- © The Mineralogical Society Of America
Seismic imaging as a tool to understand the structure and, perhaps, the dynamics of the planet at depth involves the spatially resolved, combined study of wave velocities and velocity dispersion, wave birefringence and wave attenuation (mechanical absorption). When integrated with insights from petrology, plus-or-minus input from magneto-tellurics, specific hypotheses concerning structure at depth can be formulated (cf. Karato 1993; Karato and Karki 2001). The recent advances in tomography have allowed significant improvements in spatial resolution which also have allowed ever more exacting hypotheses of structure to be articulated. Nevertheless, as is nicely illustrated by the relatively recent seismic analyses of an accreting plate margin (the East Pacific Rise —e.g., Toomey et al. 1998; Webb and Forsyth 1998), the structures inferred at depth can vary significantly: clearly, the interpretation of seismic data is limited specifically by a lack of understanding of the physical processes by which low-frequency wave absorption occurs, particularly in the cases (a) where melt is present and/or (b) where the material is being actively plastically deformed.
This chapter emphasizes the mineral physics/materials science of mechanical absorption, specifically in dense materials at elevated temperature; the interest, then, is in employing the physical study of mechanical absorption to understand natural phenomena such as those mentioned above. Beyond questions of the basic mineral physics of absorption, the guiding interest the community of scholars pursuing the ideas is in isolating actual absorption mechanisms operative in the geological setting thereby allowing for a greater discrimination in the interpretation of seismic data.
At the outset, I note that this contribution does not seek to be a comprehensive review of the ideas and work involved in the physics of mechanical absorption. There are marvelous reviews in the literature that are both authoritative and current; I refer to these frequently in contemplating my own …