Damping:

Loss of energy in wave motion due to transfer into heat by frictional forces.

Density:

The mass per unit volume of a substance, commonly expressed in grams per cubic centimetre.

Depth of an earthquake:

The value given is the depth below the surface of the mean spheroid.

Dilatancy (of rocks):

The increase in the volume of rocks mainly due to pervasive microcracking.

Dip:

The angle by which a rock layer or fault plane deviates from the horizontal. The angle is measured in a plane perpendicular to the strike.

Dip-slip fault:

A fault in which the relative displacement is along the direction of dip of the fault plane; the offset is either normal or reverse.

Dispersion (wave):

The spreading out of a wave train due to each wavelength travelling with its own velocity.

Duration (of strong shaking):

Time interval between the first and last peaks of strong ground motion above a specified amplitude.

Earthquake:

The sudden release of stored elastic energy caused by the sudden fracture and movement of rocks along a fault. Some of the energy released is in the form of seismic waves, that cause the ground to shake.

Earthquake occurrence (recurrence) interval:

The average interval of time between the occurrence of earthquakes in a particular region.

Earthquake swarm:

A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.

Earth's crust:

The layer of rock located immediately below the earth's surface. Beneath continents, it is typically about 35 km thick, and composed of granite. Under the ocean, the crust is about 5-10 kilometres thick and composed mainly of basalt.