ScienceOnline is an interactive resource for students and teachers of science.
The site provides access to a range of activities related to the New Zealand science curriculum for Years 9-11. Both Astronomy and Earth Science have been included as well as the core science areas of Physics, Chemistry and Biology. There are interactive notes and diagrams, self-marking tests, and useful links to other science websites. The site is completely up to date with NCEA requirements at Year 11 with details of achievement standards linked to topics in each of the five subject areas.
ScienceOnline has been developed by Peter Biggs, former science teacher and author of the Blue Science Book, and Sandy McGivern, science teacher, who manages the site.
An earthquake is a shock wave that moves through the Earth's crust. Earthquakes are caused by a build-up of tension between plate boundaries, as they
are forced to grind and scrape against each other by the movements of the plates.
When one huge chunk of rock is made to move next to another, it creates a fault, a crack that extends deep into the earth. Faults can be as short as a
few millimetres or as long as thousands of kilometres. When plates move apart (divergent boundary), a normal dip-slip fault occurs. When one plate
goes under the other (convergent boundary), a reverse dip-slip fault occurs. And a sideways slipping motion called a strike-slip fault occurs when two plates
slide past each other (transform boundary).
Faults give the chunks of rock the freedom to move again, if necessary, in the future. In some places, this movement is on-going, a constant "creep"
that results in frequent, moderate, earth tremors. In other areas, the movement is not constant and the pressure can build up for hundreds of years
before it gets released as a strong earthquake. After these more abrupt movements, the strong earthquake is often followed by a series of smaller quakes
called aftershocks, as the fault readjusts to its new position. The strength and frequency of these aftershocks gets smaller with time.
When an earthquake occurs, it releases energy in the form of waves that radiate out from the focus in all directions, like ripples on a pond, only
in all 3 dimensions.
The focus is the name for the point where the earthquake rupture begins, usually deep below the surface, on a fault. The epicentre
is the point on the surface directly above the focus. This is the spot that feels the effects of the earthquake the worst.
The energy waves come in three different types - primary waves, secondary waves and surface waves.
Primary waves (P waves) travel the fastest through the ground, and so are felt first. A primary wave is a compression wave which means that it squishes the ground particles in front of it, like a car gently bumping into another car in front, which then pushes forward and bumps the one in front, and so on.
Primary waves move through the earth and so don't cause a big impact on the surface.
Secondary waves (S waves) are the next fastest wave. This is the wave that moves from side to side, like the movement of a snake. This wave also doesn't
do a great deal of damage because it moves through the earth as well.
The slowest wave is the surface wave (L wave), so it is the last one to arrive. It doesn't go through the ground, but travels along the surface in a rolling
motion that has a big impact on things on top of the ground. This is the wave-type that causes the majority of the damage.
Earthquake waves are detected using a seismometer, a machine that records the shaking of the earth. It shows this on a chart called a seismograph,
which is the zig-zaggy graph that you may have seen on T.V. before. The waves arrive at seismograph stations around the world at different times and
also at different strengths. The data from each seismometer is combined to calculate the magnitude, or strength of the earthquake.
There are two main ways to measure the magnitude of an earthquake, one on a scale that indicates how much energy is being released by it, and
the other on a scale that shows how much damage the earthquake has caused.
The Richter scale is the number that we hear on the news when an earthquake has happened. It is calculated scientifically, using the data from
seismometers at different locations, and is a measure of how much energy has been released by the earthquake. The Richter scale runs from 1 to 9,
where a size 1 quake is too small for people to feel (but a seismometer will), and a size 9 quake would mean total devastation. It is a logarithmic
scale which means each whole number doesn't mean the amount of energy is one times bigger, it actually means 10 times more energy has been released.
The Mercalli scale is more subjective. It is a measure of how much damage the earthquake has done, and is a judgement by people, as opposed to a
scientific measurement. The scale runs from 1 to 12, and each point lists what people should feel or see happening, for example, a hanging light bulb
swinging a little comes under level 2, whereas if underground pipes break and buildings partly collapse, then that would be a level 9 earthquake.