Earthquake Glossary


A glossary of earthquake-related terms.

A


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.

C


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.

E


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

F


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.

H


Hypocentre
The point on the fault plane where the rupture starts.

I


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 11 point Modified Mercalli Intensity (MMI) scale.

L


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

M


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).

S


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.

T


Tectonic uplift
Elevation of the ground caused by plate movement.