Earthquakes
Plate Tectonics
Plate Tectonics is a theory that the earth's outer shell is divided into small pieces that moves above the mantle.
Major plates include:
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There are 3 types of boundary:
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Why do earthquakes occur?
When one plate subducts underneath another, or two plates move against each other in a transform boundary, earthquakes happen because of the tension created between the moving plates. These processes are not smooth and often are rough when moving by or sliding underneath a plate. These rough movement then causes the earthquakes.
How are earthquakes measured?Today, we use a device called a seismometer. A seismometer detects vibrations when there is an earthquake and draws the magnitude of the vibrations onto a seismograph. We use the Richter Scale to measure the magnitude/strength of the earthquake on a scale from 0-10.
These devices can detect the smallest tremors, even the ones we cannot feel. |
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What are the types of earthquake waves?
There are 3 types of waves: Surface wave, primary body wave and secondary body wave. Surface wave happens on the surface but only covers a small range. The primary wave is the fastest because it is compression wave. It can travel through solids, liquids, and gases. The secondary wave is slower and is shear wave, waves that vibrate perpendicular to wave direction. It can only travel through solids. Surface waves are the most destructive though.
How they can be used to locate the epicentre of an earthquake?
First, you must find the time difference between the primary wave (P) and the secondary wave (S) of which you received at the seismometer station. Then, you work backward and find the distance at which both waves were at the same place. If the distance is 10km for example, then the epicentre is somewhere on the 10km radius of the seismometer station. Then you do the same procedure to at least 2 other seismometer stations. Now you can draw out the radius you found for each station (you should have 3 circles), and the point at which all the circles intersect is where the epicentre is.
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Deep focus vs. shallow focus earthquakes
An earthquake that happened due to a transform boundary is a shallow focused earthquake because the focus and the movements of the plates are on the crust; an earthquake occurs due to a convergent boundary have a deep focus because the movements of the plates happen underground.
Shallow focus: Usually at a depth of less than 70km;Can happen at all kinds of plate boundary Deep focus: Can be at a depth of 300km to 700km;Usually happens in the presence of a subducting slab of 45 degrees |