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Questions and Answers
What is the term used by scientists to refer to earthquakes?
What is the approximate thickness of the Earth's crust on continents?
What is the term used to describe the giant sea waves caused by earthquakes?
How many people die on average each year as a result of earthquakes?
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What is the primary cause of the worst earthquakes?
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What is the primary cause of the strongest and most destructive earthquakes?
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What is the point on Earth's surface immediately above the focus of an earthquake called?
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What type of seismic waves vibrate at right angles to the direction of wave travel?
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What is the method by which scientists can determine the location of an earthquake many thousands of miles away?
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What is the name of the type of seismic waves that travel along Earth's surface?
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Study Notes
Earthquakes
- Earthquakes are sudden shakings of the ground that occur when masses of rock change position below Earth's surface.
- The shifting masses send out shock waves that can alter the surface, causing cliffs to rise and great cracks to open in the ground.
- Earthquakes occur almost continuously, but most are only detectable by sensitive instruments called seismographs.
Effects of Earthquakes
- Some earthquakes cause major catastrophes, producing dramatic effects such as destroyed cities, broken dams, landslides, giant sea waves (tsunamis), and volcanic eruptions.
- A very great earthquake usually occurs at least once a year in some part of the world.
- On average, about 10,000 people die each year as a result of earthquakes.
Causes of Earthquakes
- Most worst earthquakes are associated with changes in the shape of Earth's outermost shell, particularly the crust.
- These tectonic earthquakes are generated by the rapid release of strain energy stored within the rocks of the crust, which is about 22 miles (35 kilometers) thick on continents.
- A small proportion of earthquakes are associated with human activity, such as dynamite or atomic explosions, injection of liquid wastes, and pressures from holding vast amounts of water in reservoirs.
Faults
- The strongest and most destructive quakes are associated with ruptures of the crust, known as faults.
- Faults are present in most regions of the world, but earthquakes are not associated with all of them.
- Pressures from within Earth strain the tectonic plates that make up the crust, causing the plates to move along faults, releasing energy and shaking the ground above.
Shock Waves
- The shifting rock in an earthquake causes shock waves, or seismic waves, to spread through the rock in all directions.
- There are two broad classes of seismic waves: body waves and surface waves.
- Body waves include P (primary) waves and S (secondary) waves, which travel within the body of Earth.
- P waves compress and expand the rock, vibrating in the same direction as the wave travel.
- S waves vibrate at right angles to the direction of wave travel, causing particles to move up and down or from side to side.
- Surface waves, including Love and Rayleigh waves, travel along Earth's surface after body waves have passed through the body of Earth.
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