Earthquakes in British Columbia

Amplification

In some situations a resonance can occur in deep soil layers, markedly increasing the ground shaking resulting from an earthquake. This causes more damage than in adjacent areas.

Earthquake

A naturally occurring shaking motion of the Earth's surface. Earthquakes are caused when stress, building up within rocks of the Earth's crust, is released in a sudden jolt.

Epicentre

The position on the ground surface directly above the focus of the earthquake.

Fault

A fracture or break along which rocks slip. Faults can vary from almost horizontal surfaces to vertical. They may break through the ground surface, or be deep within the earth.

Focus

The location where energy is released and slip first occurs on a fault.

Geodetic survey

A survey using very precise levelling and horizontal positioning methods. Such surveys can monitor the subtle surface deformations that occur prior to an earthquake.

Intensity

A measure of the destructive effects of an earthquake at the surface. It is measured on the Modified Mercalli Scale.

Landslide

a general term used to describe the down-slope movement of soil, rock and organic materials under the influence of gravity. It also describes the landform that results. One of the major causes of damage resulting from an earthquake.

Liquefaction

Shaking caused by an earthquake can cause the transformation of some loosely packed, water saturated sediments, such as sand and mud, into a fluid mass. The sediments thus lose their strength and can no longer support buildings which may then sink or lean.

Magnitude

A measure of the energy released by an earthquake. Measured using the Richter scale based on the amplitude of the seismic wave recorded by seismographs.

Megathrust earthquake

A very large destructive earthquake that can occur along some converging plate boundaries where the plates are locked and do not move past each other. In such situations stress accumulates to high levels.

Modified Mercalli Scale

An arbitrary scale of 12 degrees modified from an original scale devised by the Italian seismologist Giuseppe Mercalli. The scale uses information supplied by people living in the area of the quake.

Plates

The outer shell of the Earth is made up of almost rigid plates that fit together. These plates "float" on a dense, more fluid layer just beneath them. The plates slowly move around, though we do not notice it in our everyday lives. Earthquakes occur most frequently on, or near, the edges of plates where stress is most concentrated.

Richter Scale

A scale for measuring the magnitude of an earthquake devised by the American seismologist Charles Richter. It is based on the amplitude of the seismic wave recorded by seismographs.

Seismic waves

Vibrations passing through the ground that result from an earthquake. They are of three principal types:

	Primary or P-waves - these are compression-dilation ("push-and-pull") waves that move through the Earth very quickly.
	Secondary or S-waves - transverse or shear ("shake") waves that move through the Earth more slowly.
	Long or L-waves - surface waves confined to the crust of the Earth. They travel more slowly than the P and S waves and may not be detected far from the epicentre.

Seismograph

An instrument used to detect and measure earthquakes. It senses and records the vibrations that radiate out from the earthquake focus.

Seismogram

The visual record produced by a seismograph. It is used to determine the location and magnitude of the earthquake.

Seismology

The scientific study of earthquakes.

Tsunami

A large and often destructive wave caused by a submarine earthquake. Colloquially known as a "tidal wave".

Earthquakes in British Columbia was originally produced as Information Circular 1991-6 by the BC Geological Survey of the B.C. Ministry of Energy, Mines and Petroleum Resources, in cooperation with the B.C. Ministry of Environment, the B.C. Provincial Emergency Program and with the assistance of the Geological Survey of Canada. It has been amended and updated in this WWW version.

  This page was last updated Wednesday, March 08, 2006