Active margin:

Continental margin characterized by earthquakes and volcanic activity (i.e. location of transform fault or subduction zone).

Aftershock:

An earthquake that occurs after a "mainshock" (or larger earthquake). Aftershocks occur in the same general region as the "mainshock" and result from readjustments of stress at places along the fault zone. For great earthquakes (M=8), aftershocks may occur over hundreds of kilometres. Depending on the size and depth of the earthquake, aftershocks may occur for many months after the mainshock; however, the rate of aftershock activity dies off quickly with time.

Amplitude (wave):

The maximum height of a wave crest or depth of a trough.

Array:

An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver.

Arrival:

The appearance of seismic energy on a seismic record.

Arrival time:

The time at which a particular wave phase arrives at a detector.

Aseismic:

Not associated with an earthquake, as in aseismic slip. Also used to indicate an area with no record of earthquakes: an aseismic zone.

Asperities (fault):

Roughness on the fault surface posing local resistance to slip.

Asthenosphere:

The layer below the lithosphere that is marked by low seismic-wave velocities and high seismic-wave attenuation.

Attenuation:

The reduction in amplitude of a wave with time or distance travelled.

Auxiliary fault-plane:

A plane orthogonal to the fault plane.

Barrier (fault):

An area of fault surface that is resistant to slip because of geometrical or structural changes.