EarthquakesDescribe the frequency, origin and distributionof earthquakes at mid-ocean ridges, ocean basins, subduction zones andcontinental shields. Earthquakes are happening almost everydayall over the world.
Most of the time earthquakes are not strong enoughto be felt by people, but the shaking caused by an earthquake is recordedby a seismogram. These are located all over the world at different points. Only occasionally will a larger magnitude earthquake strike and cause damageto the region. Around the world there are many faults, depending wherethese faults are plays a major factor in determining where an earthquakewill happen. It is these faults that are the reason for earthquakes.
Thetype of fault will also determine how often an earthquake will happen. A mid-ocean ridge occurs under the seaat a divergent boundary. This is where two plates are been pulled apartbecause of tension. This then allows new oceanic crust to be made in thedivergent boundary, as magma rises and eventually sets on the sea floor.
If the plates on either side of the divergentboundary continue to spread then the ocean slowly becomes larger in width,a process called seafloor spreading. Mid-ocean ridges are characterisedby a crack like valley at the divergent boundary. This crack like valleyis caused by the tension pulling the plates apart, causing normal faultingto occur a number of times in the divergent boundary. It is these normal faults that are thecause and therefore the origin of earthquakes at divergent boundaries. When the tension pulling apart the two plates becomes too much then theoceanic crust will fracture. This fracturing is caused by many normal faultshappening as shown in the diagram.
The normal faults happen because thecrust is been extended. When the tension becomes too much the faults slipvertically. 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 thesea floor and therefore the earthquakes that happen at these boundariesare distributed along the boundary. This means that the distributions ofearthquakes at divergent boundaries are at shallow depths, where the crustis been pulled apart.
The earthquakes happen at shallow depths becausethe 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 tectonicactivity in the world, meaning that at the sites of mid-ocean ridges thefrequency of earthquakes is very high. An example of a mid-ocean ridge is theMid-Atlantic ridge, there the seafloor is spreading at a rate of about3cm per year.
The frequency of earthquakes at a mid-ocean ridge will dependon how much tension is happening at that point. The more tension meansthe more seafloor spreading, resulting in a higher frequency of earthquakesat a particular mid-ocean ridge. Four major oceans make up most of the waterin the world, The Atlantic (north & south), The Pacific, The Antarcticand The Indian Ocean. Within the basins of these oceans earthquakes canhappen without been caused at Mid-ocean ridges, or a Subduction Zones.
When the earth?¦s crust isunder tensional forces the crust will become much thinner than normal,if there is no fault. This means that the crust becomes weaker as it isthinner than normal. This can happen to the oceanic crust in the oceanbasins, but will only cause an earthquake with a hot spot. A hot spot isan 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 themagma in the hot spot may hold too much pressure to be held by the thinnerweakened 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 itsway up can trigger earthquakes as it breaks through the crust. When itsbreaks through the crust at the sea bed eventually a volcanic island willbe formed in the middle of the ocean. Due to plate movements this can leadto the creation of mid-plate chains of basaltic volcanic islands, e. g. Hawaii.
The creation of these islands around theworld has happened in other places. Frequent large earthquakes do not happenalong the Hawaiian chain, it is an essentially an asesimic ridge. Thereforethe frequency of earthquakes caused in ocean basins by hot spots is verylow.The distributions of these earthquakesthat do occur
