Plate Tectonics and Natural Hazards

what are earthquakes

Movement within the earth's crust causing elastic stress to build up at points of weakness and rocks to deform

• Stored energy builds up over time in deformed rock near weakness

• When stress exceeds strength of rock = rock fractures along a fault

  • Called earthquake rupture

• Stored energy is suddenly released

  • Stress energy -> mechanical energy

  • Stress released in seismic waves

Hypocentre vs epicentre

Hypocentre / focus = place on fault plane where earthquake rupture takes place

• Energy of earthquake radiates from here

Epicentre = point on the ground directly above hypocentre

• Is not along fault line unless fault plane is vertical

what is a fault? what are the types?

ractures on which slip occurs

• Displacement is the amount of slip on a fault

• Determined by measuring distance between two ends of a marker that have been offset by fault

• Active fault = faults likely to move in the near future

• Inactive faults = faults unlikely to move again in the near future

Stages of fault deformation

1. Rock changes shape does not break

   1. Elastic behaviour -> Elastic strain

2. Rock starts to crack

   1. Small cracks grow and connect to create a fracture

3. Rock on one side of fracture slides past rock on other side -> creates a fault

   1. Fault forms = one side of fault is displace from the other

   2. Stress in rock decreases and strain = 0

   3. Rock does not return to original behaviour

   4. Permanent strain

Friction and stick-slip behaviour

• what is friction?

• what is stick slip behaviour?

• Friction = resistance to motion on a surface

  • Prevents the rock sliding on fault surface

  • Slows down and stops the fault movement

• Stick-slip behaviour = start-stop movement of a fault

  • Stick = stress builds up but fault does not slips due to friction being larger than stored stress

  • Slip = stress overcomes friction and fault slips

    • After faulting event -. Stress drops -> elastic strain decreases or disappears

    • Permanent strain -> i.e. rock does not return to original shap

How earthquake energy travels

Earthquake energy travels in the form of waves

Body waves

Surface waves

what are body waves

Pass through the interior of the Earth

• Primary waves (p) = compressional waves

  • Particles move back and forth

  • Parallel to direction in which wave moves

  • Fastest and arrive first

• Secondary waves (s) - shear body waves

  • Particles move back and forth

  • Perpendicular to direction in which wave moves

  • Slower: 60% speed of P waves

what are surface waves

Travel along the Earth’s surface

• Rayleigh waves (R) = surface waves that cause ground to ripple up and down

• Love waves (L) = surface waves that cause ground to ripple like a snake

• Slowest of all

• More destructive

  • Larger amplitude

  • Most of energy released reaches crust

  • Same or longer duration than body waves

what are seismographs

 instrument that measures wave arrival times and magnitude of ground motion

• Straight line on the rotating drum = background signal

• First wave arrives and causes frame  to move pen up

• Nex vibration causes the opposite motion

• Preserves a record of wave arrivals and the amplitude of ground motion

how to locate an epicentre

Waves always arrive in sequence:

1. P-waves

   1. The 1st arrivals of P- and S-waves vary with distance

2. S-waves

3. Surface waves

A travel-time graph the distance of each station to the epicentre

 

Data from many stations needed

1. A circle is drawn around each station

2. The radius is equal to the estimated distance of each station form the epicentre

3. Where the three circles intersect is the earthquake epicentre

how to describe earthquake size

magnitude

intensity

describing Earthquake intensity

Describes the degree of shaking based on damage

 

Mercalli Intensity Scale

• Roman numerals assigned to different levels of damage

• The higher the numeral, the higher the intensity and damage

• Damage is mapped in zones

• Damage intensity decreases with distance from the epicentre

describing earthquake magnitude

The energy released during an earthquake

 

Magnitude is estimated by:

• Max amplitude of motion from a seismogram

• Normalising the value for the seismograph distance from the epicentre

• Different magnitude scales

  • Richter

  • Moment

• Scales are logarithmic

  • Increase of 1 unit = increase in energy released by 10 times

• Smaller magnitude earthquakes happen more commonly

where do most quakes occur

80% are released in seismic sones (i.e. near plate boundaries)

• Plate boundary earthquakes = near plate boundaries

• Intraplate earthquakes = occur away from plate boundaries

 

Earthquakes within continents: continental rifts and collision zones

• Transform fault earthquakes = Shallow-focus along strike-slip faults.

  • Most earthquakes occur where brittle deformation is

• Continental rift earthquakes = shallow-focus earthquakes along normal faults.

• Collision zones = shallow-focus earthquakes along thrust faults and reverse faults.

Convergent plate-boundary earthquakes

Most earthquake occur

• Large earthquakes on thrust faults

• Shallow-focus (<20km) earthquakes occur in the downing plate along normal faults

• Intermediate-focus (20km-300km) and deep focus (300km+) earthquakes occur in downing slab

  • Define the Wadati-Benniof zone

  • Due to shearing between downing plate and mantle caused by slab pull

earthquake within continents

continental rifts and collision zones

• Transform fault earthquakes = Shallow-focus along strike-slip faults.

  • Most earthquakes occur where brittle deformation is

• Continental rift earthquakes = shallow-focus earthquakes along normal faults.

• Collision zones = shallow-focus earthquakes along thrust faults and reverse faults

Intraplate earthquakes

shallow-focus earthquakes along normal faults

• Earthquakes that occur in australia

 

Three main models:

• Stress transfer from plate boundary -> interior plate break at pre-existing faults

  • I.e. reactivation of old rift zones

• Stresses resulting from shear between lithosphere and asthenosphere

• Readjustment of crust to unloading form glaciers

Ground shaking during an earthquake: wave types and their motion

• P-waves are (mostly) perpendicular to ground surface

• S-waves cause back and forth motion

• L waves cause snake motion

• R-waves cause rolling motion

  • Last longer

  • Most damage

what does severity of earthquake damage depend on

• Magnitude (energy) of earthquake

  • Higher magnitude = more damage

• Distance from hypocentre

  • Closer = more damage

• Intensity and duration of vibrations

  • Longer duration + intensity = more damage

• The nature of subsurface material (geology)

  • Hard rock = transmit wave quickly = less damage

  • Sediment = waves bounce = amplifies damage

what is liquefaction

Waves liquefy H2O-filled sediments.

• High pore pressures force grains apart reducing friction

• Liquefied sediments flow as a slurry

• Land may slump laterally and buildings may tilt

Long-term earthquake prediction: what is it and how it works

• Probability of a certain magnitude earthquake occurring on a time scale of 30 to 100 years, or more

• Based on the premise that earthquakes are repetitive

• Requires determination of active seismic zones by

  • Mapping historical epicentres (after ~ 1950)

  • Identifying evidence of ancient earthquakes (before seismographs)

    •  In the geological record

    • In the historical record

  • Paleo-seismological studies aim to determine the earthquake recurrence interval

    • i.e. the average time between events

• Seismic gaps, places that haven’t slipped recently, are more likely candidates.

Short-term earthquake prediction: what is it and how it works

• Goal is to predict the location and magnitude of a large earthquake

• None are reliable

• Earthquakes do have precursors

  • Clustered foreshocks

  • Crustal strain

  • Water level changes in wells

  • Gases in wells

  • Unusual animal behaviour

how to prepare for earthquakes

• Understand what happens during an earthquake

• Map active faults and shake prone areas susceptible to liquefaction

• Develop construction codes to reduce building failures

• Regulate land use to control development

• Monitor earthquake activity

damage from earthquakes

tsunamis, volcanoes

what is Tsunamis

seismic sea waves

• Result when earthquakes change seafloor

  • Normal faulting drops seabed

  • Thrust/reverse faulting raises it

• Movement of seabed displaces overlying water

• Giant mound/trough forms on sea surface

• Waves are created + move rapidly

Tsunamis vs Wind waves

Wind waves	Tsunamis
Influence upper 100m	Influence entire water depth
Wavelength = 10s to 100s of metres	Wavelength = 10s to 100s of km
Wave height and wavelength -> related to windspeed	Wave height and wavelength unaffected by windspeed
Wave velocity maximum =10s of kph	Wave velocity maximum = 100s of kph
Waves break in shallow water and expend all stored energy	Waves come ashore as a raised plateau that pours onto land

Tsunami behaviour

• superfast

• Almost imperceptible in deep water

  • Low wave height (amplitude)

  • Long wavelength

• As water shallows, waves slow due to drag

• Waves grow in height reaching 10m+

• Near-field tsunamis: strike areas proximal to the earthquake epicentre.

• Far-field or distant tsunamis: cross the ocean and strike areas hundreds of kilometres away from the earthquake (more than 1,000 km).

  • May strike several hours after the earthquake

Tsunami disaster

• how destructive

• who is most at risk

• Destructive tsunamis occur frequently - about 1/yr

• Many tsunami disasters have been recorded in the history with hundreds of thousands of victims

• Future tsunami disasters are inevitable

• Growing human population in low-lying coastal areas

• Education about tsunamis can save many lives

how does tsunami prediciton work

• Scientific modelling predicts tsunami behaviour

• Tsunami detection is expanding

• Tsunami detectors are placed on the deep seafloor

• Sense pressure increases from changes in sea thickness

• Prediction / detection can save 1000s of lives.

how are tsunamis caused

• quakes

• volcano eruption

• erosion

• meteor impact on water

what are volcanic eruptions

• An erupting vent through which molten rock surfaces.

  • Result of tectonic activity

• A mountain built from magmatic eruptions

• Unpredictable and dangerous

  • Affect climate

  • Provide productive soils for farming

  • Kill humans, destroy land and property

forms of volcanic material

• Lava flows = Molten rock that moves over the ground

• Pyroclastic debris = Fragments blown out of a volcano

• Volcanic gasses = Vapour and aerosols that exit a volcan

what is lava flow?

what is it determined by?

• Can be runny or thick/sticky

• Character reflects its viscosity -> resistance to flow

 

Viscosity determined by

• Temperature - viscosity increases as temperature decreases

• Composition - Silica (SiO2), Fe and Mg content

  • Viscosity increases with silica content

  • Viscosity decreases with higher Fe and Mg content

types of lava flow

Basaltic flow	Mafic Low silica High Fe and Mg Very hot, low silica content, and low viscosity Thin and fluid Flow for long distances Shield volcanoes
Andesitic flow	Intermediate Moderate / intermediate silica content Fe and Mg are called intermediate Higher SiO2 content makes andesitic lavas viscous Do not flow rapidly Gather around the vent and flow slowly Outer crust fractures, creating rubble Flow remain close to the vent Stratovolcanoes or composite volcanoes
Rhyolitic flow	Felsic High silica content Low Fe and Mg Highest SiO2 -> most viscous lava. Rarely flows Plugs vent as lava dome Can blow lava domes to bits Forms calderas

what is pyroclastic debris

fragmental material ejected from a volcano

• Glass shards, fragmented lava in different sizes

• Ash = powdery glass shards

• Lapilli = pea to plum sized material

• Blocks/bombs = apple to refrigerator sized

  • Blocks = pre-existing rocks torn from volcano

  • Bombs = fragments of ejected lava

• Hot avalanches of hot ash

  • Race down slope fast

  • Deadly

• Incinerate everything in their path

what is a lahar

Tephra moved by water as debris flow

• Rapid speed

• Wet cement consistency

• Watery landslide

what is volcanic gas excape controled by

• Controlled by magma composition

  • Felsic magmas = gas rich

    • SiO2 trap gas

  • Mafic magmas = less gas rich

• Gases expelled as magma rises and pressure drops

• Style of gas escape controls eruption violence

  • Low viscosity = basalt = easy escape, effuse eruption

  • High viscosity = rhyolite = difficult escape, violent eruption

what are the eruptive styles

Effusive eruptions	Produce basaltic lava flows
Explosive eruptions	Produce pyroclastic flows Gas pressure in more viscous magma Calderas

Controlled by

• Viscosity -> ease of flow

  • Low viscosity = basalt lava flows away from vent = effusive

  • High viscosity = felsic lava clogs vent = increase pressure = explosive

• Gas pressure

  • Greater pressure = more explosive

  • Low viscosity = gas released easily -> can move past silica chains

  • High viscosity = traps gas

Relation of volcanism to plate boundaries and rifts

Mid-ocean ridge volcanism and Rifts	Short lived Basalt lava flows (mafic)
Convergent boundaries / subduction zones	Most of subaerial volcanoes Water and CO2 released from subducting slab = melting due to introduction of volatiles = produces magma that rises through lithosphere = erupts More intermediate to felsic Continental arc = volcanic are built on continental crust Island arc = volcanic are built on oceanic crust

list some volcanic hazards

• what are they

• impact

Lava flows	Mostly basaltic are threatening -> fast speeds Destroy objects in its path Rare to kill people since it moves slower than runni ng
Tephra	Falling ash and lapilli Bury landscapes -> kill plants and crops, destroy buildings Heavy -> collapse rooves Gritty -> abrades engines
Pyroclastic flows	Clouds of ash and gas that flow downhill Ride on top of very hot air High velocities Eases passage of flow Deadly
Lahars	Mudflows that form when water moves ash Denser than water Carries things with it -> people and buildings

volcanic activity classificaiton

Active	Erupting, have erupted in last 1000 years, likely to erupt again
Dormant	Haven't erupted in last 1000 years but could again in the future
Extinct	Will not erupt in future

how to monitor volcano eruptions

Signs that an eruption is approaching -> cannot predict timing or style of eruption

• Earthquake activity = magma flow increases seismicity

  • Seismometers near volcanoes

  • Record earthquakes live

  • Associated with magma movement

• Heat flows = rising magma heats up volcanoes

  • Thermal cameras monitor volcano temperature

• Change in shape = magma expands things

  • Satellite imaging and GPS stations to monitor shape of volcano

  • Magma can cause volcanoes to bulge

  • Can also use telemeters to record tilting of sides of volcanoes -> due to magma rising

• Emission increases = change in gas mix and volume

how to mitigate volcano hazard

• Educate people so they understand the risk

  • Have an evacuation plan and strategy to prevent deaths

    • Mapping volcanic risk

    • AOE of lahars, pyroclastic flows, landslides

    • Used for planning and zoning of land

  • Practice evacuations

• Diverting flows to prevent destruction of buildings and killing of people

  • Can direct the lava to where we want it to go

  • Explosives

  • Heavy equipment

  • Seawater