Lecture 8 - seismology

Question 1

Equations of state

Answer 1

Allow us to define mineral elastic properties as continuous functions of pressure and temperature

Question 2

Why do seismic waves and density not always increase smoothly?

Answer 2

Due to distinct chemical change (moho) or phase change (atom reorganisation) in the Earths interior

Question 3

Describe the phase changes In pyroxene?

Answer 3

Phase changes are gradual resulting in broad regions with elevated velocity gradients rather that discontinuities

Question 4

How does water get into deep earth?

Answer 4

Hydrothermal circulation at MOR and slab dehydration at subduction zone

Question 5

How does water exist in the mantle?

Answer 5

As free fluid (low pressure), hydrous silicates (core of subduction zone) and most likely as nominally anhydrous minerals

Question 6

How can water be detected in nominally anhydrous minerals (NAMs)?

Answer 6

Through observed reduction in seismic velocity (water reduces speed of P and S) and seismic anisotropy (alignment affected by water)

Question 7

How can seismic anisotropy be detected?

Answer 7

Using shear wave splitting (looking for differences in arrival times between differently polarized shear waves)

Question 8

Can we distinguish hydrous from anhydrous transition zone given the data uncertainty?

Answer 8

No, discontinuity depths are too uncertain to detect water

Question 9

Lattice preferred orientation (LPO)

Answer 9

non random distribution of axes in an aggregate of crystals. It can arise when a rock undergoes deformation - crystals align with common direction

Question 10

Uncertainties in mineral physics and sesimology?

Answer 10

Different pressure standards, experimental challenge of maintaining uniform temp across samples, Ab initio calculations - computationally expensive and uncertain, Trade off between discontinuity thickness and magnitude of velocity jump and double discontinuities - harder to detect decrease in velocity. Multiple causes