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The earthquake Richter scale, also known as the local magnitude scale and the Richter magnitude scale, assigns a single number to quantify the amount of seismic energy released by an earthquake. It is a base-10 logarithmic scale that is obtained by calculating the logarithm of the combined horizontal amplitude(shaking amplitude) of the largest displacement from zero on a particular type of seisometer. An earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0. The Richter scale has been improved upon by the moment magnitude scale, which gives similar values, but reports a fundamental property of the earthquake derived from instrument data, rather than reporting instrument data which is not always comparable across earthquakes, and does not saturate in the high-magnitude range.
The earthquake Richter scale reads from 0-10. the lower end is a small earthquake and the high end is an epic quake. Different ranges of the scale’s readings have different frequencies throughout history. Less than 2.0 is called a micro earthquake and there are about 8,000 of them per day. Minor earthquakes are divided into two categories: 2.0-2.9 are rarely felt and there are 1,000 of these each day, 3.0-3.9 are felt and there are 49,000 of these every year. Readings of 4.0-4.9 are considered called light quakes and there are about 6,200 of them each year. 5.0-5.9 are called moderate. There are about 800 of them per yer. 6.0-6.9 are considered to be strong and happen about 120 times each year. 7.0-7.9 are major quakes and there are about 120 of them yearly. Great earthquakes are divided into two categories: readings of 8.0-8.9 happen about once a year, but the destruction is massive over many miles, 9.0-9.9 Richter scale earthquakes happen about once every twenty years. The destruction from these quakes is massive and covers several thousand miles at a time. Readings of 10 on the Richter scale have never been reported or postulated in recorded history. They would be considered epic earthquakes and the destruction and effects would be felt across the globe.
The earthquake Richter scale has been superseded, but when the average person thinks about the magnitude of an earthquake it is the measuring tool that comes to their mind.
We have written many articles about the earthquake Richter scale for Universe Today. Here are some facts about earthquakes, and here’s an article about the Torino scale revised.
We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.