GeoNet: Earthquake Glossary

A glossary of earthquake-related terms.

Active fault: A fault that is likely to have another earthquake sometime in the future. Faults are commonly considered to be active if they have moved in the last 10,000 years.

Crust: The outermost layer of the earth, ranging from 10 to 65 km in thickness worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes.

Epicentre: The point on the earth's surface vertically above the hypocentre.

Fault: A fracture, along which the blocks of crust on either side have moved relative to one another in a direction parallel to the fracture.
Fissure: A long narrow crack in the ground caused by earthquakes.
Focal depth: The depth of an earthquake's hypocentre.
Focus: Same as Hypocentre.

Intensity: A measure of how strongly an earthquake manifests at the surface, based on its observable effects on people, buildings and the environment. Intensity is usually ranked using the 12 point Modified Mercalli Intensity (MMI) scale.

Liquefaction: A process in which water-saturated sediment temporarily loses its strength and acts as a fluid.

Magnitude: A measure of the energy released by an earthquake at its source. Magnitude is commonly determined from the shaking recorded on a seismograph. Each unit of magnitude on the scale represents a substantial increase in energy, for example a magnitude 5 releases 30 times more energy than a magnitude 4.
MMI: An abbreviation for Modified Mercalli Intensity (see Intensity above).

Sandblows: When waves from a large earthquake pass though wet, loose sand, patches of sand erupt from below the surface onto the ground and form sandblow deposits.
Slumping: When loosely consolidated materials or rock layers move a short distance down a slope.
Subduction zone: The area or zone where two tectonic plates come together, one riding over the other.
Swarm: An earthquake swarm is a sequence of nearby earthquakes striking in a short period of time.