hazardous environments resulting from tectonic (crustal) movements

Hazardous environments resulting from tectonic (crustal) movements

earthquakes

• An earthquake is a sudden release of energy with a series of vibrations or seismic (shock) waves which originate from the focus – the point at which the plates release their tension or compression suddenly.

• The epicentre marks the point on the surface of the earth immediately above the focus of the earthquake.

• A large earthquake can be preceded by small tremors known as foreshocks and followed by numerous aftershocks.

Surface waves and body waves

• Seismic waves can travel along the surface of the earth i.e. surface waves

• They may also travel through the body of the earth i.e. body waves

• There are two types of body waves – P-waves (primary or pressure waves) and S-waves (transverse secondary waves), which are a series of oscillations at right angles to the direction of movement.

Cont…

• P-waves travel by compression and expansion, passing through rocks, gases and liquids.

• S-waves travel with a side-to-side motion, and can pass through solids but not liquids.

• Therefore the nature of rock and sediment beneath the ground influences the pattern of shocks and vibrations during an earthquake.

Liquefaction

• Unconsolidated sediments such as sand shake in a less predictable way than solid rock.

• P-waves can turn solid sediments into fluids like quicksand by disrupting sub-surface water conditions.

• This is known as liquefaction or fluidisation and can wreck foundations of large buildings.

Earthquakes and Plate boundaries

DESTRUCTIVE BOUNDARIES• The subducting plate at a destructive boundary

creates very deep earthquakes (700 km below ground).

• Plates move at a rate of 1.5 to 7.5 cm per year but often get stuck, creating increasing pressure and stress on rocks.

• The size of the earthquake depends upon the thickness of the descending slab and the rate of movement.

CONSTRUCTIVE BOUNDARIES

• Along mid-ocean ridges earthquakes are small because the crust is very hot, and brittle faults cannot extend more than a few kilometres.

Conservative margins

• Characterised by frequent small earth tremors and occasional severe earthquakes as plates try to slide past each other.

• The San Andreas fault is the most notorious of several hundred known transform faults in California.

The Richter Scale

• The Richter Scale measures earthquake magnitude.

• It is logarithmic i.e. an earthquake of 5 is ten times more powerful than one of 4, and 100 times more powerful than one of 3.

• It is numbered from 0 to 9

The Modified Mercalli Scale

• By contrast the Modified Mercalli Scale relates ground movement to commonplace observations around light bulbs and bookcases. Its advantage is that it allows ordinary eyewitnesses to assess the strength of an earthquake.

• (see table 3.2 p264)

Factors affecting earthquake damage

• Strength and depth of earthquake, and number of aftershocks. Shallow earthquakes are potentially more dangerous than deep ones.

• Population density• Type of buildings• Time of day

Cont….

• Distance from the epicentre• Type of rocks and sediments• Secondary hazards (mudslides, tsunami, fires,

contaminated water, disease, hunger and hypothermia)

• Level of economic development

Resultant hazards from earthquakes

• Most earthquakes occur with little if any advanced warning

• Damage to buildings, structures and transport systems cause most problems

• Most deaths are caused by this damage• Aftershocks may worsen damage• Displacement may fracture gas pipes leading

to fires

Geomorphological hazards

• Landslides• Liquefaction• Tsunamis

Haiti earthquake January 2010

• Located in the Caribbean as an island shared with the Dominican republic.

• Characterised by poverty, environmental degradation, corruption and violence.

• An earthquake recording 7 on the Richter scale just south of Port-au-Prince (the capital) just 13 km deep occurred on January 12th.

Cont…

• Aftershocks as strong as 5.9 occurred just 9 km below the surface and 56 km s.w. of the city.

• 300 000 people were killed, 250 000 injured and 1 000 000 made homeless.

• Most residents live in tin-roofed shacks perched on unstable steep ravines.

• International aid poured in but distribution was hampered because the capital had been destroyed.

Cont…

• Since 2000, more than $4 billion has been given to Haiti to rebuild communities and infrastructure devastated by tropical storms, floods and landslides, but mismanagement, a lack of co-ordination and attempts by global institutions to use Haiti as an economic test-bed are believed to have frustrated all efforts.

• A foreign debt of $1.5 billion has weighed down the economy.

• The earthquake took place where the Gonave microplate is squeezed between the North American Plate and the Caribbean Plate.

Earthquakes and human activity

Humans can trigger earthquakes in three ways:• Through underground disposal of liquid

wastes• By underground nuclear testing and

explosions• By increasing crustal loading.

What should people do about earthquakes?

• Do nothing and accept the hazard• Adjust to living in a hazardous environment –

strengthen your home• Leave the area

Earthquake preparedness

• Better forecasting and warning• Improve building design, building location• Establish emergency procedures

Predicting and monitoring earthquakes

Measurements of:• Small-scale ground surface changes• Small-scale uplift or subsidence• Ground tilt• Changes in rock stress

Cont…

• Micro-earthquake activity (clusters of small quakes)

• Anomalies in the Earth’s magnetic field• Changes in radon gas concentration• Changes in electrical resistivity of rocks

Tsunamis

• The term ‘tsunami’ means ‘harbour wave’ in Japanese.

• 90% of all tsunamis occur in the Pacific ocean• They are generally caused by earthquakes but

can be caused by volcanoes (Krakatoa in 1883) and landslides (Alaska 1964).

The Boxing Day Tsunami 2004

• This was a global disaster, killing people from nearly 30 countries, many of whom were tourists.

• Between 180 000 and 280 000 people were killed.• A massive earthquake (9 on the Richter scale) took

place off the coast of Indonesia, where the Indo-Australian plate is being subducted below the Eurasian plate.

• Sumatra, an Indonesian island, suffered the most deaths (70% of the populations of some villages perished), and 31 000 people in Sri Lanka alone died.