question: Why does the earth shake in earthquakes?
comment: The ground is literally moving. So, picture the ground you are standing on as a Cinnamon Toast Crunch floating in a bowl of cereal. An earthquake is when your piece of toast crunch rubs against another piece of toast crunch.
passages: Section::::Examples.:Elephants.
   Section::::Examples.:White-lipped frog.
   Section::::Causes.:Carbon Capture and Storage.:Failure mechanisms due to fluid injection.
   There are various theories of the cause of diastrophic movement such as being the result of pressures exerted by convection currents in the mantle or the rise of magma through the crust. Other deformations are caused by meteorite impact and combinations of gravity and erosion such as landslides and slumping.
   Injecting liquids into waste disposal wells, most commonly in disposing of produced water from oil and natural gas wells, has been known to cause earthquakes. This high-saline water is usually pumped into salt water disposal (SWD) wells. The resulting increase in subsurface pore pressure can trigger movement along faults, resulting in earthquakes.
  
  One of the first known examples was from the Rocky Mountain Arsenal, northeast of Denver. In 1961, waste water was injected into deep strata, and this was later found to have caused a series of earthquakes.
   Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.
   In a long-term study of small earthquakes triggered by a large event, the 1992 Landers, California earthquake, he and his colleagues discovered an annual cycle: fall had the greatest number of earthquakes, spring the least. The team found that this pattern could be related to barometric pressure changes: less pressure meant reduced stress on the faults, which permitted them to move more frequently. More recent work by him and his collaborators suggests that changes in the state of stress of the lithosphere induced by a large earthquake can alter the strength of faults and the seismic activity in an earthquake-prone area.
   Section::::Causes.:Carbon Capture and Storage.:Monitoring.
   This is due to the fact that areas of the basin are superimposed upon one another, which some interpret to show pulses of deformation and uplift that occur at irregular times and places.
  
  Section::::Tectonic mechanisms.:Gravity spreading.
  
  The Angola Basin is also highly characterized by gravity spreading where energy is released when the center of gravity lowers as crustal material thins.
  
  This spreading mechanism requires at least some deformation as opposed to the breakup of rigid blocks.
   Most occurrences of bimodal volcanism are associated with thinning of the crust and the presence of such rocks in metamorphic sequences has been used to provide evidence for past rifting events. Most examples come from areas of active continental rifting such as the Basin and Range Province. Bimodal volcanism has also been described from areas of transtension, the early phases of back-arc basin formation and in the products of both continental and oceanic hotspots (e.g. Yellowstone and the Canary Islands).
  
  Section::::Mechanism of formation.
   The external force are the long term movements of tectonic plates or the liquid rock currents within the upper mantle, which is a continuous force applied eventually the plate will snap back or fracture relieving stress on the system to flipping it to a stable state, i.e. an earthquake. Volcanoes are similar in that the build-up of magma pressure underneath will eventually overcome the layer of dry rock on top causing an eruption. Such models can be used to predict the occurrence of earthquakes and volcanoes in active regions and predict aftershocks which are common after a large events.
   On a small scale, fractured rock behaves essentially the same throughout the world, in that the angle of friction is more or less uniform (see Fault friction). A small element of rock in a larger mass responds to stress changes in a well defined manner: if it is squeezed by differential stresses greater than its strength, it is capable of large deformations. A band of weak, fractured rock in a competent mass can deform to resemble a classic geologic fault. Using seismometers and earthquake location, the requisite pattern of micro-earthquakes can be observed.
   Section::::Formation.
   Section::::Occurrence.
   Sometimes a series of earthquakes occur in what has been called an "earthquake storm", where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.
   Section::::Examples.:Namib Desert golden mole.
   Section::::Causes.:Groundwater extraction.
  
  The changes in crustal stress patterns caused by the large scale extraction of groundwater has been shown to trigger earthquakes, as in the case of the 2011 Lorca earthquake.
  
  Section::::Causes.:Geothermal energy.
   Section::::Naturally occurring earthquakes.:Earthquake fault types.
   Burridge-Andrews mechanism".
  
  Section::::Geological effects.
   BULLET::::- Bailey Willis and William Bowie saw the sima with great strength and rigidity through the seismological studies, and tidal forces would act more on the sima (2800 to 3300 kg/m) as it is denser than the sial (2700 to 2800 kg/m) .
  
  BULLET::::- Quote, W. Van Waterschoot van der Gracht (Wilson cycle): "there may have been a pre-Carboniferous "Atlantic" that was closed up during the Caledonian orogenis" .
   On the Earth, biological processes such as burrowing or tree throw may play important roles in setting the rates of some hillslope processes.
  
  Section::::Processes.:Igneous processes.
   Section::::Influence of rock type.
   The Earth's crust is the outer brittle layer of our planet, on average thick in continental areas. This moves up and down by ten centimeters during the time of 12 and 24 hours due to the attraction of Moon and Sun, and is accompanied by a local tilting of some parts in a billion of radians.
   The ESA and NASA spacecraft SOHO records sunquakes as part of its mission to study the Sun.
  
  Section::::Starquake.
   Section::::Naturally occurring earthquakes.:Earthquakes away from plate boundaries.
  