9.1. Earthquakes Flashcards
Global Distribution of Earthquakes
- broad belts of earthquakes are associated with subduction zones
- collision boundaries are also associated with broad belts of earthquakes
- exceptions include intraplate earthquakes (found in the centre of plates that might be on old fault lines or weaknesses that are often unknown
Focus
the point where the rocks start to fracture
Epiccentre
the point on the earth’s surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins
Cause of Earthquakes
- Movement of plates on all types of plate boundaries
- When plates move and get stuck, there is friction and this builds up
- There is increasing tension, as convection currents still try to move the plates
- When the friction is released (seismic waves), the ground shakes in the form of an earthquake
- There are two types of seismic waves released: P waves and S waves
Primary Waves
- first seismic waves released from earthquakes
- longitudinal waves
- relatively weak
- causes the surface to move in a back and forth motion by stretching and compressing the material they pass through
- can pass through solids and liquids (able to move pass all layers of the Earth)
Secondary Waves
- next seismic waves to arrive from earthquake
- transverse waves
- cause the crust to move side to side or up and down as they pass
- can only pass through solids (does not go through the liquid outer core
Earthquake Magnitude
Total amount of energy released by a earthquakes
Richter scale
- Measures magnitude (energy released)
- Has a logarithmic scale
- Mercalli scale - measures damage and destruction
Mercalli scale
- Measures damage and destruction of Earthquakes
Factors affecting earthquake damage
1) magnitude
2) depth of focus
3) bedrock nature
4) location (population density, distance from epiccentre)
5) building structure
6) earthquake preparedness
7) secondary effects
8) levels of development
How magnitude of earthquakes influence their damage
- measured by RIchter scale (1 to 10)
- has a logarithmic scale
- higher magnitude –> more destruction
How depth of earthquakes influence their damage
- shallow earthquakes that occur close to the surface tend to result in a greater intensity of surface shaking and often cause the greatest loss of life and damage to property
- often associated with destructive margins
How bedrock nature influences earthquake damage
- some materials are vulnerable to become ‘jelly-like’ when shaken
- commonly associated with clay and silts
- this is liquefaction
- building foundations become unstable and slopes potentially undergo mass movements
How location (population density, distance from epiccentre) influences earthquake damage
- earthquakes are only hazards when it impacts human activity
- earthquakes that occur at places with high population densities –> more damage
- If closer to the epiccentre of the earthquake there would be more damage as waves are strongest there
How building structure influences earthquake damage
- “earthquakes don’t kill people, buildings do”
- vast majority of suffering results from the collapse of building structures such as buildings and bridges
- appropriate building designs to counteract earthquakes can reduce impacts