What causes earthquakes?

The shaking motion of an earthquake is the result of a sudden release of energy. Earthquakes are caused when stress, building up within rocks of the earth's crust, is released in a sudden jolt. Rocks crack and slip past each other causing the ground to vibrate.

Cracks along which rocks slip are called faults. They may break through the ground surface, or be deep within the earth. The location on a fault where slip first occurs is called the focus, whereas the position directly above it on the ground surface is called the epicentre.

Relationship of earthquake focus and epicentre

Relationship between the focus of an earthquake and the epicentre.

Earthquakes rarely take place at the surface of the Earth but at some depth within it. Though focal depths are usually shallow, earthquakes have been detected as deep as 720 km. Based on the depth of the focus earthquakes can be classified as:

	Shallow: when the depth of origin is less than 60 km.
	Intermediate: when the depth of origin is between 60 and 300 km.
	Deep: when the depth of origin is greater then 300 km.

Earthquakes in B. C. Home Page

Next page - How big are earthquakes?

Copies are available from:

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.

Copies are available from:

Publications Office
BC Geological Survey
Ministry of Energy, Mines and Petroleum Resources
P.O. Box 9333. Stn Prov Gov't
Victoria, B.C.
V8W 9N3

This page was last updated Wednesday, March 08, 2006