2) Seismic Hazards

interplate earthquake

= at the edge of plate margins

- most powerful ones almost always occur at the edge

intraplate earthquake

not at a plate margin
= within a plate along a fault line

- shifting of rock layers releases seismic energy

megathrust earthquakes

most powerful earthquakes
= occur along destructive plate margins

- almost always accompanied by tsunamis

as tectonic plates move over/under/against each other...

the stresses generated through FRICTIONAL DRAG build up tp break point

= results in an earthquake

causes of seismic hazards

human:
- mining
- fracking
- reservoir construction

physical:
- plate movements produce energy to move, friction along plate margins build stresses in lithosphere
- when strength of rocks under stress is suddenly overcome, they fracture along cracks called faults
= sends a series of seismic shockwaves to surface

focus/hypo-centre

= breaking point of the plates

epicentre

= the point on the surface directly above the focus

- experiences most intense ground shaking

- shaking becomes progressively less severe the further from the epicentre

seismic shockwaves

P waves, S waves, L waves, R waves

p waves

primary/pressure waves

= the fastest waves
- type of body wave
- reach surface first
- high frequency
- longitudinal
- travel through mantel + core to oppose side of earth

solids, liquids and gases

S waves

secondary/shear waves

= 1/2 as fast
- type of body wave
- second to reach surface
- high frequency
- transverse
- travel through mantel but not core

can travel through solids but NOT liquids

can be measured at a point opposite the focus/epicentre

L waves

surface Love waves

= slowest waves
- cause most damage
- travel along earths surface
- moves ground side to side

R waves

surface Rayleigh waves

= radiate from epicentre in complicated low-frequency rolling motions

aftershocks

Smaller earthquakes that follow a major earthquake

Benioff zone

= A zone of seismic activity along the upper portion of a sinking plate

elastic rebound theory

theory behind seismic energy
= explains how energy is stored in rocks

force of friction resists the movement of plates by convection currents below

Elastic strain theory

elastic materials deform when stress is applied but return to original shape when stress is removed

some materials deform permanently, even when stress is removed

elastic materials store energy as they deform = strain energy

strain energy

= energy that elastic materials store when they are deformed

...released ether when stress if removed or material undergoes brittle failure

brittle failure

earthquakes result from brittle failure of portion of earths lithosphere

1. strain energy builds up as the lithosphere is deformed by plate tectonics (convection currents and opposing frictional forces)

2. when the strain energy is too great and the rock has deformed significantly, this results in permanent deformation as the plates suddenly break apart

3. this releases large amounts of strain energy from the focus of the earthquake (the breaking point)

4. the strain energy released by an earthquake may have taken thousands of years to accumulate
- energy released in secs/mins
= high rate of energy is released

4. brittle failure of lithosphere usually occurs on planer fractures (faults), where 2 sides of fracture have slipped past each other

on planer fractures, if fault can slip easily...

... rock on either side = NOT deformed

= no strain energy and no earthquake
(aseismic slip/fault creep)

plate boundaries are defined by...

...concentrated bands of earthquakes that occur at them
(can be inter or intra plate)

all earthquakes occur in the....

lithosphere
= the only mechanical layer of the earth

why do most earthquakes occur at plate boundaries?

because plates are deformed mostly at their edges when they move past another plate
= most strain energy released

for an earthquake to occur...

... the sides of the fault must STICK TOGETHER/LOCK

= this allows the rocks to deform so that stresses become high enough for fault to slip abruptly

- releases large amounts of energy as an earthquake

hypocentre + strain energy

hypocentre = where the strain energy that is stored in the rock is first released

... marks the point where the fault began to rupture

(occurs directly beneath epicentre)

earthquakes have...

Magnitude
Frequency, Randomness + Regularity
Mitigation

Magnitude of earthquakes

measured by the Modified Mercalli scale and the Richter scale

MMI (Modified Mercalli Index)
= a survey of varying levels of damage across an area
- measures the actual intensity of damage caused by an earthquake
- subjective visual assessments by trained pros
- visual observations of ground of actual impact of earthquake
.... has URBAN BIAS as depends on damage done to an area/urban environment

MMS (modified magnitude scale)
= richter scale

MMI

uses a 12 point scale range in Roman numerals

I = imperceptible, measured only by seismometers
IV = moderate, likely to rattle doors/windows
VII = destructive, will cause considerable damage
XII = catastrophic, complete destruction

richter scale

a quantitive value based on the actual measurement of seismic energy

= a logarithmic scale that increases by a base of x10

Frequency, Randomness + Regularity of earthquakes

earthquakes show NO PREDICTABILITY
... can only predict where an earthquake could occur and which areas are at greatest risk due to plate tectonics

= impossible to predict when and where an earthquake will occur
- only able to estimate due to some plate margins being more seismically active than others (wider extent of earthquakes)

both the intensity + depth of earthquakes vary according to type of plate margin

Megathrust earthquakes have to occur at destructive plate margins with oceanic crust

earthquakes occur most often at plate boundaries = INTER Plate earthquakes
(only some occur within plates = INTRA)

Mitigation of earthquakes

warning signs
Risk assessments, contingency plans and earthquake engineering
Earthquakes proof buildings

warning signs of an earthquake

1. microquakes before main tremor
2. bulging of ground due to flexing of plate
3. decreasing radon gas concentration in groundwater
4. raised groundwater levels
5. electrical and magnetic changes within local rocks
6. increased argon gas content in soil
7. curious animal behaviour

Risk assessments, contingency plans and earthquake engineering

- buildings, roads, bridges, pipelines, etc., can be designed to withstand ground shaking

- GIS (geographic information systems) are used to prepare hazard maps that show area at greatest risk

- aid planning of urban growth and development to map areas that may be damaged the most

- public education (e.g., New Zealand earthquake evacuation drill once a year - ShakeOut)

- earthquake preparation checklist for houses/businesses

- evacuation zones created

earthquake proof buildings

1. reinforced lift shafts with tensioned cables
2. reinforced latticework foundations deep in bedrock
3. rubber shock absorbers between foundations and superstructure
4. rolling weights on roof to counteract shock waves
5. panels of marble and glass flexibly anchored to steel superstructure
6. steel frames which can sway during quake
7. automatic window shutters to prevent falling glass
8. open areas for evacuation
9. fire resident building materials
10. birdcage interlocking stee frame

TransAmerica Pyramid

San Francisco, USA

= survived a 7.1 magnitude earthquake in 1989 due to earthquake proof design
- built on a steel and concrete foundation