Terms You Should Know About Earthquakes


An earthquake of similar or lesser intensity that follows the main earthquake.


A sudden slipping or movement of a portion of the earth’s crust, accompanied and followed by a series of vibrations.

Earthquake Risk

The probable building damage and number of people that are expected to be hurt or killed if an earthquake occurs on a particular fault.


The place on the earth’s surface directly above the point on the fault where the earthquake rupture began (the hypocenter). Once fault slippage begins, it expands along the fault during the earthquake and can extend hundreds of miles before stopping.


A fracture along which plates slide over each other during an earthquake. The slippage may range from less than an inch to more than ten yards in a severe earthquake.


The point deep in the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth.


The movement of rocks and earth down the side of a slope which can be extremely dangerous to anything that is at the bottom of the hill. Can be triggered by an earthquake.


The amount of energy released during an earthquake, which is computed from the amplitude of the seismic waves.


The largest earthquake in a sequence, sometimes preceded by one or more foreshocks and usually followed by many aftershocks.

Richter Scale

A scale developed in 1935 by Charles Richter from Cal Tech to compare the size of earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. Each whole number increase in magnitude represents an increase in amplitude ten times larger than the previous number. In energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the previous number.

Seismic Waves

Vibrations that travel outward from the earthquake fault at speeds of several miles per second. Although fault slippage directly under a structure can cause considerable damage, the vibrations of seismic waves cause most of the destruction during earthquakes.


A sensitive instrument that can detect, amplify, and record ground vibrations too small to be perceived by human beings, used to record and measure earthquakes.



Earthquake Hazards Program

Science Master