The Cause of Frequency-Dependent Seismic Absorption in Crustal Rock

Peter Leary

Published July 1995, SCEC Contribution #217

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.

Leary, P. (1995). The Cause of Frequency-Dependent Seismic Absorption in Crustal Rock. Geophysical Journal International, 122(1), 143-151. doi: 10.1111/j.1365-246X.1995.tb03542.x.