Mod 6.1 - Geological Natural Disasters (Earthquakes) Flashcards
How/why do earthquakes occur?
Earthquakes are the result of sudden movement along faults within the Earth. Such movement on the faults is generally a response to long-term deformation and the build-up of stress.
The movement of plates grinding, crumpling, stretching or subducting beneath a neighbouring plate results in continual ‘elastic strain’ energy storage that is inevitably followed by energy release in the form of seismic waves which propagate through the Earth and cause the ground surface to shake.
What is the Focus?
Earthquakes occur underground at a point called the focus (or hypocentre). This is the point where the stored elastic potential energy is released due to a fault rupture. The point on the surface above the focus is called the epicentre.
Depth of earthquakes at divergent plate boundaries
With divergent boundaries, the plates are moving apart, creating transform faults which can cause earthquakes. Additionally, as new crust is created and other crust is pushed apart, the crust can crack and form faults where earthquakes occur. As there is no subduction, frictional stress builds up in the laterally moving surface rock, and this energy is usually released as low magnitude earthquakes with focus depths at less than 20km.
Depth of earthquakes at continental
collisions
With continental collisions at convergent boundaries, the colliding plates will both be made of continental crust. As such, neither plate will subduct, but instead, both plates will compress against one another, forming non volcanic mountain ranges, since the lack of subduction and melting means no volcanoes will form. In the continental crust, pressure builds up and is released as shock waves in faults within the crust. These usually will be strong earthquakes ranging from shallow to medium depth, since there is no deep subducting plate to cause deep quakes.
Depth of earthquakes at subduction zones
In subduction zones, the denser oceanic crust subducts beneath either another oceanic plate or a continental plate. The subducting slab fractures and compresses as it is exposed to increasing temperature and pressure while descending into the mantle, building up stress between the plates. Because these plates don’t melt until around 700km deep, the focus can occur anywhere between the surface and this depth, called the Benioff zone.
Depth of earthquakes at intraplate zones
In intraplate earthquakes, as a plate moves over a hotter or colder asthenosphere, the stress regime changes as the plate is stressed or compressed. If the stress builds up too much, rock strata may fracture and move across fault planes. Energy is released as earthquake waves. These are usually weak surface earthquakes.
Ground motion as a hazard
Earthquakes generate a series of seismic waves which vibrate from an earthquake’s focus in a circular fashion. The surface waves are made up of two components - Love waves that produce a rolling motion of the ground, and Rayleigh waves that cause a sideways movement. This ground motion leads to cracks and weakening of human-built structures such as buildings, bridges, roads and dams (since such structures cannot move sympathetically with the earth in all directions simultaneously).
Tsunamis as a hazard
Tsunamis are large and powerful waves caused by the displacement of water by an oceanic volcanic eruption, earthquake or landslide. They can wipe out structures and ecosystems, causing death and destruction as the surge contains large amounts of energy and can carry material and debris. Additionally, tsunamis can cause flooding across land due to the high volume of water mass.
Landslides as a hazard
A landslide is a geologic process that involves mass movements of rock, debris, and soil down a slope of land. It is caused by disturbances in the natural stability of a slope, and the movement that occurs during an earthquake can trigger them. Landslides can result in death of wildlife and humans, as well as destruction of built structures due to the energy and material involved.
Using a named example (tsunami), contrast hazard and disaster.
Tsunamis are powerful waves caused by the displacement of water after an earthquake or volcanic eruption. The hazards of these waves would be coastal flooding. It would be considered a disaster if it hit populated areas and caused many deaths and much destruction.
If a tsunami was to be generated in the open ocean and the energy dissipated significantly before reaching landfall it may not be disastrous. If the wave energy was too insignificant to inundate residential regions than it would not cause loss or life or major infrastructure damage, and therefore would not be a disaster.
