Earth science
What is the difference between magnitude and intensity?
Magnitude and Intensity measure different characteristics of earthquakes. Magnitude measures the
energy released at the source of the earthquake. Magnitude is determined from measurements on
seismographs. Intensity measures the strength of shaking produced by the earthquake at a certain
location. Intensity is determined from effects on people, human structures, and the natural
environment.
Why did the February 21st 2011 earthquake in Christchurch cause so much damage?
When a ‘strong’ fault breaks, it releases more energy than an equal-sized ‘weak’ fault would. Think
about the difference between breaking a 1cm-thick sheet of Styrofoam and a 1cm-thick sheet of
plywood, and the amount of jarring you would feel in your hands. The Canterbury crust is analogous
to the plywood, but most earthquakes are more like breaking the Styrofoam.”
In addition, seismologists have discovered that the earthquake rupture was faster than most
earthquakes of this magnitude.
The faults that failed in both the September magnitude 7.1 and February magnitude 6.3 Christchurch
earthquakes were very strong, with high amounts of friction holding the two sides together.
A recently discovered physical phenomenon explains this observation. As the huge early pulses of
energy travelled through horizontal layers of the earth beneath Christchurch, the weaker upper
layers travelled farther upward than the stronger lower ones, and so separated from them. When
these upper layers fell back under gravity, they ‘slapped’ against the lower layers coming up again,
producing very high impacts.
“These ‘slapping’ impacts generate energy that travels back up towards the surface, magnifying the
whole process."
The Christchurch earthquake occurred within kilometres of the central Christchurch leaving little
time or distance for the seismic energy to attenuate before impacting on the city. Dr fry
Subduction zone: A Subduction zone is an area of tectonic plate collision where the more dense
plate subducts, or follows a path underneath, the less dense plate.
Divergent boundaries: at divergent boundaries plates move away from each other and the gap that
this creates is filled with molten magma formed below.
Convergent boundaries: at a convergent boundary two plates collide as they move toward each
other. When an oceanic plate collides with another oceanic plate and when a continental plate
collides with an oceanic plate the more dense plate subducts into the mantle. When a continental
plate collides with another continental plate neither subducts and they create mountain ranges and
volcanoes.
Transform Boundary: when two plates slide along each other in opposite directions and due to
friction when the buckle and shift they can cause earthquakes
Magma: magma is hot fluid material below or within the earth’s crust.
How is it that new crust forms at divergent boundaries?
New crust is formed when magma rises up through the gap the plates create and solidifies.
How is it that plate material is destroyed at convergent boundaries?
The crust is destroyed because when the denser plate subducts the crust gets to hot and melts.
What do Scientists think causes the earth’s tectonic plates to move?
Scientists think that plates move because of the intense heat in the earth’s core. When the mantle
layer heats up it moves and cause the tectonic plates to move.
What plate boundary does New Zealand straddle?
New Zealand straddles the boundary between the pacific plate and the Indian- Australian plate.
Magnitude and intensity
Magnitude and intensity measure different parts of an earthquake. Magnitude measures how much
energy is released at the centre of the earthquake. Magnitude is measure by seismographs. Intensity
measures the strength of the shaking at a point. Intensity is measured by the effects it has on people,
structures and environment.
What do the plates do?
To the north of NZ: The pacific plate moves down under the Indian-Australian plate
To the south of NZ: The Indian-Australian plate is forced under the pacific plate
Within the south island: The plates rub together horizontally
What does this plate movement result in?
Plate movement results in volcanic activity in the North Island and in earthquakes that are felt
throughout the country.