The surface of the earth is broken into giant
plates of rock which each move stubbornly in different directions across the surface. When
two plates meet, as they do in New Zealand, the brittle outer layers of the earth are
folded and fractured as the rocks try to cope with the strain of the mighty collision.To
the west of New Zealand stretches the Australian plate, and to the east lies the largest
and fastest moving plate of them all, the Pacific plate. We lie jammed between the two
plates where the strain causes the buckling up of mountain ranges such as the Southern
Alps, and the fracturing of solid rock, resulting in the formation of numerous faults. To
release the strain of the colliding plates, up to 15,000 earthquakes shake New Zealand
every year.
If we plot the earthquakes which occur beneath New Zealand we see a rather
interesting pattern. The deep earthquakes form a wide band under the North Island that
reaches over 300 km depth towards the northwest.There are also a smaller number of deep
earthquakes under Fiordland on the South Island. These deep earthquakes occur within old
ocean floor that is diving beneath New Zealand (see the cutaway view in the lower
diagram). In New Zealand the plates are colliding in an east-west direction at about 40
mm/yr (about as fast as your fingernails grow. The result is that parts of New Zealand are
both being compressed and sheared sideways. The shallow earthquakes (less than 40km deep)
are largely a result of this compressing and shearing.