How Are Earthquakes measured?

Summary: How do we measure earthquakes? After an earthquake hits, we all want to know what it measured on the Richter Scale. What does it all mean?

With the recent earthquake off Indonesia and the resulting devastating tsunami, people seem to be a lot more interested in earthquakes and when and why they happen. Immediately following an earthquake, everyone wants to know “What was it?” What they really mean to ask is, “What did it rate on the Richter Scale?”

But the Richter Scale, while considered scientifically to be the most accurate rating system, is only one of many different systems for rating the intensity and magnitude of an earthquake.

First, an understanding of magnitude and intensity is necessary as they aren’t the same thing and different methods of earthquake (EQ) measurement usually concentrate on only one of these two factors.

In 1883, after publishing similar papers, Michele de Rossi of Italy and Francis Forel of Switzerland jointly collaborated on what was to be the first internationally recognized scale for measuring the intensity of an earthquake. The Rossi-Forel Scale, created in 1883, measures the seismic intensity of an earthquake taken in a given location, usually the epicenter. This, in conjunction with the qualitative data of visual damage assessment, gives a somewhat subjective rating.

In 1902 Guiseppe Mercalli created what was to replace the Rossi-Forel Scale. The Modified Mercalli Scale used eye-witness accounts of the destruction caused by a quake. In spite of it’s extreme subjectivity, it remained the primary way to record data during tremors and was not replaced until 1935.

The Richter Scale, developed by Charles Richter at the California Institute of Technology, differs from both the Rossi-Forel and Modified Mercalli as it takes its seismic measurement from the EQs point of origin. By using that information as well as the distance between the point of origin and the seismograph collecting the data an objective and quantitative measurement can be made.

While an EQ measured by the Richter Scale will display only one measurement (although sometimes an EQ can be up or down graded once more data is available and the point of origin is found), a quake measured by one or more of the scales that measure intensity can vary. In addition, intensity measurement of a single EQ at different points giving a varying results – all of which can be accurate. The Richter Scale, which obtains its data from seismic measurements and conditions, is usually referred to as the scientific standard for measuring EQs, while the Modified Mercalli Scale gets a good deal of it’s data via human observation, which is subjective. So while a single EQ can have many ratings of intensity based on the number of locations seismic activity is recorded, there would only be one magnitude rating using the Richter Scale.

The Richter Scale measurements are rated 2-10+ depending on magnitude, while the modified Mercalli is rated I-XII. The below chart demonstrates the differences between the two rating systems:

Richter
Magnitude
Mercalli
Intensity
Description

2
I
Usually not felt, but detected by instruments.

II
Felt by very few people.

3
III
Felt by many, often mistaken for a passing vehicle.

IV
Felt by many indoors, dishes and doors disturbed.

4
V
Felt by nearly everyone. People awakened. Cracked walls, trees disturbed.

5
VI
Felt by all. Many run outdoors. Furniture moves. Slight damage occurs.

VII
Everyone runs outdoors. Poorly built buildings suffer severe damage. Slight damage every where else.

6
VIII
Everyone runs outdoors. Moderate to major damage. Minor damage to specially designed buildings. Chimneys and walls collapse.

7
IX
All buildings suffer major damage. Ground cracks, pipes break, foundations shift.

X
Major damage. Structures destroyed. Ground is badly cracked. Landslides occur.

8
XI
Almost all structures fall. Bridges wrecked. Very wide cracks in ground.

XII
Total destruction. Ground surface waves seen. Objects thrown into the air. All construction destroyed.

The Richter Scale measurements increase depending on the effects of the earthquake, but the damage created by the quake seems to increase more steeply than one might expect. This is because the Richter Scale is logarithmic, meaning that a magnitude rating of three is 10 times more powerful than a rating of two and rating of four is 10 times more powerful than a rating of three which is why a Richter rating of seven covers both IX and X on the Mercalli Scale. Each whole number increase on the Richter Scale corresponds to the release of about 31 times more energy than the amount the preceding number.

Rated as a 9.4 on the Richter Scale, the tsunami created by the earthquake in the Indian Ocean was totally devastating, leveling everything in it’s wake.

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