Richter magnitude scale

The Richter magnitude test scale (or more correctly local magnitude ML scale) assigns a single number to quantify the size of an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismogram. So, for example, an earthquake of magnitude 5 is ten times greater than one of magnitude 4 and an earthquake of magnitude 8 is 10(8-4) or 10000 times greater than one of magnitude 4.

Richter magnitudes

Events with magnitudes of about 4.5 or greater are strong enough to be recorded by seismographs all over the world.

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The following describes the typical effects of earthquakes of various magnitudes near the epicenter. This table should be taken with extreme caution, since intensity and thus ground effects depend not only on the magnitude, but also on the distance to the epicenter, and geological conditions (certain terrains can amplify seismic signals).

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DescriptorRichter magnitudes

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Earthquake Effects

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Frequency of Occurrence

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MicroLess than 2.0

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Microearthquakes, not felt.

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About 8,000 per day

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Very minor2.0-2.9

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Generally not felt, but recorded.

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About 1,000 per day

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Minor3.0-3.9

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Often felt, but rarely causes damage.

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49,000 per year (est.)

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Light4.0-4.9

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Noticeable shaking of indoor items, rattling noises. Significant damage unlikely.

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6,200 per year (est.)

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Moderate5.0-5.9

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Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings.

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800 per year

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Strong6.0-6.9

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Can be destructive in areas up to about 100 miles across in populated areas.

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120 per year

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Major7.0-7.9

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Can cause serious damage over larger areas.

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18 per year

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Great8.0-8.9

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Can cause serious damage in areas several hundred miles across.

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1 per year

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Rare Great9.0 or greater

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Devastating in areas several thousand miles across.

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1 per 20 years

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(Adapted from U.S. Geological Survey documents.)

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Great earthquakes occur once a year, on average. The largest recorded earthquake was Great Chilean Earthquake of May 22, 1960 which had a magnitude (MW) of 9.5 (Chile 1960).

Related Topics:
Great Chilean Earthquake - May 22 - 1960 - Chile

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The following table lists the approximate energy equivalents in terms of TNT explosive force.

Related Topics:
Energy - TNT

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Richter MagnitudeApproximate TNT for Seismic Energy YieldExample

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-1.56 lb (3 kg)Hand grenade

Related Topics:
Lb - Hand grenade

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1.0 30 lb (14 kg)Construction site blast

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1.5 320 lb (145 kg)WWII conventional bombs

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2.0 1 metric tonWWII conventional bombs

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2.5 4.6 metric tonsWWII blockbuster bomb

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3.0 29 metric tonsMassive Ordnance Air Blast bomb

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3.5 73 metric tonsChelyabinsk nuclear accident, 1957

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4.0 1 kilotonSmall atomic bomb

Related Topics:
Kiloton - Atomic bomb

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4.5 5.1 kilotonsAverage tornado (total energy)

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5.0 32 kilotonNagasaki atomic bomb

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5.5 80 kilotonsLittle Skull Mtn., NV Quake, 1992

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6.0 1 megatonDouble Spring Flat, NV Quake, 1994

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6.5 5 megatonsNorthridge quake, 1994

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7.0 32 megatonsLargest thermonuclear weapon

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7.5 160 megatons Landers, CA Quake, 1992

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8.0 1 gigaton San Francisco, CA Quake, 1906

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8.5 5 gigatonsAnchorage, AK Quake, 1964

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9.0 32 gigatonsIndian Ocean quake/tsunami, 2004

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10.01 teraton10 km rocky bolide impacting at 25 km/s

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