Seismic Hazards

Distribution of Earthquakes:

Formation of an Earthquake:

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• Earthquakes are caused by sudden movements along a fault line.
• The movements are preceded by a gradual build-up of tectonic strain, which stores elastic energy in crustal rocks.
• When the pressure exceeds the strength of the rock, the rock fractures. The point of the rupture is called the hypocentre and can vary in depth.
• This produces a sudden release of energy, creating seismic waves that radiate from the fracture.
• The brittle crust rebounds either side of the fracture, making it shake.

The Benioff Zone and Subduction Processes

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• The Benioff Zone is an area of seismicity corresponding with a subducted piece of crust.
• The different speeds and movements of rock at this point produce numerous earthquakes.
• It is the site for deep-focus earthquakes in particular. The depth of the hypocentre of an earthquake can help to determine its magnitude.
• Locked faults are common in Benioff Zones. This is when the frictional resistance on a fault is greater than the shear stress across the fault, meaning that it doesn't move and is left to build up energy for an extended period of time.

Seismic Waves:

• There are three type of seismic wave. These can be measured with a seismometer.
• P (Primary) waves are usually caused by compression. They spread quickly from the fault at a rate of 8km/second.
• S (Secondary) waves vibrate at right angles to the direction of travel and cannot move through liquids. They are much slower than P waves and move at a speed of 4km/s.
• L (Love) waves are surface waves with a vibration occurring in the horizontal plain. These waves have a high amplitude.
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Measuring Earthquakes:

Primary Hazards of Earthquakes

• Death and destruction
• Damage to the landscape
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Secondary Hazards of Earthquakes

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• Fires
• Soil liquefaction - soil turning to a liquid causes buildings to tilt and collapse
• Tsunamis
• Landslides
• Restriction of water supplies
• Spread of disease
• Economic recession
• Increase in homelessness
• Increase in crime rates

Tsunamis

• Tsunamis are initiated by undersea earthquakes, landslides and, sometimes, volcanic eruptions. A tsunami is not a single wave but a series of waves.
• Most tsunamis occur near subduction zones, particularly the Japan-Taiwan Island Arc and South America.
• They are characterised by:
• Long wavelengths (150-1000km)
• Low amplitude (wave height) at sea - they grow bigger near land
• Fast velocities (up to 600km/h)
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Factors affecting Earthquake Hazards

• Earthquakes
• Location - For example, areas around the Pacific Ocean in particular are susceptible to tectonic hazards due to their location near the Pacific Ring of Fire. Location also plays a large role in determining climate.
• Population - A larger population will mean more people are vulnerable to natural hazards.
• Nature of settlements and the buildings within - durability of buildings within a settlement will often determine the amount of damage after an earthquake or eruption.
• Telecommunications - though lines may be damaged in an event, good telecommunication networks in an area can affect responses to an event.
• Tsunamis
• The duration of the event
• The wave amplitude, water column displacement and the distance travelled
• The physical geography of the coast - water depth and gradient of the coastline
• The degree of coastal ecosystem buffer - eg. mangroves and coral reefs
• The timing of the event (night vs day) and the quality of warning systems
• The degree of coastal development and proximity of the development to the coastline.