Seismic Waves

Section 1: Understanding Seismic Waves

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Japan Earthquake News video.jpg

Seismic wave are vibrations caused by an earthquake.  As mentioned in other sections, stress builds up inside the earth.  When that stress is released we get an earthquake.  The location at which the stress built up and then released is called the focus.  Seismic waves travel from the focus outwards in all directions.  The point on the surface of the earth at which a seismic wave first hits is called the epicenter.  The epicenter is directly above the focus.  There are instruments throughout the world that detect earthquakes.  These instruments are called seismometers or seismographs.

There are a few earthquake waves that we need to understand.  The first earthquake wave to hit a seismograph, because it is moving the quickest is called the primary waves or p-wave for short.  P-waves cause the ground to move back and forth in the same direction.  P-waves compress and stretch the rock.

p-wave animation

Click on it to see animation.

The next type of wave to hit a seismometer is called the secondary wave or the s-wave.  Particles in an s-wave move at right angles to the direction of the waves movement.  The move similar to a slinky being shaken in an up and down motion.

S-Wave Animation

Click to see animation

The last types of earthquake waves to hit seismic stations are the surface waves.  Surface waves move back and forth in a swaying motion in all directions.  These earthquake waves tend to be the most destructive because one part of a building moves up, while another part might be moving down, left or right.  These waves also just move along the surface around the earth.  There are two types of surface waves: Love Waves and Rayleigh Waves.  Below are animations of both types.

Earthquake Love Wave

Click to see animation.


Earthquake rayleigh wave
Click to see animation.


Seismologists are scientists who study earthquakes.  Understanding how seismic wave travel, the speeds at which they travel and how they move through specific material within the Earth, can help scientists locate the epicenters and depths of earthquakes.  Below is a video on how seismologists use seismometers and read their corresponding seismograms.

Reading a Seismogram Video.jpg

The farther apart earthquake waves are on a seismograph, the further the epicenter is from the seismic station.  The farther the epicenter is away, the more time a p-wave has of putting distance between itself and an s-wave.  To locate an epicenter, seismologists use a method called triangulation.  Triangulation uses data from three different seismic stations to determine the epicenter's location.  Scientists need data from a minimum of three seismographs.  However, the more seismic readings a scientist gets, the easier it is to pinpoint the exact location.  Triangulation is used in more than just seismology.  GPS works similarly and understanding triangulation helps law enforcement locate criminals with cell phones. 

Earthquake Triangulation Video.jpg

Seismic waves can also help us understand what the inside of the Earth looks like.  What is down there and how do we know?  The layers of the Earth have different densities.  Seismic wave travel at different rates of speed based on a material's density.  Hopefully you understand that the Earth has three main layers.  The crust, mantle and core.  Earthquake waves move faster through solids.  S-waves can't move through liquid at all, so area in which s-waves quit moving are considered to not be solids.  Both p and s waves slow down when a hit a specific layer called the asthenosphere.  The asthenosphere is a plastic like layer and is part of the upper mantle.  Both p and s wave speed up when they hit the solid lower mantle.  On the other side of the Earth from where and earthquake hits, there is a place called the shadow zone.  This is an area of about 35 degrees where seismic waves are not felt.  Why?, because the p-waves are deflected and slowed by the liquid outer core and the s-waves stop when they hit the outer liquid core.

Shadow Zone

The last thing that needs to be mentioned when talking about seismic waves, are the scales by which we measure their strength and intensity.  The Richter scale is used for small earthquakes and the moment magnitude scale is used for larger earthquakes.  They both measure intensity, but measure it more accurately for smaller or larger earthquake respectively.  

Richter Scale

Magnitude is a measure of the amount of energy that is released during an earthquake.  As an earthquake is happening it is shaking seismometers.  Seismograms print off pictures that look like waves.  The height or amplitude of these recorded waves help determine the magnitude of an earthquake.  Every factor of 10 in the height of the wave corresponds to about 32 times the amount of energy that is released in an earthquake.  So for every 10 times the amplitude of a wave on a seismometer the energy that is released is 32 times greater.  Lets compare a 1.0 to a 2.0 on a seismometer.  A 2.0 will be 10 times higher on the seismometer which means that a 2.0 earthquake released 32 times more energy.  A 3.0 on a scale would have a 10x10 or 100 times larger of an amplitude than a 1.0 and would release 32x32 or 1024 more energy  By the time you get to a 10.0 earthquake the numbers are both very, very large.

richter scale

Moment Magnitude Scale

This scale takes a few other things into consideration.  You calculate the seismic moment by understanding how rigid the Earth around a fault is and multiply it by the average amount that they fault shifted.

Moment Magnitude

Even though most earthquakes are measured by the Moment Magnitude Scale most of the time the media reports magnitudes based on the Richter Scale.  

This next activity will review and take you further in your understanding of measuring and locating earthquakes.

SUSD5 Student version of the Understanding Earthquakes through Analyzing and Plotting Data.

Purchase this Understanding Earthquakes through Analyzing and Plotting Data activity at Teachers Pay Teachers for $1.00.


Section 2: Current Earthquakes

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Below is a video that shows the process of liquefaction.  When and earthquake starts compressing and stretching the ground, it squeezes water out of the soil.  This can be very dangerous because it can loosen the soil around foundations.  The video demonstrates liquefaction that happened during the 2011 Japan earthquake.  If you watch carefully you can see the ground actually shifting and faulting.

Earthquake Soil Liquifaction Video.jpg

Now that you understand what an earthquake is and how it is measured, lets take a look at current earthquakes that are happening around the world.

SUSD5 student version of the Current USGS Significant Worldwide Earthquake Investigation activity.

Purchase the Current USGS Significant Worldwide Earthquake Investigation activity at Teachers Pay Teachers for 0.50 cents.

Before we move on in our journey to understand the Earth, have you ever wondered what scientists have to say about the portrayal of earthquakes in the movies.  Below is a quick video of scientists discussing the movie San Andreas.

What science says about Movie San Andreas.jpg

Just for fun, here is a sped up and edited version of the movie San Andreas.