I lived in San Diego, California from the time I was 3 until I was 13. While I was there I felt a few earthquakes, but most of them were pretty small. The most memorable earthquake happened probably around 2003 or so. I was at a friend’s house and we were eating a snack at the table. Suddenly there was a rumbling sound and the glasses and plates on the table started shaking around, so we quickly rushed to the doorframe to wait until the earthquake was over. Earthquakes don’t last very long, so it was over quickly. Where I was, nothing got damaged or broken in that earthquake, but it was strong enough to be memorable.
Because I lived in an earthquake-prone area in my childhood I was curious about earthquake magnitude and what it really meant when an earthquake was a 4.2 or a 6.6. I also wondered, why are earthquakes sometimes reported with a magnitude of 5.4 by one news source, but 5.6 by another? When I see an earthquake magnitude listed it’s normally a number, but sometimes it’s a Roman numeral, do those mean something different? If you are curious about any of these things, just like I was, then you can get the answers here.
What is earthquake magnitude?
In 1935 a man named C. F. Richter made up a way of measuring earthquakes based on how much energy each earthquake releases. If the name Richter sounds familiar it’s because the system he made is called the Richter scale and is commonly used to measure earthquake magnitude. The energy from the earthquake moves through the ground as seismic waves. The size of the very first seismic wave to arrive is used in the calculations to determine magnitude. Something interesting about magnitude is that it is on a logarithmic scale. This means that as the magnitude gets a little bit bigger the size of the earthquake gets a lot bigger. I made some graphs to show this relationship.
A logarithmic scale means that a magnitude 2 earthquake is not the same as a magnitude 1 + another magnitude 1. Instead a magnitude 2 is as big as 10 magnitude 1 earthquakes
A magnitude 2 is also as strong as nearly 32 magnitude 1 earthquakes happening at the same time.
The difference between bigger and stronger.
A magnitude 2 earthquake is 10 times bigger than a magnitude 1 earthquake. This means that the amplitude, or height, of the first seismic wave from the magnitude 2 earthquake is 10 times taller than the amplitude of the first seismic wave from the magnitude 1 earthquake. A magnitude 2 earthquake is nearly 32 times stronger than a magnitude 1 earthquake. Strength measures the amount of energy that is released. So a magnitude 2 earthquake releases the same amount of energy as nearly 32 magnitude 1 earthquakes happening at the same time.
The United States Geological Survey (USGS) has a “How Much Bigger…?” Calculator that you can use here to see just how much bigger a 6.3 is than a 5.4, or whatever other numbers you’d like. In my geophysics class we had to do all those calculations ourselves, but this handy calculator is much easier!
To really understand the differences in magnitude I’ll take two earthquakes that most people are very familiar with and compare them. Many people remember the 1994 Northridge Earthquake that happened in California because it was very destructive. Another very destructive earthquake was the 2004 Indian Ocean Earthquake, which also caused a large, destructive tsunami.
The 1994 Northridge Earthquake was a magnitude 6.7 earthquake.
The 2004 Indian Ocean Earthquake was a magnitude 9.3 earthquake.
Here are the results from the USGS “How Much Bigger…?” Calculator:
So the first seismic wave to arrive at the 2004 Indian Ocean Earthquake was 398 times taller than the first seismic wave to arrive at the 1994 Northridge Earthquake! The Indian Ocean Earthquake also released the same amount of energy as if almost 8,000 of the Northridge Earthquakes were happening at the same time in the same place!
Calculating Magnitude
Sometimes you will see different magnitudes reported for the same earthquake. Here are some examples of that using the 2004 Indian Ocean Earthquake. I found some news articles about the earthquake and took a screenshot of what they say the magnitude is. The full article is also linked for each example.
The Guardian says it was a magnitude 8.9,
the New York Times says it was a magnitude 9.0,
Earth Magazine says it was a magnitude 9.2,
and BBC says it was a magnitude 8.9.
So what’s up with these numbers?
Are these news sources going crazy? Did they not bother to do the calculations right? Are they really just guessing? The answer to all of these questions is no. This doesn’t mean that one person calculated it wrong and the other person calculated it right. There are actually a few different ways to calculate different types of magnitude and some news sources will say which calculation they used. Here is a screenshot from the Wikipedia page “List of 21st-century earthquakes”
You can see here that in the comments next to the magnitude column there a comment with either Mw or Mb. Those letters are identifying which magnitude calculation was used. Mb is a calculation of magnitude based on the seismic body waves, which is one of the types of waves that allow the energy released during the earthquake to travel through the earth. Mw is the moment magnitude, which is calculated using the seismic moment, which is a function of the area of a fault and how much the earth moved along the fault. There is another measure of magnitude, Ms, which is calculated using the seismic surface waves, another type of wave that allows the seismic energy to move through the earth. Ms is not commonly used, Mb is sometimes used, and Mw is the most common. Each different method of calculating the magnitude can give a slightly different answer. This is why you might see two different news sources reporting two different magnitudes for the same earthquake.
The Mercalli Scale
There is another way of representing how big an earthquake is, called the Mercalli scale. The Mercalli scale is a subjective scale, so it cannot be calculated mathematically like the Richter scale is. The Mercalli scale ranges from 1-12 and is usually represented by Roman numerals. Each number is associated with a certain amount of shaking and destruction. This means that an earthquake in an area that is prepared for earthquakes will rank lower on the Mercalli scale than the same earthquake in an area that is not prepared for earthquakes.
Here is an idea of what each number in the Mercalli scale represents
I – Most people won’t notice this earthquake; there is only a little bit of shaking.
II – Most people won’t notice this earthquake either. If you’re lucky and happen to be sitting very still at the top of a very tall building, doing nothing you may notice some very gentle swaying but may not recognize it as an earthquake.
III – Most people in a building will feel some shaking or swaying but may not recognize it as an earthquake. The sensation is a slight shaking or rumbling similar to when a large truck, like the garbage truck, passes by your house.
IV – You will notice shaking, and something small like a cup or a plate might rattle on the table. If you are asleep you might wake up even though you may not know why you woke up. Even if you haven’t experienced an earthquake before you would probably recognize this as an earthquake.
V – You would have to be doing something very active to not notice this earthquake. The shaking is strong enough to break a few windows or knock plates off of shelves.
VI – You definitely feel this earthquake and furniture will slide or shake around. The shaking could break small sections of walls. This earthquake will cause a small amount of damage, but nothing major.
VII – Buildings built to withstand earthquakes will not have noticeable damage. Older buildings or buildings not built to withstand earthquakes will have more significant damage and some chimneys may break.
VIII – Columns on buildings crack or fall, chimneys break. Partial collapse on most buildings. Only specially designed buildings will have a small amount of damage. Other buildings will have significant damage.
IX – Buildings collapse and shift off of their foundations. Even specially designed buildings will have a significant amount of damage.
X – Brick buildings destroyed. Most wood-frame buildings destroyed.
XI – Mostly all buildings are flattened. Bridges are broken.
XII – This is like an end-of-the-world earthquake, which causes total damage. Nothing is left standing. Everything is broken and collapsed and there is a large death toll.
The Mercalli scale is not used as frequently as the Richter scale, mostly because it is so subjective. How an earthquake ranks on the Mercalli scale really depends on where it hits and how prepared that area is for an earthquake. Here is a simple infographic to show the differences between the Richter scale and the Mercalli scale.
Sources:
Fowler, C. M. R., 2005, The Solid Earth: An Introduction to Global Geophysics, 2nded., Cambridge University Press
USGS, Earthquake Hazards Program, https://earthquake.usgs.gov/earthquakes/
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