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Question:
Explain how a study of plate tectonics helps us understand the occurrence of earthquakes. (30
marks)
Answer:
Earthquakes are caused by the movement of the world’s tectonic plates. Most earthquakes occur at
plate boundaries so the study of these boundaries can help us 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-moving convection currents in the mantle below. The convection currents
cause the plates to collide, separate and slide past each other at plate boundaries. There are 3 types
of plate boundaries in the earth’s crust and each is associated with earthquakes.
1.
Earthquakes at destructive plate boundaries
These can be divided into two groups
a. Earthquakes at subduction zones
At subduction zones, oceanic crust slides beneath lighter continental crust or another oceanic plate.
The descending plate moves in sudden bursts triggering powerful earthquakes (eg. the Asian
Tsunami quake in 2004). Because the descending plate may reach great depths before it melts, the
focus of subduction zone quakes may be deep (eg. 300 kilometres below the surface). Quakes which
occur at subduction zones are also very powerful, often measuring 8 or more on the Richter Scale
(the Asian Tsunami registered 9.0).
b. Earthquakes associated with mountain building (orogeny)
Earthquakes also occur where two continental plates collide and fold mountains are formed, eg. the
Himalayas formed when the Indian plate and the Eurasian Plate collide and this area experiences
earthquake activity.
As the plates collide land is uplifted and shallow quakes occur whose focus is less than 70 kilometres
below the earth’s surface. These quakes may not rate highly on the Richter Scale but because they
are shallow much damage is done. The 2008 Sichuan Quake in China registered 8.0, devastated huge
areas and left millions homeless. Its focus was only 19 kilometres below the surface.
2. Earthquakes associated with conservative plate boundaries/transform faults
Where two tectonic plates slide past each other, shallow earthquakes may occur. This is seen along
the boundary between the North American plate and the Pacific Plate. Sometimes they lock against
each other, and at other times they slide smoothly past each other. When the plates are locked
together, pressure may build up for centuries and then suddenly release. These quakes can be very
powerful due to the great amounts of energy released and the shallow depth of the focus.
3. Earthquakes associated with constructive plate boundaries
Here magma reaches the earth’s surface as two plates separate, eg. at 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 quakes which are not very powerful and rarely
cause damage.