Everyone has heard of earthquakes before, and news of the Nepal earthquake in April 2015 has brought this issue to full light. However, how much do you really know about earthquakes?
Earthquakes are triggered by the release of seismic energy, which when released causes vibrations on the surface of the Earth. Seismic energy can be released for many reasons, and the causes of which currently remain a subject of much interest.
The most common types of Earthquakes occur in the most brittle layer of the Earth – the crust. More specifically, the top 15km of the crust, known as the seismogenic layer, deforms in a brittle manner. As shear stresses build up to a maximum value, the tension increases. After this value is reached, rocks will deform and slip past each other along a fault plane. Such deformation can be seen on the surface of the Earth, with rocks of different type displaced relative to each other and fault scarps on the meter scale being made. Such earthquakes typically have large magnitudes and are very destructive, these include the Boxing Day earthquake of 2004. Japanese earthquake of 2011 and most recently the Nepalese earthquake of 2015 last month.
Geological setting determines the nature at which rocks build up tension. Earthquakes are common around plate margins, particularly convergent plate margins. Convergent plate margins are where two (or more) different tectonic plates are colliding. Depending on the nature of the crust involved in the collision (continental or oceanic), different features are formed. Oceanic-oceanic and oceanic-continental collisions, such as the Andes of South America for the latter, cause one of the plates to subduct and sink beneath the other. Oceanic crust always sinks beneath continental crust because it is both colder and denser than continental crust. A deep marine trench is produced at the point of buckling of the two plates, and mountain ranges, such as the Andes, are made as the overriding plate buckles. Continental-continental collisions cause the formation of high mountain ranges, such as the Himalayas, as both colliding plates override each other. This compressional strain causes stresses to build up on regional scales, and large magnitude earthquakes with shallow focuses result as rocks slip past each other and seismic energy is released.
Beneath this layer, is the Asthenoshpere. Here, rocks are partially melted, and the build-up of stresses is accommodated by ductile flow of the rocks. It is for this reason that Earthquakes due to slip motion do not occur at depths below about 15km. Deep earthquakes are thought to occur for many reasons, which include the release of energy due to phase changes between different mineral states as subducting plates move deeper into the Earth and are heated up. Deep earthquakes are much less frequent than shallow (<15km) earthquakes, however this is largely due to the inaccessibility of the deep Earth resulting into most lower magnitude deep earthquakes being drowned out against background seismic noise.
So, earthquakes are more common around plate margins, so if you live away from a plate margin you are safe, right? Not necessarily! Inter-continental earthquakes also occur, typically at far lower magnitude than those around plate margins however, large enough to cause damage. Deglaciation since the last ice age has caused pressure to be released on the crust above which glaciers once sat. The release of pressure is causing the Earth’s crust to rebound and return to its original equilibrium state. This can be thought of in the same way as jelly, when pressed down, returning to its original shape after removing your hand. The difference of course being the Earth’s surface operates on much longer length and time scales. Such earthquakes occur in Scandinavia.
Other causes of smaller, but man made earthquakes include hydraulic fracturing, also known as fracking. This is caused as drilling fluids cause rocks to fracture, and therefore release seismic energy to the surface. Sudden liquidation of fault planes can also reduce the coefficient of friction on the plane, such that rocks slip past each other when subject to lower stresses. Cases where wastewater from improperly bounded rubbish tips have infiltrated and migrated up buried faults have been known to cause earthquakes.
In summary, earthquakes can be thought of as the transfer of seismic energy. In cases of tectonic collisions, compressional stresses are released as seismic energy. When compressional strain is released above crust which was subject to previous loading, stresses are released as crust returns to the equilibrium state. Finally, human activity can also induce earthquakes when rocks are forced to deform and/or slip is caused is on faults due to the reduction of friction.
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