HOW EARTHQUAKES ARE MEASURED

This page is being adapted from DMG NOTE 32 (8/97 edition)
by Ted Smith and Lisa Chisholm.

Vibrations produced by earthquakes are detected, recorded, and measured by instruments called seismographs. These devices may amplify ground motions beneath the instruments to over 1 million times, transcribing the ground motion into a zig-zag or wiggly trace called a seismogram. From the data expressed in seismograms, the time, epicenter, and focal depth of an earthquake can be determined. Also, estimates can be made of its relative size and amount of energy it released.

The point on the fault where rupture initiates is referred to as the focus or hypocentre of an earthquake. The hypocentre of an earthquake is described by its depth in kilometers, its map location in latitude and longitude, its date and time of occurrence, and its magnitude (a measure of the amount of energy radiated as seismic waves). The term epicentre, which is more commonly used to refer to an earthquake location, is the point on the earth's surface directly above the hypocentre. The description of an epicenter is the same as for a hypocentre except that the depth is omitted. The depth of an earthquake's focus may vary from a few km to almost 700 km. Where do deep earthquakes occur? Shallow earthquakes are more common with foci < 60 km.

In Jamaica, most seismic events have foci which are less than 30 km deep.