The following terms are used throughout the website. We have defined them here to help further your understanding about earthquakes. The list has been arranged in an order we think you will find most useful if you are planning to read all the definitions. If you are looking for a specific term, they are listed alphabetically in the side bar.

Earthquake: Ground shaking caused by tectonic or volcanic processes

Aftershock: An earthquake that follows a previous earthquake, called the mainshock. The aftershock is related to the mainshock in both time and place.

Hypocenter: The place where the earthquake starts. Synonym: focus

Epicenter: The place on the earth's surface directly above the hypocenter

Intensity: A qualitative (subjective, non-numerical) way to measure an earthquake. It is measured using the Modified Mercalli Scale. This scale is based upon what is felt by people and the damage done to buildings. Each earthquake can have many intensities, as the amount of shaking and damage vary from place to place.

Magnitude: A quantitative (numerical) way to measure an earthquake. There are several different magnitude scales, which is why you may hear different numbers reported by the news after an earthquake. Each earthquake has only one magnitude (for each scale), and it is the same everywhere.

Fault: A crack in the earth, where rock on one side has moved relative to the rock on the other side. A plane of weakness in the earth.

Rupture: A fault related movement where the ground is offset. It is always associated with energy release along the fault plane. The longer the rupture the larger the earthquake.

Lithosphere: The top 100 km of the earth. It is hard and solid.

Asthenosphere: The layer below the lithosphere. This layer is soft and probably molten. Earthquakes cannot happen in this layer.

Tectonic Plate: A large rigid piece of the Earth's surface, about 100 km thick.

Plate Boundary: The border between two tectonic plates.

Transform Plate Boundary: A plate boundary where the plates on either side are sliding past each other.

Divergent Plate Boundary: A plate boundary where the plates on either side are moving away from each other

Convergent Plate Boundary: A plate boundary where the plates on either side are moving towards each other.

Subduction Zone: Occurs at a convergent plate boundary. One plate is being pushed underneath the other plate. The world's deepest and largest earthquakes occur here.

Seismogram: A seismogram is a visual recording of the motion of the ground, following an earthquake. Earthquakes generate seismic waves which cause the ground to shake, and seismograms are recordings of that ground motion. Originally, seismograms were recorded by pen and ink on paper wrapped around a rotating drum. Currently, most seismograms are recorded by computers, and their signals are stored and displayed digitally.

Seismic Wave: Seismic waves are waves of energy that travel through Earth and along its surface. The waves are produced by earthquakes, explosions, or some other disturbance.

P Wave: An abbreviation for primary wave, P waves are a type of seismic wave that travels through Earth's interior. They are a type of body wave. P waves cause contraction and expansion as they travel through a material. P waves are the fastest of all seismic waves.

S Wave: S waves are body waves, a type of seismic wave that travels through Earth's interior. S waves generate motions within a material that are perpendicular to the direction that the wave is traveling. S waves are the second fastest of all seismic waves, which is how the name S wave, or secondary wave, originated.

Liquefaction: Liquefaction is the temporary change of soil from a solid state to a liquid state during an earthquake. For liquefaction to occur there needs to be a lot of water in the ground and loose soil.

Retrofit: Retrofit is adding or replacing a part belonging to an object after the object has been built. This is done to buildings and other structures so that they will be safer during an earthquake.