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1 Tectonic Processes and Hazards > EQ1 - Why are some locations more at risk to tectonic hazards? > Flashcards

EQ1 - Why are some locations more at risk to tectonic hazards? Flashcards

1
Q

Where are 70% of earthquakes found?

A

Pacific ‘Ring of Fire’

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2
Q

Where do the most powerful earthquakes occur?

A

At convergent or conservative boundaries, however rare intra-plate earthquakes can occur.

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3
Q

What determines the violence of a volcanic eruption?

A
  • Amount of dissolved gas in magma
    -How easily the gases can escape
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4
Q

How is a seismic hazard generated?

A

When rocks within 700km of the Earth’s surface come under such stress that they break and become displaced.

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5
Q

What are intra-plate earthquakes?

A

Ones that occur in the middle or interior of tectonic plates - much rarer than boundary earthquakes

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6
Q

What are the 3 types of Plate Boundary?

A
  1. Divergent (Constructive)
  2. Convergent (destructive)
  3. Conservative
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7
Q

What are the characteristics of Divergent (constructive) plate margins?

A
  • frequent shallow focus + generally low magnitude earthquakes
  • Most are submarine - low hazard risk
  • Creates new oceanic crust - denser than continental crust
  • Do not usually trigger tsunamis
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8
Q

What are the characteristics of Convergent (destructive) plate boundaries?

A
  • One plate slides under the other
  • strain builds in subduction zone - until friction between plates is overcome, releasing energy
  • actively deforming collision locations (crumple zone)
  • Subducting plates melt, creating magma - causing frequent earthquakes and volcanoes
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9
Q

What are the characteristics of Conservative plate margins?

A
  • one plate slides against another
  • relative movement is horizontal, sinistral (left) or dextral (right)
  • Creates a zone of friction
    -Lithosphere (land) is not created or subducted
  • no volcanic activity, site of extensive shallow focus earthquakes, occasional large magnitude
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10
Q

Where are volcanoes formed?

A
  1. Destructive plate boundaries
    -2 plates move together
    -creating a subduction zone/continental collision
  2. Divergent Boundaries
    - Create rift volcanoes
    - plates diverge at thermally buoyant ocean ridge
  3. Hotspot Volcanoes
    -in the middle of tectonic plates
    -fed by underlying mantle plumes
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11
Q

How is a subduction zone formed?

A
  1. dense oceanic plate collides with less-dense continental plate
  2. Oceanic plate thrust underneath - due to greater buoyancy of continental plate
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12
Q

How do hotspot volcanoes create landforms?

A
  1. Heat rises as a hot thermal plume
  2. High heat + low pressure allow for melting of lithosphere
  3. Molten material erupts through cracks - forms volcanoes
  4. As plates move, volcanoes are rafted away from the hotspot - these cool and subside
  5. This creates islands/chains, atolls and seamounts
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13
Q

How are earthquakes and volcanoes formed from subduction zones?

A
  1. strain builds as oceanic plate is subducted
  2. friction between the 2 masses is overcome - releasing energy

The magma for volcanoes is produced by melting of the subducting plate

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14
Q

What are the 2 different types of crust and how do they vary?

A

Thin Oceanic Crust - underlies ocean basics, composed mostly of basalt

Thicker Continental Crust - underlies continents, composed primarily of granite

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15
Q

Where is the earths mantle at it’s hottest?

A

Highest temperatures where mantle is in contact with heat-producing core

Increase in temperature with depth

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16
Q

What is radioactive decay, and how does it lead to tectonic movement?

A

This is heat derived from the earths core.

It rises within the mantle to drive convection currents

These convection currents in turn move tectonic plates

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17
Q

How does gravity also contribute to tectonic plate movement?

A

It is responsible for the subduction of the denser Oceanic Crust at subduction zones

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18
Q

What role does magma play at conservative (divergent) margins?

A

Magma gap fills, where the two plates have spread apart, creating new crust once it cools.

Magma does NOT act as a force pushing the plates apart

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19
Q

What is ‘sea floor spreading’ ?

A
  • Occurs at divergent (spreading) boundaries under the sea
  • continuous input of magma forms a mid-ocean ridge

On land, a ‘rift valley’ would be formed

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20
Q

What technique can be used to date the age of new tectonic crust?

A

Palaeomagnetism

  • Zone of magma ‘locks in’ the Earths polarity when it cools
  • Scientist use it to see historic tectonic activity + create a geo-timeline
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21
Q

What is the Benioff Zone?

A

Area of seismicity corresponding with the slab being thrust downward in a subduction zone

22
Q

Why are numerous earthquakes produced in this zone?

A

Different speeds and movements of rock causes frequent earthquakes

  • They are often intermediate/deep focus

-

23
Q

What is the name given to the contact between plates in a subduction zone?

A

Thrust/megathrust fault

24
Q

How do megathrust earthquakes occur?

A
  1. Where plates are locked together, frictional stress builds
  2. Failure occurs along the fault once the stress exceeds a certain threshold
  3. Strain energy and seismic waves are released
25
Q

What is the ‘locked fault’

A

A fault that is not slipping as frictional resistance is greater than the shear stress across the fault

  • Can be locked for extended periods, storing lots of strain
  • often results in a high magnitude earthquake once finally overcome
26
Q

What is the hypocentre?

A

The focus point (point of rupture) WITHIN the ground, where strain energy is first released

  • Most damaging events are usually shallower than 40km from surface
  • Can range as deep as 700km
27
Q

What is the epicentre?

A

The location on the earths surface directly above the earthquakes focus

  • The distance between hypocentre and epicentre is known as focal length
28
Q

Why is ground shaking experienced on the surface?

A
  1. Tectonic strain stores elastic energy in crustal rocks
  2. strain exceeds the strength of the fault - seismic waves are released + rocks fracture
  3. Brittle crust rebounds either side of the fracture
29
Q

What device is used to measure the amount of ground shaking during an earthquake?

A

A seismometer - measures both the vertical and horizontal movement of the ground.

30
Q

What are P waves?

A

Vibrations caused by compression - like a shunt along a line of train carriages

31
Q

What are S waves?

A

These move more slowly and vibrate at right angles to the direction of travel

Often more destructive due to larger amplitude and energy force.

Cannot pass through liquids (unlike P waves)

32
Q

What are L waves (also known as Q waves)?

A
  • Surface waves
  • vibration occurring in the horizontal plane.
  • Have a high amplitude
33
Q

Define ‘secondary hazard’ of an earthquake?

A

Side effects, often just as significant/destructive as the primary hazard

34
Q

What is soil liquefaction?

A

Process where water-saturated material temporarily loses it’s strength and behaves like a liquid, under the pressure of strong shaking

  • Earthquakes cause the water pressure to increase to a point where soil particles, when saturated, can move easily
  • This is especially common in poorly compacted sand/silt
35
Q

What damage can liquefaction cause?

A
  1. Buildings tilt - buildings settle, tilt (sometimes up to 60 degrees) and even collapse
  2. Lateral spreading - land near to rivers on steep slopes can slide under low friction conditions
  3. Damage to infrastructure
    - roads + bridges
    - services (gas, electricity)

These all contribute to costs of (short term) aid + (long term) rebuilding

36
Q

What other secondary hazard of an earthquake causes slopes to weaken and fail?

A

Landslides (also avalanches, rock falls)

37
Q

Why are landslides so hazardous?

A

They can travel several miles, grow in size and pick up debris (trees, cars etc.)

  • many destructive earthquakes occur in mountainous areas
  • 70% of all earthquake deaths (excluding shaking, building collapse + tsunami’s) caused by landslides
  • However, rarely occur at magnitudes less than 4
38
Q

What other conditions were found to affect the severity of landslides in the 2015 Nepal Earthquake?

A
  • Summer monsoon rainfall
  • previous landscape disturbance affects future vulnerability
39
Q

What are the characteristics of a Tsunami

A
  • long wavelengths (150-1000km)
  • low amplitude (wave height) 0.5-5m
  • high velocity (up to 600km/h in deep water)
40
Q

Why are tsunami’s hard to detect out at sea?

A

They are often below 30cm in height.

41
Q

What name is given to the series of waves in a Tsunami?

A

Wave train - caused by seabed displacement - water pushed upward

42
Q

Why does a greater wave period present a higher risk?

A

People return home thinking the Tsunami is over.

Wave period - amount of time between successive waves.

  • Often only few minutes but in some rare cases, can be hours
43
Q

Where do 90% of all Tsunami events occur?

A

The Pacific Basin

  • Mostly generated in subduction zones (convergent margins)

e.g Japan-Taiwan Island Arc

44
Q

What factors affect the physical impact of a Tsunami?

A
  1. Duration of event
  2. Wave amplitude (height), water column displacement and distance travelled
  3. Physical Geography - water depth, gradient of shoreline
  4. Ecosystem buffers - e.g mangroves, coral reef
  5. Timing of event (night vs day) + quality of early warning systems
  6. Coastal development - harm to tourists etc.

Most serious events occur when human and physical factors interact - produces a disaster

45
Q

Name the volcanic hazards. (4 primary, 2 secondary)

A

PRIMARY
1. Pyroclastic Flows
2. Tephra
3. Lava flows
4. Volcanic gases

SECONDARY
1. Lahars
2. Jokulhlaups

46
Q

What is a Pyroclastic Flow?

A

The frothing of molten magma in the vent of a volcano

Eject hot gases and pyroclastic material (glass, pumice, ash etc.)

  • Can be up to 1000’C
  • Responsible for most deaths
    -Most hazardous when close to the ground
47
Q

What is Tephra?

A

The material (rock fragments) ejected into the air during an eruption

  • Larger materials can cause roofs to collapse + start fires on the ground
  • Dust can reduce visibility + affect air travel
48
Q

How quickly can lava flows travel?

A

On steep slopes, can reach 15m/s, quicker than you can run.

  • viscosity is determined by amount of silicon dioxide
  • widespread damages to crops can induce a famine
49
Q

Why is carbon dioxide the most deadly volcanic gas?

A

It is hard to detect

Colourless and odourless and can accumulate in valleys

  • Water vapour, suplhur dioxide, hydrogen and carbon monoxide also present
50
Q

What is a Lahar?

A

Volcanic Mudflows composed of fine sand and silt

  • Triggered by heavy rainfall
  • This remobilises deposited Tephra on steep slopes, creating a mudflow
51
Q

What are Jokulhlaups?

A

A Glacial Outburst Flood

  • occur very suddenly
  • rapid and large volume of discharge of water, ice and debris
  • Initiated following the failure of an ice or moraine damn
52
Q
A

1 Tectonic Processes and Hazards (3 decks)

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