answer to week 8 question

What is an earthquake?
New Zealand lies on the boundary of the Pacifi c
and Australian tectonic plates.
Tectonic plates are always on the move. Tension
builds up as they scrape over, under or past each
other. In some places movement between the
plates is happening all the time, causing frequent
small or moderate earthquakes. Other areas,
where the movement is not constant, are prone
to stronger quakes separated by longer periods of
time.
Most (though not all) earthquakes occur at faults,
which are breaks extending deep within the earth,
caused by the movement of these plates. The point
under the ground where the earthquake actually
begins is called the hypocentre or focus, while the
place directly above it on the surface is known as
the epicentre.
Earthquakes cause vibration waves to travel
though the ground. The fi rst sign of a quake
is often the rumbling sound caused by the ‘P’
(primary or push) waves travelling at about 20,000
kilometres an hour, twenty times faster than a jet
aircraft. The ‘S’ (secondary or shear) waves follow
along at about 10,000 kilometres an hour, and
cause the main rolling and shaking effects of an
earthquake.