Crash Course: How do Seismologists Study Earthquakes?

Seismologists study everything related to the causes, effects, and future prediction of earthquakes. Take some time to learn about how seismologists do their jobs.

• Collecting Data and
• Monitoring Geologically Active Areas

The most important data collection tool used by seismologists is the seismograph. These sensitive instruments measure even the tiniest vibrations created by the crustal movement during an earthquake. A global network of seismometers is able to record the characteristic vibrations created by earthquakes along with other ground-shaking events such as quarry blasts or even the detonation of nuclear weapons. There are even several seismographs on the Moon; they were placed there by the Apollo astronauts in order for planetary geologists to study “moonquakes.” Much can be learned from reading and analysing seismographs.

Each instrument is equipped with a solid base meant to move with the Earth’s vibrations and a small mass suspended by a spring or pivot that is free-floating. It is not affected by any movements at all. Attached to this mass is a recording pen that “graphs” any difference in motion between the mass and the instrument’s base. When the earth is not vibrating as a result of an earthquake, there is no difference and pen will record a line like this:

During an event that causes vibration, the seismometer will shake with the Earth, causing the recording pen to draw a oscillating line. The greater the shaking, the more the line varies from the original horizontal shape:

Interpreting the seismograph is as simple as follows.

1. The size of the oscillating wave describes the intensity of shaking.
2. The length and location of the oscillating wave describes when the shaking started and how long it lasted.

Questions

1. This particular vibration started at second:

   1. 1

   2. 2

   3. 3

   4. 4
   Answer

   b

2. This vibration lasted for how long?

   1. 0.7 seconds

   2. 2 seconds

   3. 2.7 seconds

   4. 3 seconds
   Answer

   a

So far, we have observed very simple models of vibrations. Vibrations are known as seismic waves. These always radiate outward from the location of the earthquake (its epicentre). There are several types of seismic waves, each with unique properties. Two important waves are known as P waves (for preliminary waves) and S waves for (secondary waves). Preliminary waves always move faster (6.2km/sec) than secondary waves (3.65km/sec) and thus always reach a seismograph first. Therefore, they are recorded first.

Conveniently for seismologists, P waves and S waves travel at specific speeds which allow them to know how far the waves had to travel to reach the seismic recording station. A nearby earthquake would have S waves that closely follow the arrival of P waves, whereas a distant earthquake would have S waves that arrive at a much more delayed time after the P wave arrived. This is because of their slower speed and the P waves’ faster speed. By calculating the difference in the arrival time of P and S waves, seismologists can use a RATIO to determine how far away the earthquake was. The image below shows this relationship in the form of a graph.

The relationship can also be expressed using mathematical formulas.

P wave Travel	S wave Travel
How long would it take for a P wave to travel 100km? Time = distance/speed P wave distance = 100 km P wave speed =  6.2 km/sec P wave time =   100/6.2 P wave time = 16.1 seconds	How long would it take for an S wave to travel 100km? Time = distance/speed S wave distance = 100 km S wave speed =  3.65 km/sec S wave time =   100/3.65 S wave time = 27.39 seconds

Once seismologists know the travel time of the waves, they can calculate the difference in arrival time. They can then use this information to calculate the exact distance the earthquake’s epicentre (definition:the point on the Earth's surface vertically above the focus of an earthquake) was from the recording station.

Difference in arrival time =   S wave time  -  P wave time

Difference in arrival time =  27.39 - 16.1

Difference in arrival time = 11.29 seconds

 

Epicentre Distance =      22.63 (difference in arrival time)/2.55

Epicentre Distance =      22.63 (11.29)/2.55

Epicentre Distance =      255.49/2.55

Epicentre Distance =      100.19km

The epicentre of this earthquake was approximately 100.19km from this seismic station.

 

The final step for a seismologist to pinpoint the location of an earthquake would be to repeat the above steps for two other seismic stations. Seismologists would then use circles with diameters equaling the calculated distances to triangulate the location where each distance overlaps. Triangulation is required because the formula only provides a distance away, without a direction, so theoretically, the distance calculated could have come from any direction. When three seismic recording stations readings are observed together, a clear picture of distance and direction are achieved.

 

Organizing, Classifying, and Categorizing Earthquakes

Almost everyone is familiar with the system for classifying earthquakes known as the Richter Scale. However, this scale has limitations and was replaced in the 1970s by a scale known as the Moment Magnitude Scale. It does a better job of measuring and comparing the intensity of all earthquakes. Interestingly enough, popular news sources still mistakenly refer to current earthquakes using the name Richter, when, in fact, the values (between 0-10) published are Moment Magnitude values. The difference between both scales is minimal to the average person and is likely a reason why the use of “Richter” has persisted. The Moment Magnitude Scale is known in mathematical terms as “logarithmic” which relates to exponential increases as opposed to arithmetical. The table below illustrates the importance of understanding how magnitude is calculated and interpreted by a seismologist.

Arithmetic Increase	Logarithmic Increase	The Moment Magnitude Scale
1 + 0 = 1 1 + 1 = 2 2 + 1 = 3 3 + 1 = 4 4 + 1 = 5 1 is one unit larger than 0 2 is one unit larger than 1 3 is one unit larger than 2 4 is one unit larger than 3 5 is one unit larger than 4     Using our example above the total range of “size” is 5. 5 is 5 times bigger than 1. Using numbers in this way does not work for measuring events like earthquakes because small earthquakes would have to be measured using extremely small numbers like (0.00000001) in order to measure large earthquakes at a convenient value between 1 - 10.   Conversely, if one were to use a value of between 1-10 for small earthquakes, the equivalent measure of a large earthquake would require numbers into the billions like 1000000000 which is neither convenient nor easy to write.   So how do we make it so that all earthquakes can be measured using numbers between 1-10? LOGARITHMS.	Logarithms use exponents to cover more “mathematical ground” than simple arithmetic allows. When reading the magnitude of an earthquake what you are really reading is the exponent to which a number has been raised.   Example 100  = 1 101  = 10 102  = 100 103  = 1000 104  = 10000 105  = 100000 106  = 1000000 107  = 10000000 108  = 100000000 109  = 1000000000 1010 = 10000000000   So when you interpret an earthquake's magnitude, it is important to understand how much stronger one quake is compared to another. For example, an increase of one value (say 2 to 3) on a simple logarithmic scale is not 1x  larger but rather 10x larger. An increase of two values (say from 2 to 4) on a logarithmic scale is 100x larger.	The logarithmic scale used to measure earthquake magnitude is not as simple as our first logarithm scale that used 10 as the base. Ignoring the unnecessary mathematical details, this formula results in the following measurements of intensity.   An earthquake measuring 3.0 on the Moment Magnitude scale is roughly 32 times stronger than an earthquake measuring 2.0.   An earthquake measuring 5.0 on the Moment Magnitude scale is roughly 1000 times stronger than a 3.0.   So…   An increase of 1 = 32 times more energy released.   An increase  of 2 = 1000 times more energy released.   An Increase of 3 = 31600 times more energy released.   An Increase of 4 =  1000000 times more energy released. Being able to calculate this is not within the scope of this course but for comparative purposes, a 10.0 earthquake would release 3.1 trillion times as much energy as an earthquake measuring 1 on the Moment Magnitude Scale!

 The Magnitude Scale

Take a moment to explore how the magnitude scale works:

Fill out this chart using this convenient conversion app created by the United States Geological Survey.

Submit Your Work

 

 

Seismologists don’t just want to know where on the surface or how intense an earthquake is. They also measure the depth of the quake. This is known as the focus. Simply put, the depth of a quake has an effect on its strength. Depth can tell experts about what is happening along a fault line or plate boundary. Depth is categorized simply as shallow (0-70 km), intermediate (70-300 km), and deep (300-700 km). Distance is only measured as far as 700 km because this is the depth to which rigid subducting plates can exist. Below this point, Earth’s interior loses the rigidity required to cause a quake. Earthquake depth is calculated by interpreting the characteristics of P and S waves as they are recorded on seismograms.

Setting Up and Managing Early Warning Systems

Once seismologists have mastered the study of earthquakes and tectonic processes, the most important role in their job can begin. It is their responsibility to communicate with the media and public to educate people about earthquakes and design earthquake preparedness strategies. During or after a quake, they inform government about damage to critical infrastructure, help coordinate emergency response groups, and as mentioned, communicate with media.

Check out the sophisticated technology that Canadian seismologists are implementing off the coast of British Columbia to keep people safe.

 

Or read this article, The race to get an earthquake early warning system in B.C. from CBC News (Original article)

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