Seismic Wave Attenuation and the EAGBS Model for the MLD and LAB

Abstract:

The lithosphere-asthenosphere boundary (LAB) and the mid-lithosphere discontinuity (MLD) are both seismic features where seismic wavespeeds drop by 2-10% over a short depth range. Partial melt, frozen melt of hydrous rock, anisotropic twists and physical dispersion from elastically accommodated grain-boundary-sliding (EAGBS) have all been cited as causative factors. The transition from "unrelaxed" to "relaxed" rheology occurs under conditions of maximal frictional dissipation at mineral grain boundaries, suggesting that seismic-wave attenuation could help distinguish EAGBS from other mechanisms to explain the LAB and MLD. This dissipation behavior was claimed by Ma et al (2020) to be detectable in NOMELT OBS data from central Pacific. We adapt 1-D reflectivity algorithms to compute and compare both body waves and surface waves from (1) elastic non-dispersive velocities, (2) non-dissipative velocities that exhibit EAGBS physical dispersion, (3) dissipative non-dispersive velocities, and (4) velocities that EAGBS dissipation and physical dispersion. For a standard Basin-and-Range upper-mantle velocity profile we estimate attenuation as a first-order perturbation, and we insert physical dispersion directly into the reflectivity computation. We focus on both dispersive and dissipative behavior in these models, in order to expand the range of possible constraints one can obtain from seismic data.

Ma, Z., C. A. Dalton, J. B. Russell, J. B. Gaherty, G. Hirth, and D. W. Forsyth (2020), Shear attenuation and anelastic mechanisms in the central Pacific upper mantle, Earth and Planetary Science Letters, 536, 116148, doi.:10.1016/j.epsl.2020.116148.