Seismic Attenuation: Scaling from the Lab to the Earth

In a recent study, attenuation and modulus dispersion were measured experimentally using a novel material that has a melting temperature near room temperature. This organic crystalline proved to be a very good analogue to mantle rock. The relatively low melting temperature was easy to work with and allowed a much broader range of frequencies to be explored than in previous studies. When normalized by the Maxwell frequency, all data from the study (and previous studies) collapse to a single master curve (below), regardless of grain size or temperature or even gross material differences, proving the universality of the anelastic response of quite possibly all crystalline materials. It was found that anelasticity is a unique function of normalized frequency fn=f/fM, where fn is a function of frequency, grain size, temperature, and even melt fraction.

These new findings, while consistent with previous experimental observations, represent a significant change in the way that experimental data should be scaled to be applied to seismic observations. In the rock mechanics lab we will be working directly with in-house seismologists to begin to directly apply this new scaling of experimental data to seismological observations. With this project we hope to use seismic data to locate regions of high temperature and melting within the earth.

Relevent publications:

McCarthy, Takei and Hiraga (2011) Experimental study of attenuation and disperson over a broad frequency range: 2. The universal scaling of polycrystalline materials, JGR 116 B09207  PDF

McCarthy and Takei (2011) Anelasticity and viscosity of partially molten rock analogue: Toward seismic detection of small quantities of melt, GRL 38 L18306 PDF

Measured attenuation versus normalized frequency from various studies with gross material differences [from McCarthy et al., 2011]