Earth's Internal Structure

Question 1

Factors that Seismic velocities depend on

Answer 1

Seismic velocities depend on material properties (composition, mineral phase & structure, temperature, pressure) of media through which waves pass.

Question 2

Seismic Waves Travel..

Answer 2

• Quicker through denser materials, so fasted with depth
• Slower through anomalously hot areas
• Slower through a liquid than a solid
• P waves slower through molten areas; S waves stopped as shearing motion can’t be transmitted through liquid

Question 3

Describe Seismic Discontinuities

Answer 3

• Sudden jumps in seismic velocities across a geological boundary.
• Results in reflections, refraction (bending), and the production of new wave phases (e.g. an S wave produced from a P wave).

Question 4

Mohorovicic Seismic Discontinuity = the Moho

Answer 4

*Crust-Mantle Boundary depicted by seismic velocity jump.
-Felsic/Mafic crust (seis. vel. ~6km/s) vs denser,
ultramafic mantle (seis. vel ~8km/s)

• Mohorovici (1909): Seismic waves recorded beyond 200km depth from earthquake source had passed through a lower layer with much higher seismic velocity
• Seismic stations within ~200km of an earthquake report travel times that increase regularly with distance from source (i.e. passing through crust)
• But beyond 220km, seismic waves arrive sooner than expected (i.e. meet the mantle and hence travel faster)

Question 5

Describe Continental Crust

Answer 5

• Depth to Moho ~35km (range 20-70km)
• Composition: felsic, intermediate, and mafic igneous, sedimentary and metamorphic rocks
• Age: 0 to 4 b.y.

Summary: thicker, less dense, heterogenous, old

Question 6

Describe Oceanic Crust

Answer 6

• Depth to Moho ~7km below seafloor (i.e. ocean crust is about 7km thick)
• Composition: mafic igneous rock (basalt & gabbro) with thin layer of sediments on top
• Age: 0 to 200 m.y.

Summary: thin, more dense, homogenous, young

Question 7

Low Velocity Zone (LVZ)

Answer 7

• Seismic velocities increase with depth in the mantle due to increasing pressure/density
• Zone of low seismic at ~100-250km depth interpreted to be 1% or less molten (i.e. >99% solid) i.e. is “soft”
• Represents zone of weakness in the upper mantle called the asthenosphere.
• Uppermost mantle and crust above the asthenosphere is called the lithosphere
• Lithosphere can move (glide) over the weak asthenosphere => Plate tectonics

Question 8

670km Seismic Discontinuity

Answer 8

• Results from a change in crystal structure at high pressure (large depth) from Crystalline structure of olivine to tighter packing structures
• 670km discontinuity is a major boundary separating less dense upper mantle from a more dense lower mantle

Question 9

The Core Gutenberg Seismic Discontinuity

Answer 9

• Core-mantle boundary
• At arc distances of ~103-143°C no P waves are recorded
• S waves have an even larger ‘shadow zone’
• Gutenberg (1914)- result of a molten core beginning at ~2900km depth. S (shear) waves could not penetrate this molten layer and P waves would be severely slowed and refracted

Question 10

The Core Lehman Seismic Discontinuity

Answer 10

• The inner core
• Between 143-180°C from an earthquake another refraction is recognised
• Results from a sudden increase in P wave velocities at a depth of 5150km
• This velocity increase is consistent with a charge from a molten outer core to a solid inner core