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.

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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.


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

The distributions of these earthquakes
that do occur


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