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### Course: Algebra (all content)>Unit 11

Lesson 31: Logarithmic scale (Algebra 2 level)

# Richter scale

Sal explains the basics of the Richter scale (Moment magnitude scale) and uses this to compare the magnitude of 4 famous earthquakes. Created by Sal Khan.

## Want to join the conversation?

• Why is there nothing >10
• The rock layers can not hold enough potential energy (that is stored in the rock layers) that can convert to kinetic energy that would cause >10 earthquake.
• Why is the richter scale not used as much anymore?
• It is used mostly. But as Sal says, At , the moment magnitude scale is used more often now by geologists. The moment magnitude scale (abbreviated as MMS; denoted as MW or M) is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped. The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale (ML). Even though the formulae are different, the new scale retains the familiar continuum of magnitude values defined by the older one. The MMS is now the scale used to estimate magnitudes for all modern large earthquakes by the United States Geological Survey.
• Has there ever been a 10.00 earthquake?
• No, there has never been a 10.00 earthquake in recorded history. The largest earthquake ever recorded was the 1960 Valdivia earthquake in Valdivia, Chile. It was a 9.5 magnitude earthquake on May 22, 1960.

The largest United States earthquake was the 1964 Alaska earthquake in Prince William Sound on March 27, 1964. It was a 9.2 magnitude earthquake.

The March 11, 2011 earthquake in Sendai, Japan was Japan's largest earthquake in Japan's recorded history. It was registered as a 8.9 magnitude (reports range from 8.9 - 9.1) earthquake.
Princessky
• Is there a 20.0 earthquake
• No. The rock layers that move to make the earthquakes can not hold enough potential energy (that is stored in the rock layers) that can convert to kinetic energy that would cause >10 earthquake, thus ruling out anything 20. Hope this helps
• How does an earthquake change time on earth?
• A very large earthquake doesn't "change time", but it can impact Earth's rotation. For example, if a large chunk of Earth were to shift closer to Earth's center, it might speed the rotation a bit (like what happens to a spinning skater when they pull their arms and legs in) which would make the astronomical "day" a bit shorter.

Even large events like the 8.9 earthquake that caused the 2011 Japanese Tsunami shortened the "day" by only 1.8 microseconds (or little less than 1/500,000 th of a second) so it won't really be noticeable.
• So, everything up is to a power of ten. i.e., 6.0 vs 5.8. 6.0 is 10^0.2 more than 5.8?
• yup
• What is the unit of earthquakes??
• I think Sumit meant the physical units that are used in the Richter scale. If I remember correctly, in my plate-tectonics and sismology class, the professor told us that that practically all the earthquake scales use a reason or proportion between the energy released and the amplitude of the waves, wich are several different ones that are released (Rayleigh, Love, p and s).
So, maybe the physical units are Joules and Kilometers, then converted and scaled into the logarithmic Richter scale formula.
• So if the strength of an earthquake differs by a certain amount (example: loma prieta vs. japan, differs by a factor of 100), that means that range ground movement was 100 times bigger. Is it that the range is expanded in all directions? (up/down, left/right, forward/backward)?
• I think that means that it is 100 times stronger throughout the crust, so forward/backward, left/right because it is measuring the magnitude of how strong it was throughout the earth and not the air.