Measuring Earthquakes – a short guide to Richter Scale

Whenever a major earthquake is in the news, you’ll probably hear about its Richter scale rating. You might also hear about its Mercalli Scale rating, though this isn’t discussed as often. These two ratings describe the power of the earthquake from two different perspectives. The Richter magnitude scale is a scale of numbers used to tell the size of earthquakes. Charles Richter developed the Richter Scale in 1935. His scale was based on theseismogram measured by a particular type of seismometer at a distance of 100 kilometres (62 mi) from the earthquake.

Charles Richter
Charles Richter

Earthquakes 4.5 or higher on the Richter scale can be measured by tools all over the world.

The scale is logarithmic, with a base of 10. The amplitude of an earthquake that scores 3.0 is about 10 times the amplitude of one that scores 2.0. The energy that is released increases by a factor of about 32. 

A majority of quakes register less than 3 on the Richter scale; these tremors, called microquakes, aren’t even felt by humans. Only a tiny portion — 15 or so of the 1.4 million quakes that register above 2.0 — register at 7 or above, which the threshold for a quake being considered major [source: USGS]. The biggest quake in recorded history was the 9.5 quake that struck Chile in 1960. It killed nearly 1,900 people and caused about $4 billion in damage in 2010 dollars [source: USGS]. Generally, you won’t see much damage from earthquakes that register below 4 on the Richter scale.

Descriptor

Richter Magnitude number

Damage caused by the earthquake

Frequency of occurrence

Micro

Less than 2.0

Micro (very small) earthquakes, people cannot feel these.

About 8,000 each day

Very minor

2.0-2.9

People do not feel these, but seismographs are able to detect them.

About 1,000 per day

Minor

3.0-3.9

People often feel these, but they rarely cause damage.

About 49,000 each year

Light

4.0-4.9

Objects inside houses are disturbed, causing noise. Nothing is damaged.

About 6,200 each year

Moderate

5.0-5.9

Buildings that are not built well may be damaged. Light objects inside a house may be moved.

About 800 per year

Strong

6.0-6.9

Moderately powerful. May cause a lot of damage in a larger area.

About 120 per year

Major

7.0-7.9

Can damage things seriously over larger areas.

About 18 per year

Great

8.0-9.9

Massive damage is caused. Heavy objects are thrown into the air and cracks appear on the ground, as well as visible shockwaves. Overhead highways may be destroyed, and buildings are toppled.

About 1 per 20 years

Meteoric

10.0+

There are no records of anything of this size. The vibration is about the same as that of a 15 mi meteor.

Unknown

(Adapted from U.S. Geological Survey documents)

The earthquake with the biggest recorded magnitude was the Great Chilean Earthquake. It had a magnitude of 9.5 (approximately 9.5 on the Richter scale) and occurred in 1960. About 6,000 people died because of the earthquake.

Richter ratings only give you a rough idea of the actual impact of an earthquake, though. As we’ve seen, an earthquake’s destructive power varies depending on the composition of the ground in an area and the design and placement of man-made structures. The extent of damage is rated on the Mercalli scale. Mercalli ratings, which are given as Roman numerals, are based on largely subjective interpretations. A low intensity earthquake, one in which only some people feel the vibration and there is no significant property damage, is rated as a II. The highest rating, a XII, is applied to earthquakes in which structures are destroyed, the ground is cracked and other natural disasters, such as landslides or tsunamis, are initiated.

Modern Seismograph
Modern Seismograph

Richter scale ratings are determined soon after an earthquake, once scientists can compare the data from different seismograph stations. Mercalli ratings, on the other hand, can’t be determined until investigators have had time to talk to many eyewitnesses to find out what occurred during the earthquake. Once they have a good idea of the range of damage, they use the Mercalli criteria to decide on an appropriate rating

More examples

Approximate Richter Magnitude number

Seismic energy equivalent: Amount of TNT

Example event

0.5

5.6kg

Large hand grenade

1.5

178kg

Bomb used in WWII

2

1 metric ton

Large Bomb used in WWII

2.5

5.6 metric tons

Blockbuster bomb (dropped from airplanes) in WWII

3.5

178 metric tons

Chernobyl accident, 1986

4

1 kiloton

Small atomic bomb

5

32 kilotons

Nagasaki atomic bomb

Lincolnshire earthquake (UK), 2008

5.4

150 kilotons

2008 Chino Hills earthquake (Los Angeles, United States)

5.5

178 kilotons

Little Skull Mtn. earthquake (NV, USA), 1992

Alum Rock earthquake (CA, USA), 2007

6.0

1 megaton

Double Spring Flat earthquake (NV, USA), 1994

6.5

5.6 megatons

Caracas (Venezuela), 1967

Rhodes (Greece), 2008

Eureka Earthquake (Humboldt County CA, USA), 2010

6.7

16.2 megatons

Northridge earthquake (CA, USA), 1994

6.9

26.8 megatons

San Francisco Bay Area earthquake (CA, USA), 1989

7.0

32 megatons

Java earthquake (Indonesia), 2009, 2010 Haiti Earthquake

7.1

50 megatons

Energy released is equivalent to that of Tsar Bomba, the largest thermonuclear weapon ever tested

1944 San Juan earthquake

7.5

178 megatons

Kashmir earthquake (Pakistan), 2005

Antofagasta earthquake (Chile), 2007

7.8

600 megatons

Tangshan earthquake (China), 1976

8.0

1 gigaton

San Francisco earthquake (CA, USA), 1906

Queen Charlotte earthquake (BC, Canada), 1949

México City earthquake (Mexico), 1985

Gujarat earthquake (India), 2001

Chincha Alta earthquake (Peru), 2007

Sichuan earthquake (China), 2008 (initial estimate: 7.8)

1894 San Juan earthquake

8.5

5.6 gigatons

Toba eruption 75,000 years ago; the largest known volcanic event.[1]

Sumatra earthquake (Indonesia), 2007

9.0

32 gigatons

2011 Sendai, Japan Earthquake and Tsunami,Lisbon Earthquake (Lisbon, Portugal), All Saints Day, 1755

9.1

67 gigatons

Indian Ocean earthquake, 2004 (40 ZJ in this case)

9.2

90.7 gigatons

Anchorage earthquake (AK, USA), 1964

9.5

178 gigatons

Valdivia earthquake (Chile), 1960

13.0

108 megatons = 100 teratons

Yucatán Peninsula impact (causing Chicxulub crater) 65 Ma ago.

Sources:
https://simple.wikipedia.org/wiki/Richter_scale
Harris, Tom, and Patrick J. Kiger. “How Earthquakes Work” 16 January 2001. HowStuffWorks.com. <http://science.howstuffworks.com/nature/natural-disasters/earthquake.htm> 03 January 2016.

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