Earthquakes
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An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves
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About a million earthquakes occur each year
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Few Earthquakes are deadly because they must be large and occur in populated areas
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An earthquakes size can be calculated using a seismometer, also known as a seismograph
Elastic Rebound Theory
- As the tectonic plates move past each other, the rocks at the boundary stretch
- When the rocks stretch past their elastic limit, they jump back to release energy
- this sudden movement causes vibrations to travel through the Earth
Focus: where the rocks break
Epicenter: on the Earth's surface directly above the focus
Types of Waves
Primary Waves
Fastest
push/pull or compress waves
travel through solids and liquids
- Secondary Wave
- Slower than P Waves
- Transverse or travel side to side
- Travel through only solids
- Slowest type of wave
- Travels up/down or side to side
Measuring Earthquakes
- Mercalli Scale
- Based on intensity
- Qualitative and subjective
- Scale from 1 to 12
- Measures amount of damage
- Richter Scale
- Measures magnitude
- Quantitative
- Logarithmic
Effects of Earthquakes
- Damage is dependent on population density, construction, soil types and rocks
- It isn't the earthquake that kills, it is the effects:
- Buildings collapsing
- Fires
- Landslides
- Tsunamis
- Liquid faction (when soil and other soft sediment turns to liquid while the Earth is vibrating)
Predicting Earthquakes
- Long-term prediction
- Based on historical dates of previous earthquakes
- Measurements of plate movement
- Can only give vague estimates
- Distinguish high potential areas from low potential areas
- Short-term prediction
- To pinpoint time and location
- Almost impossible
- Seismologists look for signals:
- Foreshocks
- Odd Animal Behaviour
- Changes in land
- Changes in water table
- Radon gas detected
One of the most significant earthquakes of all time was The Great 1906 San Francisco earthquake. It was estimated to have a magnitude of 7.8 on the Richter scale.
The streets of San Francisco after the devestating earthquake.
Types of Earthquakes
There are two types of Earthquake : Earthquakes take place within the interior of a plate
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Interplate Earthquakes - Earthquakes that occur in the fault zones at plate boundaries
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Intraplate Earthquakes - Earthquakes take place within the interior of a plate
InterPlate Earthquake :
Interplate earthquakes occur along the three types of plate boundaries :
* Ocean spreading ridges
* Subduction zones
* Transform faults
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Ocean spreading ridges are places in the deep ocean basins where the plates move apart. As the plates separate, hot lava from the earth's mantle rises between them. The lava gradually cools, contracts, and cracks, creating faults. Most of these faults are normal faults. Along the faults, blocks of rock break and slide down away from the ridge, producing earthquakes. Near the spreading ridges, the plates are thin and weak. The rock has not cooled completely, so it is still somewhat flexible. For these reasons, large strains cannot build, and most earthquakes near spreading ridges are shallow and mild or moderate in severity.
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Subduction zones are places where two plates collide, and the edge of one plate pushes beneath the edge of the other in a process called subduction.
Because of the compression in these zones, many of the faults there are reverse faults. About 80 per cent of major earthquakes occur in subduction zones encircling the Pacific Ocean. In these areas, the plates under the Pacific Ocean are plunging beneath the plates carrying the continents. The grinding of the colder, brittle ocean plates beneath the continental plates creates huge strains. The world's deepest earthquakes occur in subduction zones down to a depth of about 700 kilometres. Below that depth, the rock is too warm and soft to break suddenly and cause earthquakes.
Transform faults are places where plates slide past each other horizontally. Strike-slip faults occur there. Earthquakes along transform faults may be large, but not as large or deep as those in subduction zones. One of the most famous transform faults is the San Andreas Fault. The slippage there is caused by the Pacific Plate moving past the North American Plate. The San Andreas Fault and its associated faults account for most of California's earthquakes.
Intraplate earthquakes are not as frequent or as large as those along plate boundaries. The largest intraplate earthquakes are about 100 times smaller than the largest interplate earthquakes. Intraplate earthquakes tend to occur in soft, weak areas of plate interiors. Scientists believe intraplate quakes may be caused by strains put on plate interiors by changes of temperature or pressure in the rock. Or the source of the strain may be a long distance away, at a plate boundary. These strains may produce quakes along normal, reverse or strike-slip faults.
Earthquakes can be further broken down and classified as: tecotonic, volcanic, and explostion.
Tectonic earthquakes - These occur when rocks in the earth's crust break due to geological forces created by movement .
Volcanic earthquakes - Earthquakes that result from tectonic forces which occur in conjunction with volcanic activity.
Collapse earthquakes - Are small earthquakes in underground caverns and mines.
Explosion earthquakes - Result from the explosion of nuclear and chemical devices.
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