Describe the frequency, origin and distribution of earthquakes at mid-ocean ridges, ocean basins, subduction zones and continental shields.

Earthquakes are happening 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 is recorded by a seismogram. These are located all over the world at different points. Only occasionally will a larger magnitude earthquake strike and cause damage to the region. Around the world there are many faults, depending where these faults are plays a major factor in determining where an earthquake will happen. It is these faults that are the reason for earthquakes. The type of fault will also determine how often an earthquake will happen.

A mid-ocean ridge occurs under the sea at a divergent boundary. This is where two plates are been pulled apart because of tension. This then allows new oceanic crust to be made in the divergent boundary, as magma rises and eventually sets on the sea floor.

If the plates on either side of the divergent boundary continue to spread then the ocean slowly becomes larger in width, a process called seafloor spreading. Mid-ocean ridges are characterised by a crack like valley at the divergent boundary. 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.

It is these normal faults that are the cause and therefore the origin of earthquakes at divergent boundaries. When the tension pulling apart the two plates becomes too much then the oceanic crust will fracture. This fracturing is caused by many normal faults happening as shown in the diagram. The normal faults happen because the crust is been extended. When the tension becomes too much the faults slip vertically. They move a large distance in a relatively short space of time, this is the cause of the earthquakes at divergent boundaries.

Divergent boundaries mostly occur on the sea floor and therefore the earthquakes that happen at these boundaries are distributed along the boundary. This means that the distributions of earthquakes at divergent boundaries are at shallow depths, where the crust is been pulled apart. The earthquakes happen at shallow depths because the normal faulting occurs near the sea floor, as a result of the tension. The normal faults are the cause of the earthquakes at these divergent boundaries.

The seafloor sees the most intense tectonic activity in the world, meaning that at the sites of mid-ocean ridges the frequency of earthquakes is very high.

An example of a mid-ocean ridge is the Mid-Atlantic ridge, there the seafloor is spreading at a rate of about 3cm 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.

Four major oceans make up most of the water in the world, The Atlantic (north & south), The Pacific, The Antarctic and The Indian Ocean. Within the basins of these oceans earthquakes can happen without been caused at Mid-ocean ridges, or a Subduction Zones.

When the earth??s crust is under tensional forces the crust will become much thinner than normal, if there is no fault. This means that the crust becomes weaker as it is thinner than normal. This can happen to the oceanic crust in the ocean basins, but will only cause an earthquake with a hot spot. A hot spot is an abnormal hot rising area of the mantle that supplies the lava for volcanoes. If at the same time a hot spot is directly below a thinned crust then the magma in the hot spot may hold too much pressure to be held by the thinner weakened crust. If this is the case then 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 its breaks through the crust at the sea bed 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, e.g. Hawaii.

The creation of these islands around the world has happened in other places. Frequent large earthquakes do not happen