Understanding the Richter Scale

One of the most misunderstood things about the topics of earthquakes is the Richter Scale. While it is a very important measurement, it actually gives us very different information from what most people think.

This week, you will need:

  • a large plate
  • cake
  • Jello or something similar

Anytime there is an earthquake, the news media makes a big point of talking about the Richter Scale, using it to indicate how bad the earthquake was. As we will see, the Richter Scale does not tell us nearly as much about surface effects as people think it does. Instead, it tells us how much energy was involved in the quake. That may seem like a small point until you think about it for a minute. An earthquake from a fault that is deep in the Earth may not cause as much damage as an earthquake from a fault at the surface, even if they release the same amount of energy.

Even at the same depth, there are other things that can cause major differences in the surface effects of an earthquake. Place a large, dinner plate on a flat surface. Cut a square of cake and place it on one side of the plate. Cut a square of Jello the same size and place it on the other side of the plate. Imagine that the plate is part of the crust of the Earth. (Insert your own joke about plate tectonics here.) Then imagine a tiny city built on top of each of the two squares. Shake the plate from side to side to simulate an earthquake. Which of the cities do you think would have the most surface effects?

Think of the city on the cake as a city built on solid rock, while the Jello represents a city built on layers of sand, gravel and clay. With the same amount of energy, differences in geology can give you very different results.

Another thing that many people don't realize about the Richter Scale is that it is a logarithmic scale. WAIT! Don't run away! I promise to keep this simple. Because the intensity of earthquakes varies so much, they use a logarithmic scale to make it manageable. It simply means that the vibrations of an earthquake with a 3 rating are ten times stronger than for one with a rating of 2. A 4 is ten times stronger than a 3, and so on. If you look at the total energy, it gets even more impressive. Because the energy radiates in all directions, the energy difference between each number on the scale is more than 30 times greater. That means that while the vibrations of a 4 are ten times stronger than a 3, the total energy released is more than 30 times stronger!

This means that as you go up the scale, the points become much more important. When the news media reported on the recent quake, some said it was a 9, while others said it was an 8.9. That does not sound like much difference until you look at the way the scale works. The difference between 8.9 and 9 is bigger than the difference between a 1 and an 8.

There is a different scale which is much more useful to the average person. The Modified Mercalli Scale is based on surface effects, so it gives you a much clearer idea of the impact of the quake. Instead of a single rating for each earthquake, you get a map with zones to show how much impact it had on each area. You can find a copy of the scale at:

https://earthquake.usgs.gov/learn/topics/mercalli.php

I know that it will take a while to digest all this information, so I suggest you digest the cake and Jello at the same time.

Have a wonder filled week.