scattering, QLgc appears to be primarily governed by intrinsic Qµ in the upper crust. Both upper crustal Qµ and QLgc values correlate with the time that has elapsed since the most recent tectonic activity in continental regions. A tomographic image of the variation of QLgc values across Africa shows reduced Q values which correspond to recent tectonic activity in the East African rift system and other regions of Mesozoic or younger age. Reductions of QLgc that correlate with tectonic activity that occurred in the early Paleozoic during the coalescence of the cratons which formed that continent can also be detected. Qµ increases rapidly at midcrustal depths, in a range which appears to coincide with the transition to the plastic lower crust. In the lower crust and upper mantle, Qµ decreases with increasing depth, possibly by progressive unpinning of dislocations with increasing temperature. Observed regional variations in upper mantle Qµ at depths of about 150 km can be explained by differences in temperature alone, but those at crustal depths cannot. Regional variations of Qµ in the upper crust are most easily explained by differences in the density of fluid‐filled fractures in which fluids can move during the propagation of seismic waves. Studies of the regional variation of Qµ and QLgc indicate that crack density is greatest during and immediately following tectonic activity in a region and that it decreases with time. Permeability determinations in deep wells show that fluid movements in those cracks may be largely restricted to zones of crustal fracturing. That situation will 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 retrograde metamorphism, causing a reduction in the number of open cracks and a concomitant increase in Qµ.