Most earthquakes can be caused by two main events a volcanic eruption or plate tectonic movement. However a small earthquake can be caused by other events such as isostatic rebound, large undersea rock falls or even just waves hitting a nearby shoreline. However the latter would not be detected by humans.
Firstly earthquakes can be caused by volcanos erupting. Volcanoes are mostly found on areas of weaker crust so this can mean that just the mass of the volcano adds to the strain on the already fragile crust. This can cause fault lines or cracks to appear and when these cracks move or are put under any more pressure they can release this pressure in the form of seismic waves creating an earthquake. Volcanoes also can cause earthquakes due to the changing pressure underneath a volcano in its magma chamber, which is caused by the magma entering and leaving the magma chamber during an eruption. After an eruption and when the magma have left the magma chamber an empty space is left, which needs to be filled. This empty space is often filled by collapsing rocks, which can create seismic movements in the earths crust. However these earthquakes are usually small and therefore don’t often cause any damage.
However most earthquakes in the world are caused by plate tectonics. Earthquakes can happen on all of the 4 types of plate boundaries, but often the largest and most destructive occur on destructive plate boundaries. At these plate boundaries, the oceanic plate is pushed underneath the continental plate, as the oceanic plate is denser. As this happens they can scrape and catch each other due to friction. when this happens pressure builds up until it reaches the point where it has to be released, and the oceanic plate is forced under the continental plate further. When this occurs the energy is released in the form of seismic waves that travel out from the focus (the point at which the energy is released), creating an earthquake.
Small earthquakes can be caused by lots of things but often we don’t even notice them because they are too small for humans to detect, however seismometers can detect them. In Britain we do not have any active fault lines near us to cause any major earthquakes which is why we rarely experience them. However, we do occasionally get earthquakes of around 4th or 5th magnitude. These can sometimes be explained by a movement of the UK called isostatic rebound. When large volumes of ice covered the northwest of the UK in the last ice age the crust was pushed down with the weight of it. As it melted this pressure was released causing the northwest of the country to begin to rebound or rise up again and the south-east to sink. This is still happening today and every so often this can cause a sudden movement and hence a small earthquake. Although there aren’t any active fault lines near us some old undersea canyons can cause earthquakes near and around the UK due to the occasional large undersea rock fall or cliff collapse, the resulting earthquakes are often too small to be felt by humans though. Also if you look a seismometer you will notice that it is very rarely still, I noticed this when looking at my schools seismometer. My geography teacher explained to me that this is because any small movement can cause the earths crust to ‘quiver’. For example, waves crashing on to a beach 10 miles away from my school could explain the reason for why the seismometer is never still as waves release energy that can travel inland in the form or small vibrations.
- information on earthquakes caused by volcanoes sourced from: http://www.pnsn.org/outreach/earthquakesources/volcanic
- information on earthquakes caused by plate tectonics sourced from: http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/earthquakes_rev1.shtml
- information on the causes of smaller earthquakes and isostatic rebound: my own personal knowledge.