## The Richter Scale measures the magnitude of an earthquake. It assigns a number to quantify the size of an earthquake, not the intensity.

Richter Scale magnitudes are based on a logarithmic scale (base 10). This means that for each whole number you increase on the Richter scale, the amplitude of the ground motion recorded by a seismograph increases 10 times. Using this scale, a magnitude 5 earthquake would result in 10 times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). These numbers can increase quickly. Magnitude 1 seismic waves release as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Fortunately, most earthquakes are magnitude 2.5 or less, too small to be felt by most people.

With some modern day adjustments, the Richter Scale is more correctly known as the local magnitude scale. It still assigns a single number to quantify an earthquake and uses a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismogram. The decrease in amplitude of the seismograph recording, due to distance between the earthquake epicenter and the seismometer, is corrected for by subtracting the logarithm of the expected amplitude of a magnitude 0 event at that same distance. This correction is intended to make the local magnitude an absolute measure of earthquake size.

Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of 1 micrometre on a seismograph recording for every 100 km the seismometer was from the earthquake epicenter. This choice was made to prevent negative magnitudes from being assigned. However, the Richter Scale has no upper or lower limit, and modern seismographs routinely record earthquakes with negative magnitudes.

Because of the limitations of seismometers used to develop the scale, the original local magnitude scale cannot be calculated for events larger than about 6.8. Many investigators have proposed extensions to the local magnitude scale, the most popular being the surface wave magnitude and the body wave magnitude scale.

The Richter Scale magnitude does not easliy relate to physical characteristics of the earthquake source. Furthermore, there is a saturation effect near 8.3-8.5 causing traditional magnitude methods to yield the similar magnitude estimates for events clearly of different size.

By the beginning of the 21st century, most seismologists considered the traditional magnitude scales obsolete. These scales have been replaced by more physically meaningful ones which include a measurement called the seismic moment. The seismic moment is more directly related to the physical parameters of an earthquake, such as the dimension of the earthquake rupture, and the energy released from the earthquake. In 1979, seismologists Tom Hanks and Hiroo Kanamori proposed the moment magnitude scale. This scale provides a way of expressing seismic moments in a form that can be approximately related to traditional seismic magnitude measurements.

Magnitude is not the same as intensity. Intensity scales are used to describe relative earthquake effects. The intensity of earthquakes is sensitive to many local site conditions and is not an absolute measurement of earthquake size.

The following table describes typical earthquake effects associated with various magnitudes near the epicenter. This should be taken with caution, as intensity and ground effects depend on the magnitude, the distance from the epicenter, and geological conditions.

Descriptor/Magnitudes/Effects/Frequency

Micro/Less than 2.0/Not felt/8,000 per day

Very minor/2.0-2.9/Not felt but recorded/1,000 per day

Minor/3.0-3.9/Often felt but rarely causes damage/49,000 per year

Light/4.0-4.9/Noticeable shaking of indoor items, Minor damage/6,200 per year

Moderate/5.0-5.9/Major damage to poorly constructed buildings. Slight damage to well dsigned buildings/800 per year

Strong/6.0-6.9/Can cause destruction over a 100 mile area in populated areas/120 per year

Major/7.0-7.9/Serious damage over large areas/ 18 per year

Great/8.0-8.9/Serious damage over several hundred miles/1 per year

Rare Great/9.0 or greater/Serious damage over thousands of miles/1 every 20 years

Events with magnitudes of about 4.5 or greater are strong enough to be recorded by seismographs all over the world. On average, Great earthquakes occur once a year. The largest recorded earthquake was the Great Chilean Earthquake of May 22, 1960, which had a magnitude of 9.5.