We have developed a mathematical formulation of two mechanisms of
compressional wave attenuation, which can occur within the solid rock
frame prestressed up to its yield stress in part of its volume. Energy
losses are attributed to two distinct processes: irreversible plastic
yielding and formation of radial microfractures around microscopic
cavities. Small-amplitude waves propagating through the rocks
prestressed at their yield point would cause nonelastic strain to avoid
building local stresses above the yield limit and attenuate some
fraction of their energy per every loading cycle. New mechanisms of
microscale yielding and microfracturing give rise to
frequency-independent attenuation due to rate-independence of plasticity
formulation. Quality factor Q predicted by the model is independent of
strain amplitude for small strains and decreases with increasing
amplitude for large strains. We found that attenuation can be high even
for small seismic strains (10(-9)-10(-5)). Thus, Q = 12...20 is achieved
at effective pressures greater than twice the yield strength of the
solid matrix for the plastic yielding mechanism and at overpressures
exceeding half tensile strength for microfracturing.