Dispersive Elastic Moduli and Frequency-Dependent Attenuation due to Wave-Induced Fluid Flow in Metapelite

Abstract

Seismic waves are used to interpret geologic structure, composition, and environmental conditions in the Earth. However, rocks are not perfectly elastic and their viscoelasticity can dissipate energy during wave propagation. Wave-induced fluid flow mechanisms can cause viscoelasticity resulting in frequency-dependent attenuation, velocities, and elastic moduli (dispersion) in saturated rocks. Dispersion and attenuation are hypothesized to be important in subduction zones, where regions of high fluid content are interpreted below the seismogenic zone. However, this has not been well-tested because of a lack of measurements on relevant lithologies and under saturated conditions. We measured the Young's and shear moduli and the attenuation of a greenschist facies metapelite with the forced oscillation technique at frequencies between 2 x 10-5 and 30 Hz. The moduli and attenuation are frequency-dependent under saturated conditions and depend on the effective pressure. At relatively low effective pressure, the Young's and shear moduli increase by over 50 % between 2 x 10-5 and 30 Hz. We use Standard Linear Solid viscoelastic models to investigate the relationship between the attenuation and dispersion in the Orocopia schist. The models are consistent with the experimental data and demonstrate that viscoelasticity can cause significant dispersion and attenuation in subduction zones.