Investigating earthquakes – key terms
Earthquakes help shape New Zealand and are a constant threat in many parts of the country. The more we understand about what causes them, the more we can be prepared.
This resource provides explanations of the key concepts encountered when exploring earthquakes – the ‘basics’ that every student should understand.
Australian plate
The main land masses on this plate are Australia and India, so the plate is also called the Indo-Australian plate. Movement of this plate has caused earthquakes and volcanoes across regions from Indonesia to New Guinea and the Pacific, and it has created the Himalayan Mountains.
Building standards
In New Zealand, buildings have to be constructed to a certain legal standard. Part of the aim is to keep damage low in an earthquake, and to help keep people safe inside affected buildings.
Earth structure
The Earth has four main layers:
Inner core – solid, mostly iron
Outer core – liquid, mostly iron
Mantle – viscous rock (solid, but can flow)
Crust – solid, rigid rock
Convection currents
These are currents that occur in heated fluids, including the Earth’s mantle. Hot areas of mantle are less dense and rise up. As the hot areas meet the lithosphere, they transfer heat energy to it, cool and sink back down. There is also horizontal movement, which drags tectonic plates with it.
Earthquake
An earthquake occurs when a fault suddenly gives way and creates a break-point in the rock under the Earth’s surface. This generates seismic waves that, if strong enough, cause damage from shaking when they reach the surface. The closer the break-point is to the surface and the greater the energy, the more destructive the earthquake.
Faults
These are areas of crust (much smaller than a tectonic plate) that are moving in relation to each other. Because rock doesn’t move easily, strain can build up in these areas, and in an earthquake, the rock suddenly gives way at weak points. A fault trace is what we see of the surface features of a fault. Often this can be a ridge or a rift (a gap where the ground has pulled apart).
Fluid
A substance that can flow like a liquid or gas. The fluid part of the earth's mantle is solid but, as it can flow, we call it “plastic”.
Geophysics
The part of Earth science that deals with the structure and evolution of the Earth. This includes earthquakes, volcanoes and Earth’s gravitational and magnetic fields.
Lithosphere
The rigid rocky outer layer of the Earth, composed of the crust and the upper part of the mantle.
Locking interface
Where one tectonic plate is driven down beneath another one, they are not free to pass each other smoothly. In some places, they grate slowly and steadily past each other, but often they lock together to some extent. The area of the plates that touch and lock is called the locking interface.
Pacific plate
The features related to this plate include most of the Pacific Ocean, but no major landmasses. Earthquakes and volcanoes are associated with regions on the edge of the Pacific plate, like Japan, the West Coast of North America and New Zealand.
Richter scale
This is a scale used to show the strength of the seismic waves in an earthquake. In the Richter scale, an increase of one in earthquake magnitude (e.g. from magnitude 6 to magnitude 7) represents 30 times increase in energy released.
Seismic waves
These are vibrations from earthquakes that travel through the Earth. Different types of seismic waves include P-waves and S-waves, which can sometimes be felt before the main earthquake. The waves that cause the damage in earthquakes are called surface waves and have both up and down and side to side movement.
Seismograph
An instrument that detects and records earthquakes. A seismometer generates an electrical current when the ground shakes. This is digitised by a seismograph and transmitted to a to a computer. Older types record directly onto paper.
Subduction zone
At a plate boundary where one tectonic plate is moving towards another, one plate can sink below the other, where it is eventually recycled into the mantle. This is called subduction. Along the east of the North Island, the Pacific Plate is subducting (sinking under) the Australian Plate, but along the southwest of the South Island the Australian Plate is subducting the Pacific Plate.
Tectonic plate
This is a piece of the Earth’s lithosphere that moves on top of the fluid part of the mantle. There are seven main plates and a number of smaller ones.
