What Causes An Earthquake?
An Earthquake is a sudden movement of the earth's crust, which originates naturally at or below the surface of the earth.
The TWO most common cause of earthquakes are:
1.)they can be linked to explosive volcanic eruptions. earthquakes very common in areas with volcanos where they are either active or non active.
2.)they can be triggered by Tectonic activity with plate margins and faults.Most earthquakes world wide are this kind.
Volcanic Earthquakes:
The Result of a Volcanic Earthquake
Volcanic earthquakes are much less common than Tectonic ones.
They are triggered by the explosive eruption of a volcano.
Not all volcanoes erupt violently, most are 'quiet' for most of the time,this is the reason that they are extremely rare.
When a volcano explodes, it is likely that the earthquake effects will be limited to an area 10 to 20 miles around the baseof the volcano,but a tectonic earthquake may be felt around the globe.
The volcanoes which are most likely to explode violently are those which produce acid like lava. Acid like lava cools and sets very quickly when it touches the air.
This tends to clog the volcanic vent and block pressure from coming out. In the case of Mt Pelee, the lava became hard before it could flow down the sides of the volcano. Instead it formed a shield of solid rock within the volcano vent.
The only way that a blockage like that can be removed is by the build up of pressure until the lava is exploded out of the way.
The weakest part of the volcano will be the part which breaks away, sometimes leading to a sideways explosion like the Mt St.Helens eruption.
When very high levels of pressure grow, the result is an explosion that can be devastating,
Making an earthquake of large size.
Volcanoes that make free flowing basic lava almost never cause earthquakes.
The lava flows out of the vent and down the sides of the volcano, releasing pressure evenly. Since pressure doesn't build up, violent explosions do not happen.
Tectonic Plate Earthquakes:
Tectonic earthquakes are started when the crust begins to strain, and after a while it moves.
Plate tectonics explains how the crust of the Earth is made of several plates or large areas of crust which float on the Mantle of the Earth.
Since these plates are free to slowly move, they can either drift towards each other, away from each other or slide past each other.
Many of the earthquakes which we feel are located in the areas where plates collide or try to slide past each other.
San Adreas Fault,Callifornia
Maybe one of the most famous example of plates sliding past each other is the San Andreas Fault in California.
Here, two plates, the Pacific Plate and the North American Plate, are both moving in a roughly northwesterly direction, but one is moving faster than the other.
The San Francisco area is subjected to hundreds of small earthquakes every year as the two plates grind against each other.
Sometimes, a much larger movement occurs, starting a very violent Earthquake.
There might be short period of time which there is no shock after the major shock also known as foreshocks where the two plate temporalily stick and stop the earthquake and soon starts it again.
This might take the form of more frequent minor shocks as the rocks begin to move,called foreshocks , or a period of less frequent shocks as the two rock masses temporarily 'stick' and become locked together.
After the main shock there might be more movement of plate that settle into place. This is known as "aftershocks" these can cause major problems for resuce crew when they knock down an already weak building.
There are different types of shockwaves:
Primary Waves(P-waves): Are identical to sound waves. They are high frequency, short-wavelength waves which can pass through both solids and liquids. The ground is forced to move forwards and backwards. This produces small displacements of the ground compared to other shockwaves.
Primary waves under certain circumstances can change into Secondary Waves.
This is an example of a Primary Wave
Particles are push inward and outward
Secondary Waves (S-Waves) travel more slowly than P-Waves and arrive at any given point after the P-Waves. Like P-Waves they are high frequency, short-wavelength waves.They move in all directions away from the source, at speeds which depend upon the thickness of the rocks that they move through. They cannot move through liquids. On the surface of the Earth, S-Waves are responsible for the sideways displacement of walls and fences, leaving them 'S' shaped.
This is an example of a Secondary Wave
S-waves move particles at 90° angle to the direction of the wave.
Surface Waves (Su-Waves) are low frequency vibrations with a long wavelength. They are created close to the outer part of the crust. They are responsible for the majority of the building damage caused by earthquakes. This is because Su Waves have a movement almost the same as the waves in the sea. The ground is made to move in a circle motion, causing it to rise and fall as visible waves move across the ground. Together with other disasters such as landslides, fires and tsunamis these waves account for the loss of about 10,000 lives and over $100 million a year.
This is an example of a Surface Wave
Su-Waves move Particles in a circle motion across the ground
