Question:

• Explain how the study of plate tectonics helps us to understand the occurrence of earthquakes

Answer Layout:

• Earthquakes are caused by the movement of the world’s tectonic plates. Most earthquakes occur at plate boundaries so a study of these boundaries can help us to understand the depth, location and magnitude (on the Richter scale) of earthquakes. • The theory of plate tectonics explains how the earth’s crust is split

into huge tectonic plates. The plates are moved by slow convection currents in the mantle below. • The convection currents cause the plates to collide, separate and slide

past each other at plate boundaries.

Location and depth of earthquakes: page 20

Earthquakes occur at divergent, convergent and transform plate boundaries

• Earthquakes are more common at destructive plate boundaries where you have subduction taking place (oceanic-continental plate collision/oceanic-oceanic plate collision) and Mountain building taking place (Continental-continental)

Destructive Plate Boundaries

• (A)Earthquakes at subduction zones• As plates collide the heavier oceanic plate will sub-duct beneath the lighter continental

plate.• As the oceanic plate sub-ducts, layers of rock are scraped off it by the continental plate

to form mountains, e.g. The Rocky Mountains. However, as the plate sub-ducts, it may become JAMMED against the continental plate. Convection currents will continue to attempt to move the oceanic plate downwards, which leads to a large build up of pressure. • Eventually the PRESSURE will become too great and the oceanic plate will break free

suddenly, releasing energy/seismic waves which lead to a powerful earthquake at the surface. • The Asian tsunami earthquake measured 8 or more on the Richter Scale. (Subduction

between the Burma and Indian Plate)

• (B) Earthquakes associated with Mountain Building.• Earthquakes also occur where 2 continental plates collide and fold

mountains are formed, e.g. the Himalayas formed where the Indian and Eurasian plate collide and this experiences earthquake activity.• As the plates collide land is uplifted and shallow quakes occur whose

focus is less than 70 km below the surface. These quakes may not rate highly on the Richter scale but because they are shallow much damage is done. • The Sichuan Quake in China registered 8. Its focus was only 19 km

below the surface.

Transform Plate Boundaries• Earthquakes associated with transform plate boundaries• A second location where earthquakes occur is at a transform plate boundary.• When 2 plates are moving roughly parallel to each other.• Such areas are usually free of volcanic activity but are extremely prone to seismic activity or

earthquakes. • As the plates are kilometres thick, the rocks within the plates tend to grind against each other

as they slide past and the plates may become JAMMED.• Convection currents will continue to push the plates in their respective directions which leads

to a build up of PRESSURE and puts tremendous stress on the rock within the plate which may build up for centuries.

• These rocks can stretch up to a point (elastic rebound) but will eventually snap along a fault line. This is known as a transform fault. It releases seismic waves to the surface to to cause an earthquake.

• The San Andreas Fault which lies between the North American Plate (moving north west at a rate of 1 cm per year) and the Pacific plate (moving in the same direction at a rate of 6 cm per year)

• Cities such as San Fran and San Diego are affected by seismic activity along this line. These cities lie directly above the focus, they are known as the epicentre and suffer the most damage.

Constructive Plate Boundaries

• Earthquakes associated with constructive plate boundaries• Here magma reaches the Earth’s surface as 2 plates separate, e.g. the

mid ocean ridge between the North American and Eurasian Plates. • The vibration of rising magma and bursting gas bubbles within the

magma chamber causes frequent shallow earthquakes which are not very powerful and rarely do much damage.

Test tomorrow:

• Examine with reference to actual examples, the measurement and effects of earthquakes.• Read sample answer and tsunami Japan and Haiti