Wave velocity and attenuation in rocks undergoing polymorphic transformations

Abstract

The effect on seismic transmission of phase transitions in which the transformation of one solid to another involves a change in density is analyzed theoretically. Stress exerted by the seismic wave is assumed to transform a minute amount of material at a rate proportional to the local stress relative to the transformation value. The analysis shows that such transitions, even though the amount of material affected is very small, can have an appreciable effect on velocities and attenuation if the regions transforming are in the shape of thin planar areas. The effect is negligible for regions with more equant dimensions. The hydrostatic component of the response is found to be that of a standard linear solid; the response in shear is more complicated but can be approximated as a standard linear solid also. The characteristic frequency of the transition from nickel olivine to nickel spinel is found to be 0.01 Hz at 1000°C and 0.001 Hz at 900°C. The ratio of compressional wave velocity to shear wave velocity at frequencies when this mechanism is operative is lower than normal, whereas the ratio is higher than normal when grain boundary sliding occurs.