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)
• 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 motiondirection (loosely corresponding to the depth of a gradualdecrease in shear wave velocity). Why would that matter?
Yuan and Romanowicz, Nature, 2010
Anisotropy:Mantle shear waveanisotropy is “usually”interpreted as alignmentof the fast axis with theflow direction… But recallwhere this comes from!Hooke’s law generalizesto:
And there are 81 terms inthe fourth-order elasticitytensor, with as many as21 different elasticconstants needed to fullydescribe the response!
€
σ ij = cijklε kl
€
σ ij = cijklε kl
Hence e.g.the shearrigidity ofa mediumcan bedifferent inonedirectionthan it isin another.If the directionaldependenceis consistentover “large
enough” scales, this can result in separation of S arrivalspolarized along the fast and slow axis directions…€
VS =μ
ρ
On the scales of sampling of a seismic wave, anisotropy canoccur for any of several different reasons… Common typesinclude Shape Preferred Orientation (SPO), which iscaused by inhomogeneous distribution of materials…
Simple examples of this type of anisotropy include stacks ofhorizontal layers with differing velocity, or an otherwiseuniform medium that contains fluid-filled cracks.
Lattice PreferredOrientation (LPO) isused to describe amedium that is innatelyanisotropic (i.e., rightdown to the sub-grainscale of individualmineral crystals).
This type dominates mostupper mantle anisotropy!
In many instances though observed anisotropy includes a mixof both shape-preferred and lattice-preferred orientations,which of course tends to complicate both the modeling andthe interpretation of anisotropy.
