6.16: Scales that Represent Earthquake Magnitude
How do scientists measure earthquakes?
This 6.3 magnitude earthquake in Christchurch New Zealand in 2011 caused 181 deaths and thousands of injuries. Earthquakes and the damage they cause can be measured in a few different ways based on the damage they cause or the energy of the quake.
Measuring Earthquakes
People have always tried to quantify the size of and damage done by earthquakes. Since early in the 20^{th} century, there have been three methods. What are the strengths and weaknesses of each?
Mercalli Intensity Scale
Earthquakes are described in terms of what nearby residents felt and the damage that was done to nearby structures. What factors would go into determining the damage that was done and what the residents felt in a region?
Richter Magnitude Scale
Developed in 1935 by Charles Richter, this scale uses a seismometer to measure the magnitude of the largest jolt of energy released by an earthquake.
Moment Magnitude Scale
This scale measures the total energy released by an earthquake. Moment magnitude is calculated from the area of the fault that is ruptured and the distance the ground moved along the fault.
Log Scales
The Richter scale and the moment magnitude scale are logarithmic scales.
- The amplitude of the largest wave increases ten times from one integer to the next.
- An increase in one integer means that thirty times more energy was released.
- These two scales often give very similar measurements.
How does the amplitude of the largest seismic wave of a magnitude 5 earthquake compare with the largest wave of a magnitude 4 earthquake? How does it compare with a magnitude 3 quake? The amplitude of the largest seismic wave of a magnitude 5 quake is 10 times that of a magnitude 4 quake and 100 times that of a magnitude 3 quake.
How does an increase in two integers on the moment magnitude scale compare in terms of the amount of energy released? Two integers equals a 900-fold increase in released energy.
Moment Magnitude Scale is Best
Which scale do you think is best? With the Richter scale, a single sharp jolt measures higher than a very long intense earthquake that releases more energy. The moment magnitude scale more accurately reflects the energy released and the damage caused. Most seismologists now use the moment magnitude scale.
The way scientists measure earthquake intensity and the two most common scales, Richter and moment magnitude, are described along with a discussion of the 1906 San Francisco earthquake in Measuring Earthquakes video (3d): http://www.youtube.com/watch?v=wtlu_aDteCA (2:54).
Vocabulary
- logarithmic scale: A scale in which each unit is an exponential increase over the previous unit.
Summary
- Mercalli Intensity Scale depends on many factors besides the amount of energy released in the earthquake including the type of basement rock and the quality of the structures built in the area.
- The Richter scale is a logarithmic scale that measures the largest jolt of energy released by an earthquake.
- The moment magnitude scale is a logarithmic scale that measures the total amount of energy released by an earthquake.
Practice
Use this resource to answer the questions that follow.
1. How is earthquake strength measured?
2. What is magnitude?
3. What do scientists use to measure earthquakes?
4. How is magnitude calculated?
5. What is intensity?
6. What does intensity depend upon?
7. How does geology affect intensity?
Review
1. Under what circumstances might the Mercalli Intensity Scale be useful today? Why was it replaced by the Richter and then the moment magnitude scales?
2. Why do scientists prefer the moment magnitude scale to the Richter scale?
3. How much difference is there between the 5.8 magnitude quake that struck Virginia and the 9.0 quake that struck Japan, both in 2011, in their energy released and largest wave amplitude?
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- Describe how scientists express the size and intensity of an earthquake.