Richter Magnitude:
Most people have heard of the Richter Scale for measuring the 'size' or 'strength' of an earthquake. This scale is quantitative and based on the amount of energy released by an earthquake.
The inferred energy of a quake is a function of both the amplitude and the duration of a single wave. The seismogram below shows waves with a wide range of amplitude (up to +/- 350) and duration (the first big wave cycle - up, down, back to zero - started just after 8 minutes on the chart and ended nearly 40 seconds later; other waves lasted only seconds).
So when you do all the math, here is what you get.
One unit on the Richter Magnitude Scale corresponds to a tenfold increase or decrease in the amplitude of the wave on the seismogram - 350 in this example would become either 3500 or 35! This change, when summed over all the wavelengths and wave types, translates to a 30 times increase in energy.
So a seismograph (the instrument) that recorded a 1 cm deflection on a seismogram (the tracing) for a magnitude 5 earthquake would show a 100 cm deflection for a magnitude 7 quake that released 30x30 = 900 times as much energy. You do the math for a magnitude 8 quake!
To make these measurements you need to use a calibrated instrument that is located 100 km from the quake epicenter (or make corrections to data gathered at other distances).
Mercalli Intensity:
The Modified Mercalli Intensity Scale is a qualitative assessment of the kinds of damage done by an earthquake. To avoid confusion with the Richter Magnitude scale, the Mercalli scale is reported using Roman Numerals on a scale from I to XII. For example:
I: Not felt except by a very few people under very special circumstances.
V: Felt by nearly everyone, people awakened, some windows and dishes broken, some tall objects disturbed.
VIII: Damage slight in specially designed structures, considerable damage in normal buildings with partial collapse, heavy furniture overturned.
XII: Total destruction of man-made structures, waves seen on the ground, objects thrown in the air.
