Measuring Earthquakes
There are two ways in which we can quantify the size of earthquakes: magnitude and intensity. Magnitude is a measure of the amount of energy released during an earthquake. Almost everytime, when an earthquake occurs, we hear news reports on the magnitude of the earthquake using the Richter scale. Magnitude of an earthquake is a single number regardless of where it originates and where it’s felt but intensity will vary from place to place.
Using a simple analogy to illustrate the intensity and magnitude, is dropping a stone into a water pond, the difference between magnitude and intensity is similar to the difference between the height of the splash exactly where the stone hit the water and the height of the waves over the pool.
Therefore in general, the intensity is greater near the epicenter than at large distances from the epicenter.
This decrease in intensity with distance is known as attenuation. Earthquakes are measured by seismographs.