Viscoelastic material:

A material which can behave as an elastic solid on a short time scale and as a viscous fluid on a long time scale.

Volcano:

An opening in the crust that has allowed magma to reach the surface.

Volcanism:

Geological process which involved the eruption of molten rock.

Wadati-Benioff zone:

See Benioff zone.

Wavefront:

Imaginary surface or line that joins points at which the waves from a source are in phase (e.g., all at a maximum or all at a minimum).

Wavelength:

The distance between two successive crests or troughs of a wave.

Quaternary:

The geological period covering about 2 million years ago to the present.

Rayleigh wave:

A type of surface wave having a retrograde elliptical motion at the Earth's surface. These are the slowest, but often the largest and most destructive, of the wave types caused by an earthquake. They are usually felt as a rolling or rocking motion and, in the case of major earthquakes, can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence.

Recurrence Interval:

The approximate length of time between earthquakes in a specific seismically active area.

Reflect:

To bounce back from a surface that represents an acoustic impedance contrast.

Refract:

To bend or change direction due to an acoustic impedance contrast.

Reverse faulting:

The rock above the fault plane (the "hanging"wall) moves up and over the rock below ("foot" wall).