Strike of fault:

The line of intersection between the fault plane and the surface of the Earth. Its orientation is expressed as the angle west or east of true north.

Strike-slip fault:

A fault whose relative displacement is purely horizontal.

Strong ground motion:

The shaking of the ground near an earthquake source made up of potentially damaging seismic waves of various types.

Subduction Zone:

A region where the earth's plates collide, with one plate sliding beneath the other. The world's largest earthquakes occur along this type of plate boundary. The Cascadia subduction zone, extending from northern California to the north end of Vancouver Island, is one such area. The subducting ocean plate is about 40 km beneath Victoria, BC, and about 70 km beneath Vancouver.

Surface Waves:

Waves that move over the surface of the Earth. Rayleigh and Love waves are surface waves.

Surface-wave magnitude: MS

Magnitude of an earthquake estimated from measurements of the amplitude of surface waves.

Swarm: (or Earthquake swarm)

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

Tectonic earthquakes:

Earthquakes resulting from sudden release of energy stored by major deformation of the Earth.

Gutenberg discontinuity:

Discontinuity in seismic velocity that marks the boundary between the core and the mantle; named after seismologist Beno Gutenberg.

Hazard:

A situation that has the possibility of occurring.

Hertz:

The unit of frequency equal to 1 cycle per second, or 2 PI radians per second.

Holocene:

The current geological time period that started about 10,000 years ago.