Week 3, Earthquake Geology, Seismology, and Engineering

Question 1

Plate tectonics are driven by Earth’s internal heat energy through several mechanisms:

Answer 1

Ridge Push
Slab Pull
Mantle Convection

Question 2

Ridge Push

Answer 2

Outward push of upwelling hot mantle material

Gravitational sliding of lithosphere down-ridge

Question 3

Slab Pull

Answer 3

Lithosphere spreads, cools, and contracts, eventually becoming denser than underlying asthenosphere.

Cold, dense lithosphere sinks at subduction zones, pulling on plate.

Question 4

Mantle Convection

Answer 4

Thermally driven convective cells in mantle pull plates along like a conveyor belt.

Question 5

Spreading Centres

Answer 5

Plates pull apart, magma upwells from asthenosphere creating new oceanic lithosphere.

Characterized by mid ocean ridges.

Small earthquakes.

Question 6

Stages in Evolution of a Spreading Margin

Answer 6

1. Centering: Continent centres over hot region in the mantle.
2. Doming: Heat causes expansion, uplift, and fracturing.
3. Rifting: Gravity causes fractures to fail, forming faults, sagging centre, volcanism.
4. Spreading: Deep rift floods, continued divergence forms new oceanic crust.

Question 7

Subduction Zones Seismicity, 3 types of Earthquakes:

Answer 7

Crustal
Inter-plate
Intra-slab

Question 8

Subduction Zone Crustal Earthquakes

Answer 8

Compressional or tensional earthquakes due to deformation of plate from convergence.

Shallow

Up to M8

Question 9

Subduction Zone Intra-Slab Earthquakes

Answer 9

Occur in cold, brittle interior of subducting plate since edges are to hot and plastic.

Faster subduction leads to deeper earthquakes.

Max depth up to 700m, up to M8

Compressional and tensional earthquakes

Question 10

Subduction Zone Inter-Plate Earthquakes

Answer 10

Occurs at the boundary between two tectonic plates.

Megathrust Earthquakes.

Question 11

Earthquakes and volcanism happen along

Answer 11

edges of plates.

Question 12

3 types of plate boundaries:

Answer 12

Transform (shear) 
/\  |
 |   |
 |  \/
Tension

Compression
–>

Question 13

Rifting can be described as when

Answer 13

you break apart continents and form new ocean. Like the Red Sea, Gulf of Aden, or the East African Rift.

Question 14

Continent-continent collision

Answer 14

Low density continental crust resists subduction.

Results in double thick crust.

Question 15

Transform faults in general are _____ boundaries. We are not ____ or _____ material. This puts a limit on how big ______ can be.

Answer 15

Transform faults in general are conservative boundaries. We are not making or destroying material. This puts a limit on how big earthquakes can be.

Question 16

Do subduction zones destroy or create?

Answer 16

Destroy.

Question 17

Do divergent zones destroy or create?

Answer 17

Create.

Question 18

Trench plate boundaries are

Answer 18

convergent/under compression.

Question 19

Spreading plate boundaries are

Answer 19

divergent/under tension.

Question 20

Where do the majority of transform faults occur? What does it do to mid-ocean ridge segments?Are they harmful to humans?

Answer 20

The majority of transform plate boundaries occur on the sea bed. This offsets mid-ocean ridges segments. Earthquakes on these faults are rarely harmful to humans.

Question 21

Mantle Plume

Answer 21

Pathway for hot material that’s partially melting as it rises to the surface. Creates bubble, creates fractures in crust. Can build new pathways for magma.

Question 22

Hot spot earthquakes can be due to

Answer 22

Rock expansion/fracture due to hot magma: small earthquakes.

Moving magma causing harmonic tremors: small continuous earthquakes, not as snappy as an earthquake.

Failure on faults caused by uplift/subsidence from repeated magma injection/withdrawal: large earthquakes.

Question 23

Tension = ____ faulting

Answer 23

Tension = Normal faulting

Question 24

Compression = _____ faulting

Answer 24

Compression = Reversal thrust faulting

Question 25

Where is the Juan de Fuca plate?

Answer 25

Nestled between San Andreas and Queen Charlotte fault. Subducting under the North America Plate and diverging away from the Pacific plate.

Question 26

Why is there no trench for the Juan de Fuca plate?

Answer 26

Crust is still young and not that cool.