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LEC.171 > Lecture 2: Seismology & the Earth's Interior > Flashcards

Lecture 2: Seismology & the Earth's Interior Flashcards

1
Q

What is a body wave?

A

A seismic wave that travels through the body of the Earth

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2
Q

What is a surface wave?

A

A seismic wave that travels on the surface of the Earth

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3
Q

Which body wave is transverse?

A

S-waves

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4
Q

Which body wave is longitudinal?

A

P-waves

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5
Q

What 2 factors influence seismic wave velocity?

A

Density and elasticity

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6
Q

What is the shear modulus in liquids and gases?

A

0

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7
Q

What is a seismic discontinuity?

A

A boundary between 2 of the Earth’s layers that have different properties

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8
Q

What 3 simultaneous events occur when a seismic wave hits a discontinuity?

A

Wave refraction, reflection, and transformation (P-wave -> S-wave and vice versa)

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9
Q

If seismic wave velocity is constant, what happens to the motion of the seismic wave?

A

It travels in a straight path

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10
Q

If seismic wave velocity is gradually increasing, what happens to the motion of the seismic wave?

A

It travels in a curved path

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11
Q

When was the Moho discovered?

A

1909

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12
Q

How was the Moho discovered?

A

Shallow refraction surveying

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13
Q

What is the typical thickness of oceanic crust?

A

7km thick

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14
Q

What is the typical thickness of continental crust?

A

35km thick

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15
Q

What is the maximum thickness of continental crust?

A

80km under Himalayas

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16
Q

What layers of the Earth’s interior are considered the lithosphere?

A

Crust and rigid upper mantle

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17
Q

What is the typical thickness of oceanic lithosphere?

A

70km thick

18
Q

What is the typical thickness of continental lithosphere?

A

150 km thick

19
Q

Why is the asthenosphere considered a ‘weak’ layer?

A

It has low elasticity to be able to ‘flow’ and allow the vertical and horizontal movement of the lithosphere

20
Q

Why can S waves pass through the asthenosphere?

A

It is partially molten, but a lot of their energy is absorbed because of this

21
Q

What happens to the seismic velocity at the base of oceanic lithosphere?

A

It decreases

22
Q

How thick is the ‘low velocity zone’?

A

About 100km thick

23
Q

Why is the Mantle completely solid?

A

The melting point gradient is higher than the temperature gradient

24
Q

Which rock type is the Mantle completely comprised of?

A

Peridotite

25
Q

Which 4 main minerals is peridotite made from?

A

Olivine, clinopyroxene, orthopyroxene, and an aluminium rich mineral dependant on depth

26
Q

How do we know that the mantle is comprised of peridotite?

A

Ophiolite suites, xenoliths, and meteorites

27
Q

What are ophiolite suites?

A

Fragments of old oceanic crust uplifted onto continental crust

28
Q

What are xenoliths?

A

Fragments of mantle peridotite contained within explosive volcanic rocks (torn from the mantle during a volcanic eruption)

29
Q

How do meteorites suggest that the mantle is comprised of peridotite?

A

Many meteorites are comprised of peridotite and originate from asteroids which are thought to have a similar interior to Earth

30
Q

What is the boundary between the lithosphere and asthenosphere called?

A

The Lithosphere-Asthenosphere Boundary (LAB)

31
Q

How does seismic velocity change between 200-400km deep?

A

It increases steadily

32
Q

How does seismic velocity change at 400km deep?

A

Increases more rapidly

33
Q

At 400-450km deep, what happens to the main mantle mineral, olivine?

A

It changes to a higher presser form (same atoms but a more densely packed structure, mimics structure of spinel)

34
Q

At 670-700km deep what happens to the main mantle mineral, olivine?

A

Structural change from 4-fold to 6-fold, even more densely packed, mimics structure of perovskite)

35
Q

By how much does the density increase over the MTZ?

A

Around 11%

36
Q

Why does the density increase in the MTZ NOT cause a decrease in seismic velocity?

A

The elasticity increases which allows the seismic velocity to increase

37
Q

True or False? The lower mantle (below the Mantle Transition Zone) is homogeneous with a constant uniform structure and composition.

A

True

38
Q

How deep is the Core-Mantle Boundary (CMB)?

A

2900km deep

39
Q

How do P waves move?

A

Compressions and dilations

40
Q

How do S waves move?

A

Peaks and troughs

41
Q

What does seismic wave velocity depend on

A

Density and elasticity of the material its travelling through

42
Q

How thick is the transition zone of the upper mantle?

A

400km

LEC.171 (8 decks)

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