The cause of frequency‐dependent seismic absorption in crustal rock

Abstract

The absorption of 5 to 200 Hz seismic wave energy propagating at depth in fracture‐bearing crystalline basement in southern California is observed to increase as a power law in wave frequency as ?f0.43±0.1, so that energy absorption per unit frequency Q−1 decreases with frequency as ?f−0.57, and the seismic quality factor Q increases with frequency as f0.57; the value of Q at 10 Hz is ?1000. Crustal fracture density, which elastic scattering and borehole logs show to vary with fracture dimension L as a power law L0.4, cannot easily explain the observed frequency dependence of absorption. A likely hypothesis identifies the f0.43±0.1 frequency dependence of seismic absorption with the f1/12 frequency dependence of a thermal diffusion process. Seismic absorption computed for thermal gradients induced by localized strain concentrations depends on a single free parameter, the thermal absorption domain characteristic size D. If D? 0.2 mm the computed magnitude of absorption agrees with the observed Q? 1000 at 10 Hz. Thin sections from deep borehole rock samples show that ?0.2 mm is the characteristic dimension of microfracture shear‐fabric and crystalline grains. Where seismic elastic scattering occurs over a wide range of fracture sizes, seismic absorption occurs predominantly at the dimension of microfractures and granularity. Systematic wide‐band borehole seismic observation of seismic absorption magnitude and particularly frequency dependence, combined with thin‐section analysis of the rock microfabric, can test the hypothesis for a range of crustal rocks. The evidence of thermal activity at the scale of microfractures suggests that static stress concentrations possess a free energy that can act to align microfractures, thus explaining the shear‐wave birefringence widely observed in the crust. Copyright © 1995, Wiley Blackwell. All rights reserved