Great Land of Alaska

Earthquake Scales

Richter Magnitudes (ML) are based on the movement of an instrument needle and increases logarithmically, 10 times for each number jump, so ML 8 is not twice as large as ML 4, it is 10,000 times as large! Richter Magnitude is an open-ended scale.

Moment Magnitude (MW) is the modern version of the Richter Magnitudes. Moment Magnitude is based on the energy released by an earthquake and is also logrithmic, but by a factor of 32 not 10. MW 4 releases 65,000,000 Btu while MW 8 releases 69,000,000,000,000 Btu. The largest Moment Magnitude ever recorded in world history was 9.5 and occured in Chile in May of 1960. The second largest ever recorded was 9.2 and occurred in Alaska on Good Friday of 1964.

Mercalli Intensity (MM) is based on actual observations of the resulting damage and therefore can not be measured on instruments.

Moment MagnitudeRichter MagnitudeMercalli IntensityDescription
1.0-3.0 1 I Usually not felt, detected by instruments.
3.0 2 II Felt by few, especially on upper floors of buildings, detected by instruments.
3.9 3 III Felt noticeably indoors, vibration like a passing vehicle, cars may rock.
4.0 - IV Felt indoors by many, outdoors by few, dishes & doors disturbed, vibration like heavy truck nearby, walls-cracking sound.
4.9 4 V Felt by most people, slight damage; some dishes & windows broken, some cracked plaster, trees disturbed.
5.0 5 VI Felt by all, damage minor to moderate.
5.9 5 to 6 VII Much damage to poorly designed buildings, minor damage to well-designed buildings, some chimneys broken, noticed by people driving cars.
6 6 VIII Damage is moderate to major. Damage minor in well-designed structures, major in poorly designed buildings; chimneys, columns, & walls fall, heavy furniture turned, well water changes; sand & mud ejected.
6.9 7 IX Major damage in all structures, ground cracked, pipes broken, shifted foundation.
7.0+ 7 & 8 X Major damage, most masonry & frame structures destroyed, ground badly cracked, landslides, water sloshed over river banks, rails bent.
- 8 XI Almost all masonry structures destroyed, bridges fall, big fissures in ground, land slumps, rails bent greatly.
- 8 & above XII Total destruction. Ground surface waves seen, objects thrown up in air. All construction destroyed.

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