Sheet 12.3 Revision Earthquakes
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Sheet 12.3 Revision
Earthquakes
When blocks of land are pushed together, pulled apart or twisted the movement may be slow
(forming folds) or fast (causing earthquakes). This movement causes the surface of Earth to
ripple in various ways. These ripples, the vibrations sent out by earthquakes, are known as
seismic waves.
Volcanic eruptions may also cause seismic waves, but these seismic waves
behave differently.
Figure 4.12: Types of earthquake waves
seismic wave Ripples or vibrations travelling through Earth, such as those sent out by earthquakes
and volcanoes
Types of earthquake waves
There are three types of earthquake waves. These are P (primary) waves, S (secondary)
waves and L (surface) waves. P waves travel the fastest and arrive first. They push through
solids (e.g. land) and liquids (e.g. oceans) at a rate of 4 to 13 kilometres per second. The best
way to understand how these waves (also called compression waves) act is to use a metal or
plastic coil (such as a slinky). Lay the coil on a flat surface with a person holding each end,
and push one end towards the other. The coil does not move up or down or side to side.
Rather, it causes a compression wave that pushes through the centre of the coil. S waves
travel at a rate of 2.5 to 7.5 kilometres per second and arrive after P waves. Their movement
is a transverse motion (side-toside, up and down, or a mixture of both). This can cause a lot
more damage than a P wave. They move only through land not water. Surface waves move in
Science for the New Zealand Curriculum Years 9 and 10
© Donald Reid, Catherine J. Bradley, Des Duthie, Catherine Low, Matthew McLeod, Colin Price 2010
Published by Cambridge University Press www.nzscience.co.nz www.cambridge.edu.au
Sheet 12.3 Revision Earthquakes
2
a similar way to the side-to-side movement of S waves. However, they move on or just below
Earth’s surface, including through water. These waves may make several trips around Earth
before stopping.
Measuring earthquakes
In 1935 Charles Richter introduced the Richter scale for measuring the energy released by an
earthquake. The scale ranges from zero to about 10, with each level having 10 times the
movement of the previous level. For instance, a magnitude 5 earthquake has 10 times the
movement (amplitude) of a magnitude 4 earthquake. Likewise, magnitude 6 earthquakes will
have 100 times the movement of magnitude 4 earthquakes. However, with each increase the
energy released increases 31.6 times. Therefore, a magnitude 6 earthquake has almost a
thousand times more energy (31.6 × 31.6) than a magnitude 4 earthquake. Magnitude 2
earthquakes are seldom felt. Magnitude 8 earthquakes occur regularly in New Zealand.
Earthquakes in New Zealand
The movement caused by an earthquake can also cause cracks in the surface of Earth. Major
cracks, known as fault lines, can cause the formation of valleys such as the Hutt Valley near
Wellington. Earthquakes can also cause uplifting, where parts of the surface rocks are moved
upwards. An example of this is Napier's Meeanee Quay. This was underwater until 1931,
when a 7.8 magnitude earthquake forced it up. Another example is in the Wairarapa, where
an 8.2 magnitude earthquake in 1855 lifted some areas up by 6.4 m.
Figure 4.13: The steep bank just above the people in this
photograph is the Wairarapa Fault, where evidence of movement
that occurred in the 1855 earthquake can be seen
Science for the New Zealand Curriculum Years 9 and 10
© Donald Reid, Catherine J. Bradley, Des Duthie, Catherine Low, Matthew McLeod, Colin Price 2010
Published by Cambridge University Press www.nzscience.co.nz www.cambridge.edu.au