Aftershock A lesser tremor that takes place after the main earthquake. There may be many aftershocks, sometimes continuing for months afterwards.

Crust The rocky outer layer of the Earth.

Earthquake A shaking or trembling of the ground, caused by the sudden movement of rocks in the Earth’s crust. Most earthquakes happen along the edges of tectonic plates, when plates collide or slide past each other. The two plates lock together before the pressure becomes too much and the rocks suddenly snap along a crack called a fault.

Epicentre The point on the Earth’s surface directly above the focus of an earthquake.

Fault A crack in the Earth’s surface. The rocks move in opposite directions on either side of the fault, causing a quake. Faults form when rocks are stretched or bent and crack along a weak line.

Focus The point in the Earth’s crust where the rocks suddenly break, releasing shock waves.

Landslide The sudden and rapid movement of soil and rock down a slope.

Liquefaction The process by which moist sediments, when shock waves are passed through them, become almost like a liquid.

Magnitude Scale A measure of the magnitude (size or strength) of an earthquake. An increase of 1 point means that an earthquake is 30 times stronger. An earthquake with a magnitude of less than 4 is minor; 4–5 light; 5–6 moderate; 6–7 strong; 7–8 major; 8–9 great. This has replaced the Richter Scale.

Mercalli Scale A scale used to measure the amount of damage caused by an earthquake. The Mercalli scale ranges from 1, an earthquake so small it can barely be felt, to 12, an earthquake that causes total devastation and changes the landscape.

P(rimary) wave A seismic wave released from the focus of an earthquake. P-waves cause the rocks to shake back and forth in the direction of wave movement.

Richter Scale See Magnitude Scale

S(econdary) wave A seismic wave released from the focus of an earthquake. S-waves cause the rocks to shake from side to side and up and down at right angles to the direction of the wave movement.

Seismic waves Waves of pressure that move through the Earth, also called shock waves. They are caused by earthquakes. Primary waves and Secondary waves are both kinds of seismic waves.

Seismograph A device that used to be used to record the shaking of earthquakes. A seismograph plots the vibrations using a pen on a rotating drum.

Seismometer A modern device for detecting earthquakes. A seismometer uses electronics to record the shaking and its intensity.

Shock wave A wave of energy released from the focus of an earthquake. Also called a seismic wave.

Tectonic plates The large slabs into which the Earth’s surface is divided. Each tectonic plate moves slowly, either pushing into another plate, pulling away from it, or sliding underneath it.

Tremor A shaking of the ground.

Tsunami A massive wave caused by an earthquake deep under the seabed. In the open ocean, tsunami waves are barely noticeable, but as they near the shore they grow much bigger—up to 30 metres (100 feet) high. The wall of water surges ashore with an unstoppable force, quickly flooding coastal areas.

Consultant: Ian Fairchild