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.

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.

(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.

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

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.