Managing Natural Hazards

Earthquakes and Volcanoes

• The structure of the Earth:

  

• Tectonic plate: a piece of lithosphere that moves slowly. It is made of crust and upper mantle.

• Where the convention currents rise to the surface, the plates move away from each other (and vice-versa).

• Plate boundaries: where two or more plates meet.

• Constructive (divergent) plate boundary:

  • Two plates move away from each other.

  • When two oceanic plates move away, magma rises to the surface (convection current) and solidifies when it comes in contact with cold ocean water.

  • The magma turns to lava and forms a new basaltic ocean crust.

  • They can also form shields or basic volcanoes (submarines) and have non-explosive eruptions.

  • This is known as sea-floor spreading or ridge push.

  • Small Earthquakes are triggered.

  • If two continental plates move away from each other, a rift valley may form.

• Destructive (converging) plate boundary:

  • Two plates move towards each other.

  • When an oceanic plate and continental plate move towards each other, the denser (oceanic) plate is forced down (subducted) under the lighter (continental) plate.

  • This happens in the subduction zone, and an ocean trench is formed.

  • The friction between the plates triggers Earthquakes.

  • The heat produced due to friction turns the descending plate into magma.

  • The magma starts to rise and erupt (due to pressure) through a weakness in the crust as an explosive composite volcano.

  • Fold mountains are also formed.

  • The magma that erupts at the surface forms a chain of volcanic islands called an island arc.

  • If two continental plates move toward each other, the sediments between the two plates are compressed (collision zone) and pushed upwards to form fold mountains.

  • Earthquakes occur, but no volcanic activity as there’s no subduction of oceanic plates.

• Conservative plate boundary:

  • Two plates slide past each other.

  • They move at different speeds.

  • The plates get locked together, and pressure builds up until it is released as an Earthquake.

• The magnitude (strength) of an Earthquake is measured using a seismometer on the Richter scale.

• Distribution and causes of volcanoes:

• Caused due to the tectonic activity (refer to 6.1; Plate boundaries).

• Found on constructive and destructive plate boundaries and hotspots.

Distribution and causes of Earthquakes:

• Caused due to the tectonic activity (refer to 6.1; Plate boundaries).

• Occur mostly on the destructive and conservative plate boundaries (and sometimes on the constructive plate boundaries).

• Earthquakes:

• Focus: where the Earthquake begins underground.

• Epicenter: point on the surface above the focus.

• Seismometer: an instrument used to measure the magnitude of an Earthquake (on the Richter scale).

Factors that affect the impact of an Earthquake:

• Location of the epicenter;

• Time of the Earthquake;

• Geology of the area;

• Relief of the area;

• Severity of aftershocks;

• Level of development of human settlement;

• Population density;

• Building density and strength.

Tropical Cyclones

• Causes of tropical cyclones:

• Ocean surface temperature of at least 27°C.

  • Warm water provides the energy to evaporate more water that rises, and condenses, releasing huge amounts of energy.

• Ocean depth of at least 60m deep.

• These conditions occurring between 5° and 20° north and south to have sufficient Coriolis effect, making the air spin.

• Very little wind shear (change in wind speed or direction).

  • Allows the vertical development of the storm.

• Distribution of tropical cyclones:

• Between 5° and 20° north and south.

• They do not form on the equator because the Coriolis effect there is 0.

  • The air at the equator tends to flow straight from high pressure to low pressure, without any rotation.

Flooding

Drought

• Causes of drought:

• Lack of rain caused by prolonged high pressure:

  • Air in a high-pressure system sinks and doesn’t form rain clouds.

• Effect of El Niño Southern Oscillation and La Niña:

  • El Niño causes the surface water in the Pacific Ocean along South America to be warmer.
    

  • These warmer waters alter storm patterns and can cause droughts in Australia.

  • Whereas, La Niña causes the temperature of the water along South America to

  • decrease.
    

  • The cooler conditions cause drought in parts of North and South America.

  • (refer to Section 5.2 World Fisheries; El Niño Southern Oscillation (ENSO))

• Effect of climate change:

  • Warmer worldwide temperatures cause the rainfall to decrease in some parts of the world, leading to drought.

The Impacts of Natural Hazards

Impacts of tectonic events:

• Damage to buildings and infrastructure;

• Fires from ruptures of gas pipes;

• Tsunamis hit coastlines;

• Landslides cover buildings and roads;

• Destruction of farmland, leading to starvation;

• Loss of wildlife habitats;

• Water-related diseases because victims are in temporary accommodation with no sanitation or clean water;

  • Water is also contaminated by broken sewage pipes or untreated sewage.

• Loss of life;

• Trauma, poor mental health;

• Financial losses when repairing the damage.

Impacts of tropical cyclones:

• Flooding from storm surges and heavy rainfall;

• Loss of life;

• Damage to buildings and infrastructure;

• Disruption of electricity, transport and water supply;

• Water-borne diseases;

• Economic loss as production is halted;

• Damage to crops, food shortages and loss of export earnings;

• Loss of wildlife habitats.

Impacts of flooding:

• Loss of life;

• Damage to buildings and infrastructure;

• Contamination of water supplies leading to disease;

• Loss of crops and livestock leading to food shortages;

• Deposition of silt from the flood waters;

• Recharge of groundwater stores;

• Rivers may change course;

• Financial losses when repairing the damage.

Impacts of droughts:

• Water sources dry up, forcing people to travel long distances to fetch water;

• Decline in crop yields;

• Loss of crops, livestock, plants and wildlife;

• Decrease in land prices as production declines and farmers lose money;

• Migration from rural to urban areas;

• Unemployment;

• Increase in food prices;

• Health problems due to malnutrition;

• Soil erosion, leading to desertification;

• Increased risk of wildfires and poor air quality;

• Conflicts over water usage and food.

Managing the Impacts of Natural Hazards

• Volcanoes:

• Prediction:

  • Seismometers can be used to monitor tremors caused by rising magma;

  • Satellites using heat-seeking cameras can be used to monitor increasing ground temperatures;

  • Tiltmeters (measure very subtle changes in the surface of the Earth as magma accumulates) and GPS can be used to monitor changes in volcano shape;

  • Emissions of steam and gas (sulfur dioxide) can be monitored.

• Preparation and protection:

  • Volcano hazard map (study past eruptions);

  • Lava diversion channels and lava barriers ;

  • Spraying lava with water;

  • Halting lava advance by dropping concrete slabs into the flow;

  • Building reinforcements (sloping roofs to protect against ashfall).

• Earthquakes:

• Prediction:

  • Monitor tremors (using seismometers), groundwater levels, and radon gas;

  • Epicentres and frequencies of past events can be mapped to check if a pattern is developing;

  • Measurement of local magnetic fields;

  • A hazard zone map can be drawn (geological info and ground stability);

  • Unusual animal behaviour.

• Preparation and protection:

  • Earthquake-proof or aseismic buildings. Older buildings can be modernised;

  • Smart meters to switch off gas supplies, preventing fires;

  • Land-use planning: important services (schools, hospitals) must be built in low-risk areas.

• Tropical cyclones:

• Prediction:

  • Tracked using satellites.

• Preparation and protection:

  • Cyclone shelters;

  • Embankments along the coast;

  • Preserve mangrove swamps to absorb the energy of storm surges.

• Flooding:

• Prediction:

  • Monitoring the amount of rainfall and river discharge using an ADV;

  • Using the features of the drainage basin and type of storm to determine the severity of the flood.

• Preparation and protection:

  • Complex engineering projects (levees, flood barriers and dams);

  • Soft engineering projects (afforestation and storage basins);

  • Increasing the river channel (clearing vegetation);

  • Land-use planning to restrict development on floodplains;

  • Use of sandbags and pumps;

  • Adapt houses to position power sockets 1.5 m above ground level to prevent electrocution.

• Droughts:

• Prediction:

  • Monitoring precipitation and temperature.

• Preparation and protection:

  • Increase water supplies (dams, reservoirs, wells, percolation ponds, aquifers, pumps, water transfer by pipeline, and desalination);

  • Water conservation (storage tanks, spray irrigation, drought-tolerant crops, recycling water and reducing deforestation);

  • Agricultural improvements (shelterbelts to decrease wind and evaporation, bunds to increase infiltration and fencing to control overgrazing);

  • The government stockpiling supplies of water, food and medicine.

Opportunities Presented by Natural Hazards

• Individuals may want to be near family and friends.

• Confidence in prediction, preparation and protection.

• Employment opportunities e.g. tourism.

• No choice in moving if there is pressure on land or if it is too expensive to move.

• After a volcanic eruption, fertile soils are created that produce high crop yields.

• The scenery can be spectacular;

• Geothermal energy can be obtained easily;

• Possibility of mining minerals such as sulfur, diamonds and gold.

• Living near rivers may provide a source of food, water for drinking and irrigation.

• Communication may be easier;

• Flat land on either side is available for building on.