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 along the Hawaiian chain, it is an essentially an asesimic ridge. Therefore the frequency of earthquakes caused in ocean basins by hot spots is very low.
The distributions of these earthquakes that do occur happen at shallow depths. This is because the origin is in the crust, which has been thinned because of tension.
A subduction zone is where two plates collide and one is forced below the other, they occur at convergent boundaries. They collide because of compression forces, pushing them into each other. One plate is subducted below the other into the mantle, where it will be recycled. An example of this is shown below with the Pacific plate subducting under the Eurasian plate.
The two plates want to travel in opposite directions, they want to go straight into each other. This causes the pressure to build up over a long period of time, as the two plates push at each other. As time progresses one of the plates will start to be bent downward under the other one because of the extreme force, however does not slip, just bends. This is because of the friction between the two plates is enough to allow them to bend, without slipping. This is a very slow but continuous movement, maybe only a few millimetres 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. The fault then ruptures, moving a large distance in a short space of time. The plates then snap back into a new position, forcing the already undercutting plate to dive down even further under the other. The collisions of two plates generally produce large forces in the plates. These forces result in the triggering of the earthquakes within the subduction zones.
The frequency of earthquakes in Subduction zones is about the same as that in the mid-ocean ridges. This is because the plates cover the globe, and if they separate in one place then in another place one-plate sinks below another. This means that the triggering of an earthquake at a divergent boundary triggers an earthquake at a convergent plate. Meaning that the frequency of earthquakes at Subduction zones is the same as at Mid-ocean ridges, which is very high.
The earthquakes at convergent boundaries are distributed at different points. The deep focus earthquakes occur along the already subducted plate. Shallow focus earthquakes occur just at the point where one plate starts to be thrust under the other. These earthquakes tend to be more common than the deeper earthquakes. This is shown on the diagram on the left. The red dots show the distribution of earthquakes at a convergent boundary.
Continental shields are extensively flat tectonically stable interiors of the continents, composed of ancient rocks. Most of the stress that builds up by tectonic movements is released in earthquakes at the plate boundaries. However stress can also build up in the interiors of plates. Old fault lines in the plates are weaker than the surrounding rocks, these old fault lines cover many continents, crossing all over each other. The old faults can slip if the stress becomes too much from recent plate movements, which will cause an unexpected earthquake. This can be a problem as many old fault lines are not known, and many are away from modern plate boundaries that exist today. This is potentially dangerous as many modern settlements may be at risk from earthquakes, even though they are not near modern day faults.
The distribution of earthquakes at continental shields is not yet known, as scientists do not know whether these earthquakes will strike the same region within a plate.
The strength of these inter-plate earthquakes are relatively small, compared to boundary earthquakes. The frequency is also very small, the last major inter plate earthquake was in Latur-India in 1993. However they can catch regions totally unexpected because they can affect areas with no previous earthquake history. Also the energy of the earthquake is spread out further without losing as much. Due to the older hard rocks that transmit energy better, than the deformed broken younger rocks. This can cause more damage to a larger region.
Earthquakes are common events and are happening all the time. They can be caused by many different factors within the earth¡¦s interior. Depending on the type of area that they happen in will determine the strength of the earthquake, and the frequency of earthquakes within the region. The distribution of earthquakes within an area will much depend upon
what caused the earthquake to happen in the first place. We understand today how earthquakes are caused, and we can record where they happen every day of the year. This has helped us to learn and understand about earthquakes in much detail. We now only miss one important factor that we all would like to know, when and where the next one will be. In truth it must be said that today we are still not close to predicting earthquakes even with all the technology that is available.
Understanding Earth 2nd edition by Frank Press and Raymond Siever.
Microsoft Encarta Encyclopaedia 1998.