Geology 5640/6640 Introduction to Seismology 15 Apr 2015 © A.R. Lowry 2015 Read for Fri 17 Apr: S&W 177-184 (§3.6) Last time: Structure of the Deep Earth’s.

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Geology 5640/6640 Introduction to Seismology 15 Apr 2015 © A.R. Lowry 2015 Read for Fri 17 Apr: S&W (§3.6) Last time: Structure of the Deep Earth’s Interior Interpretation of seismic layering relies on seismology & lab measurements & geochemistry of magmas/xenoliths & geodynamics Significant boundaries occur at the Moho (compositional), 410 km (phase), 670 km (phase), ~2700 (compositional and/or phase!), CMB (compositional), IOCB (phase) Mineral species: Upper mantle = olivine/pyroxene/garnet; transition zone = wadsleyite/garnet (to 520) & ringwoodite/ garnet; lower mantle = ferropericlase +perovskite; lowermost mantle has(?) post-perovskite; core is Ni-Fe + S(?) O(?) H? Those pesky deeper layers are still a topic of some debate…

Reminder: Please fill out your IDEA online evaluations! The Final Exam is now posted on the course website… Due Friday, May 1, at 8:30 am!

Previously we briefly touched on the topic of anisotropy, the observation that velocity sometimes depends on the dip & azimuth at which a wave propagates: Here, LAB has been defined partly in terms of a rotation of the anisotropy fast axis toward the absolute plate motion direction (loosely corresponding to the depth of a gradual decrease in shear wave velocity). Why would that matter? Yuan and Romanowicz, Nature, 2010

Anisotropy: Mantle shear wave anisotropy is “usually” interpreted as alignment of the fast axis with the flow direction… But recall where this comes from! Hooke’s law generalizes to: And there are 81 terms in the fourth-order elasticity tensor, with as many as 21 different elastic constants needed to fully describe the response!

Hence e.g. the shear rigidity of a medium can be different in one direction than it is in another. If the directional dependence is consistent over “large enough” scales, this can result in separation of S arrivals polarized along the fast and slow axis directions…

On the scales of sampling of a seismic wave, anisotropy can occur for any of several different reasons… Common types include Shape Preferred Orientation (SPO), which is caused by inhomogeneous distribution of materials… Simple examples of this type of anisotropy include stacks of horizontal layers with differing velocity, or an otherwise uniform medium that contains fluid-filled cracks.

Lattice Preferred Orientation (LPO) is used to describe a medium that is innately anisotropic (i.e., right down to the sub-grain scale of individual mineral crystals). This type dominates most upper mantle anisotropy!

In many instances though observed anisotropy includes a mix of both shape-preferred and lattice-preferred orientations, which of course tends to complicate both the modeling and the interpretation of anisotropy.