Acceleration
Is the change from one speed or velocity to another. The ground movement during an earthquake also changes is speed or velocity and thus experiences acceleration. The peak acceleration is the largest increase in velocity recorded by a particular station during an earthquake
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 one or more times in the last 10,000 years.
Active Volcano
A volcano that is erupting or is in an ongoing eruptive phase.
Accretionary wedge
This forms where oceanic and continental plates collide. Sediments accumulate as they are scraped off the top of the oceanic plate (moving downwards) and are affixed to the edge of the continental plate
Aftershocks
A series of smaller earthquakes which may follow a large earthquake. Aftershocks can continue over a period of weeks, months, or years.
Alluvium
Loose material — clay, silt, sand, gravel, and larger rocks — washed down from hills and mountains and deposited in low areas.
Amplification
Shaking at a site during an earthquake can be increased or amplified due to the geometry of the sediments at the surface and subsurface.
Amplitude
Refers to the height of the waveform on the seismograph (earthquake recording).
Andesite
Volcanic rock or lava with a 54-62% silica content and moderate amounts of iron and magnesium. It is characteristically medium dark in colour. Most of the islands in the arc have andesitic volcanoes, which tend to produce explosive eruptions and lava domes.
Arc
A chain of volcanic islands (volcanic arc) that forms at subduction zones – where oceanic and continental plates collide. The oceanic plate being denser goes below the continental plate. The resulting mixing and melting causes submarine volcanoes to form and then islands when they breach the sea surface.
Array
An ordered arrangement of seismometers, 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.
Arrival time (tsunami)
Time of the first maximum of the tsunami waves.
Aseismic
A fault on which no earthquakes have been observed.
Asthenosphere
Is just below the lithosphere and includes the upper mantle. It is ductile in nature and is about 180 km thick.
Attenuation
The decrease in amplitude of waves as they move away from the point source. Seismic waves also become attenuated as they move away from the earthquake source.
Ash
The fine material (<2mm in diameter) that is ejected from a volcano and convected upwards in the eruption column before settling out downwind to form air fall deposits. It can cause severe respiratory problems and can cause roofs and vegetation to collapse.
Ash Fall
When volcanic ash settles out of an eruption column it forms ashfall. Ash falls can blanket the entire landscape for kilometres around a volcano. Close to the eruption vent, ash may be thick enough to collapse buildings and destroy vegetation
