Earthquakes

Earthquakes

Earthquakes occur almost everyday all over the world. Most of the time earthquakes are not strong enough to be felt by people, but the shaking caused by an earthquake can be recorded by a seismometer. These machines are located all over the world to ensure detection of earthquakes of all strengths. Only occasionally will a larger magnitude earthquake strike and cause damage to the region. There are many faults around the world and depending on where these faults are plays a major factor in determining where an earthquake will occur. It is these faults that are the reason for earthquakes. The information seismologists know about past earthquakes and earthquakes in general give them a limited ability to generally predict when and where earthquakes are going to occur.

An earthquake is the shaking of Earth's surface caused by rapid movement of the earth's rocky outer layer. "Earthquakes occur when tension stored in rocks suddenly releases" (Vogt 12). Faults occur at these places where rocks on either side of the crack have moved. Oceans are very common places for major tectonic plates to shift. When two plates separate, new oceanic crust is made near the fault as magma rises and eventually sets on the sea floor. If the plates on either side of the fault continue to spread then the ocean slowly becomes larger in width. This is called seafloor spreading. Mid-ocean ridges are characterized by a crack like valley at the fault. This crack like valley is caused by the tension pulling the plates apart, causing normal faulting to occur a number of times in the divergent boundary.

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The most common type of fault is a normal or dip-slip fault. These occur when two blocks of earth are thrust toward each other, causing one to ride up over the other (Britt 1). "The hanging wall moves downward relative to the footwall" (Tarbuck 244). A reverse fault is the opposite of a normal fault. "The hanging wall moves upward relative to the footwall" (Tarbuck 244). A very powerful type of fault is the strike-slip fault that occurs when two plates slide past one another. The San Andreas Fault in California is a very good example of this type of fault. A great deal of damage is done when any type of fault shakes the soil under structures in a low-lying, waterlogged areas, causing liquefaction. Liquefaction occurs when an earthquake shakes the wet, sandy soil near a body of water. The soil is forced to sink while the water is pushed up causing the surface to become very weak and dangerous.

The seafloor sees the most intense tectonic activity in the world. The frequency of earthquakes is very high at the sites of mid-ocean ridges. An example of a mid-ocean ridge is the Mid-Atlantic ridge where the seafloor is spreading at a rate of about 3 cm per year. The frequency of earthquakes at a mid-ocean ridge will depend on how much tension is happening at that point. The more tension means the more seafloor spreading, resulting in a higher frequency of earthquakes at a particular mid-ocean ridge.

When Earth's crust is under tensional forces, the crust will become much thinner than normal and cause it to become weak. This can happen to the oceanic crust in the ocean basins, but will only cause an earthquake in a hot spot. A hot spot is an abnormal rising area of the mantle that supplies the lava for volcanoes. If a hot spot is directly below a thinned crust, then the magma in the hot spot may produce too much pressure to be held by the thinner weakened crust and if this happens, the magma can penetrate the lithosphere, and eventually erupt on the surface. The action of the magma forcing its way up can trigger earthquakes as it breaks through the crust. When magma breaks through

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the crust in the sea, eventually a volcanic island will be formed in the middle of the ocean. Due to plate movements this can lead to the creation of mid-plate chains of basaltic volcanic islands like Hawaii.

A subduction zone is where two plates collide and one is forced below the other. They collide because of compression forces, pushing them into each other. One plate is pushed below the other into the mantle, where it will be recycled. An example of this is the Pacific plate subducting under the Eurasian plate.

Convergent boundaries occur when the pressure between two plates builds up over a long period of time as they push at each other (Vogt 26). As time progresses, one of the plates will start to bend downward under the other one due to extreme force. The friction between the two plates is high enough to allow them to bend without slipping. This is a very slow but continuous movement, maybe only a few millimeters every year. Every fraction moved by the plates increases the build-up of elastic strain energy within the rock. The rock continues to store this energy from a few decades to a few thousand years. "An earthquake will happen when the strain in the rocks exceeds that of the limit of the rocks" (Plate 1). The fault then ruptures,