Over the past several years, students and I developed methods to measure surface-wave attenuation using both station pairs and single stations. It began by using fundamental-mode surface waves at periods between several seconds and 50-100 seconds, but soon also included higher-mode surface waves. The inclusion of higher-mode data allowed us to observe a variation of crustal shear-wave Q with frequency. We soon found that the severity of this frequency dependence varied from region to region and that it is most often significantly greater in high-Q regions.

Early inversions of the attenuation data led to models in which shear-wave Q in the upper crust is much lower than in the lower crust, with a rapid increase occurring at depths corresponding to the brittle-ductile transition. Later inversions have revealed that lower crustal Q is as low, or nearly as low, in the lower crust as it is in the upper crust in regions of plate convergence. Upper crustal shear-wave Q varies systematically, with the value of Q being directly proportional to the length of time that has elapsed since the most recent episode of tectonic or orogenic activity in a region.

Mitchell, B.J., Regional Rayleigh wave attenuation in North America, J. Geophys. Res., 35, 4904-4916, 1975.

Mitchell, B.J., N.K. Yacoub, and A.M. Correig, A summary of seismic surface wave attenuation and its regional variation across continents and oceans, Am. Geophys. Union Mono. 20, The Earth's Crust, its Nature and Physical Properties, edited by J.G. Heacock, 405-425, 1977.

Canas, J., and B.J. Mitchell, Lateral variation of surface wave anelastic attenuation across the Pacific, Bull., Seism. Soc. Am., 68, 1637-1650, 1978.

Mitchell, B.J., Frequency dependence of shear wave internal friction in the continental crust of eastern North America, J. Geophys. Res., 85, 5212-5218, 1980.

Cheng, C.C., and B.J. Mitchell, Crustal Q structure in the United States from multi-mode surface waves, Bull. Seism. Soc. Am., 71, 161-181, 1981.

Canas, J., and B.J. Mitchell, Rayleigh wave attenuation and its variation across the Atlantic Ocean, Geophys. J. Roy. Astr. Soc., 67, 159-176, 1981.

Mitchell, B.J., Regional variation and frequency dependence of Qb in the crust of the United States, Bull. Seism. Soc. Am., 71, 1531-1538, 1981.

Kijko, A., and B.J. Mitchell, Multimode Rayleigh wave attenuation and Qb in the crust of tghe Barents shelf, J. Geophys. Res., 88, 3315-3328, 1983.

Hwang, H.J., and B.J. Mitchell, Shear velocities, Qb, and the frequency dependence of Qb in stable and tectonically active regions from surface wave observations, Geophys. J. Roy. Astr. Soc., 90, 575-613, 1987.

Cong, L., and B.J. Mitchell, Frequency dependence of crustal Qb in stable and tectonically active regions, Pure and Appl. Geophys., 127, 581-605, 1988.