How are Earthquakes Measured? – What is Magnitude?

How are Earthquakes Measured?

I’ve explained how earthquakes were caused in my recent post so hopefully we have a better understanding of that. Now let’s discuss how they are located and measured. How do scientists actually know exactly where the earthquake occurred? Lets find out.

Earthquake Waves

First off, an earthquake emits waves which seismographs (seismic devices) detect and process. There are two main types of earthquake waves. P-Waves (Primary Waves) and S-Waves (Secondary Waves).

P-Waves (Primary)

P-waves are vibrations that transfers energy and are longitudinal in nature, which means that they go in the same direction as the direction of travel. They are called primary waves because they reach the detector first. P-Waves are compression waves and travel faster than S-waves. They can travel through solids (like the earth), liquids (like the ocean), gases (like the air) whereas S-waves can only travel through solids.


Think of it as using a slinky and moving it back and forth. You’ll notice some areas in the slinky will compress as it travels to the other side.

S-Waves (Secondary)

S-waves are vibrations that transfers energy and are transverse in nature, which means that they move up and down as they travel through the earth. They are called secondary waves because they reach the detector second. S-Waves are shear waves that can only travel through solids and are much slower than P-waves.


Again, think of it as using a slinky but this time shaek it up and down. You can see the wave moving up and down as it travels to the other side.


Locating Earthquakes

Now that we know about earthquake waves, let’s see how scientists use them to locate where the earthquake actually occurred.

As we know, P-waves and S-waves travel at different speeds and a seismograph can detect them. Because of this difference, scientists use math to calculate how far the earthquake occurred from the seismograph location but they are unable to detect which direction it came from. That means it could be any direction from the location which would make a circle but that’s not very helpful. So they use a method called triangulation to pinpoint exactly where the earthquake occurred. It’s called triangulation because a triangle has 3 sides and it takes 3 different seismographs to pinpoint the earthquake.


Courtesy USGS

To illustrate this, you will need 3 seismographs and calculate the distance the earthquake waves traveled from each. Since we don’t know which direction it came from, draw a circle using the calculated distance as the radius. Once you do this from 3 seismograph locations, there will be a point where all three circles intersect. This is the epicenter. Pretty cool huh? I guess math does come in handy!


What is Magnitude?


Magnitude is the measure of released energy during an earthquake. The size of the earthquake is often estimated by the amplitude of the earthquake waves discussed earlier. You might have heard of the Richter scale that was invented by Charles F. Richter in the 1930’s who introduced the concept of earthquake magnitude.  His method of calculating magnitude involved knowing the distance of a Wood-Anderson seismograph to an earthquake and analyzing the maximum signal amplitude. This method was capped at recording earthquakes of magnitude 7.0 so it was replaced with the current scale called the Moment Magnitude Scale for larger earthquakes. This method relates the amount of movement and the size of the area that slipped and is more precise.

Magnitude scales are logarithmic, meaning that an increase of one unit is equivalent to 10 times the increase in amplitude recorded by a seismograph and about 30 times more energy. So for example, a magnitude 6.0 releases 30 times more energy than a 5.0 and a magnitude 7.0 releases 900 times (30 x 30) more energy than a magnitude 5.0. That’s pretty scary considering the largest earthquake was a magnitude 9.5 that devastated Chile back in 1960.

As we can see, there’s a lot of science involved in detecting earthquakes. Unfortunately we aren’t able to predict earthquakes so the best defense is to know what to do during an earthquake and be prepared.

Be Prepared for an Earthquake


  1. Roger


    YES, math CAN come in handy! During my younger years, I was very much into math, but it’s been a long time since then, so thank you for explaining how triangulation is used to determine the location of the epicenter of an earthquake. I knew they used detectors, but no idea three were used to make this calculation. And I assume that MORE can be used, and still they would indicate the same spot?

    The logarithmic increase of a factor of 10 IS pretty scary! Even though I lived in San Francisco for 17 years, and experienced multiple earthquakes (including the 1989 quake), I never realized how a single point or 2 meant so much more power!

    Thanks for the information.

    And it would certainly be awesome if earthquakes COULD be predicted. Is there any chance that will someday be possible in the future?


    1. Andy (Post author)

      Thanks Roger! Yes, you can have more detectors but they will all point to the same spot. The reason why we have so many is we don’t know where an earthquake will occur so we cover our bases. Plus of course, the closer they are to the earthquake the faster we can detect it.

      It definitely save a lot of lives if earthquakes could be predicted but I think we are still far away. Nature can be very scary and unpredictable. This is one of those cases but hopefully one day scientists will get closer.

  2. Erna

    Thanks for your article. Related to magnitude scales are logarithmic, it is scary! I experienced earthquakes in my hometown. Though the scales were informed low and in very short time, they were scary.

    I agree with your statement saying that the best defense is to know what to do during an earthquake and be prepared. Many people do need your further information about that.

    Thank you and all the best.

    1. Andy (Post author)

      Thanks Erna! Yes, the best way to stay protected is to know what to do during an earthquake and be prepared. I built this site for that purpose. To spread awareness and to protect others.


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