THE CAUSE OF FREQUENCY-DEPENDENT SEISMIC ABSORPTION IN CRUSTAL ROCK

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 approximate to f(0.43+/-0.1), so that energy absorption per unit frequency Q(-1) decreases with frequency as approximate to f(-0.57), and the seismic quality factor Q increases with frequency as f(0.57); the value of Q at 10 Hz is approximate to 1000. Crustal fracture density, which elastic scattering and borehole logs show to vary with fracture dimension L as a power law L(0.4), cannot easily explain the observed frequency dependence of absorption. A likely hypothesis identifies the f(0.43+/-0.1) frequency dependence of seismic absorption with the f(1/2) 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 approximate to 0.2 mm the computed magnitude of absorption agrees with the observed Q approximate to 1000 at 10 Hz. Thin sections from deep borehole rock samples show that approximate to 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.