LAB G: Earthquakes & Earth's Interior  PRE-LAB (for printing)

 

The purpose of this exercise is to help you gain a basic understanding of earthquakes before completing the in-lab exercises.  While people usually fear earthquakes as destructive, geologists view them as constructive.  Earthquakes are the result of much pressure within Earth’s crust that has built up over time eventually causing the rocks to break.  When the rocks break, the pressure is released as energy waves that move through the crust towards the surface. 

 

The breaking of the rocks usually results in a fault, and the movement of Earth’s crust along faults produces landforms such as mountains and valleys over a long period of time.  Thus, earthquakes result from constructive forces within Earth’s crust.

 

EARTHQUAKES:

Earthquakes are massive vibrations in Earth’s crust that occur when pressures building in the rocks overcome the rocks’ resistance to the breaking point.  Since rocks are inherently stationary, the force that is applied to the rocks is stored within the rocks.  This stored energy is referred to as potential energy.  Once the forces acting on the rocks overcome the rocks’ resistance, the rocks break.  At this point, the potential energy is free to move through the crust, thus converting into kinetic energy.  These are the vibrations that we feel.  Seismology is the study of earthquakes waves.  The word “seismos” = shake and “ology”  = the study of.  Therefore, whenever you see the root word “seismos” in another word, it refers to earthquakes.  The term seismic wave refers to the vibrations felt when energy is rapidly released within Earth’s crust. 

 

An analogy using a common object would be bending a stick until it breaks.  As you take a stick in your hands and bend it, the work that your muscles are doing creates energy that must be stored in the stick.  As long as you continue to apply that energy, the stick stays bent.  The energy in the stick is the potential energy.  Eventually, you will apply enough force that the stick breaks.  Immediately, you hear a loud “snap”.  That “snap” is the energy stored in the stick now moving freely (kinetic energy) through the air as sound waves.  Likewise, stored energy in the rocks moves through the crust, creating vibrations that we feel.

 

A fault is any break in the rocks along which movement has occurred.  To identify a fault, rock layers have to show displacement. 

 

 

FOCUS AND EPICENTER

The focus of an earthquake is the point within Earth’s crust where the rocks break, sending seismic waves radiating outward in all directions. 

 

 

These waves are analogous to dropping a pebble in a calm pond and watching the ripples radiate out from its source.  The focus of an earthquake can be located at any depth within Earth’s crust, depending on where the pressure is building.  Those earthquakes that occur at great depths within the crust are called deep-focus earthquakes.  When the focus is closer to the surface, it is called a shallow-focus earthquake.  

 

 

Even though the seismic energy decreases with increasing distance from the focus, sensitive instruments can record the event throughout the world.  Go to the USGS web page http://earthquake.usgs.gov/earthquakes/world/seismicity/.  The colors on the map represent the depth to the focus of the major earthquakes for each region.  The color code is given to the right of the image.  After examining the United States and South America, write a brief description on the pre-lab activity describing the differences in the depth of their foci (foci = plural of focus).  You may also link to the individual continents from the web site to see each continent in more detail.

 

The epicenter is the point on the surface directly above the focus.  In a shallow-focus earthquake, the focus and the epicenter can be very close together. 

 

 

SEISMIC WAVES

The energy released when rocks break travels in all directions away from the focus are called seismic waves.  These waves can be divided into two main categories:  surface waves and body waves.  Surface waves (also called L-waves) travel along the ground, moving the ground and anything resting on it, similar to ocean swells tossing a ship.  These waves have the potential for causing the most damage to the foundation of structures.  Body waves radiate from the focus and move through Earth’s interior.

 

There are two types of body waves:  primary waves (P-waves) and secondary waves (S-waves).  Although both result from energy released at the same time from the focus, P-waves and S-waves are distinguished by the manner in which they move through materials.  P-waves move in a compressional manner, pushing (compressing) and pulling (expanding) against the material.   Imagine holding a coiled spring.  When you push against the spring, it compresses.  But since it resists volume change, when you release the pressure, it expands back to its original shape.  This is the manner in which P-wave energy moves through material in a fairly straight line.  S-waves shake the material they’re traveling through at right angles to their path of motion.  Imagine lying a rope on the ground and shaking it left to right at one end.  The rope’s motion will be left to right as the energy passes down the length of the rope.

 

Since the energy from S-waves is not traveling in a straight line, it takes longer for the energy to reach the surface than for P-waves to arrive.  Thus, when looking at a seimsogram, there is a major difference in the seismic waves.  P-waves always arrive at the recording station first, then S-waves, and finally surface waves.

 

Seismic recording stations are called seismographs.  Seismographs are instruments attached to the ground above which a pen is suspended by a large weight.  As the ground shakes, the instrument shakes beneath the suspended pen.  It is an optical illusion that makes the pen look as if it is moving across the paper.   If you are interested in learning more about how seismographs work, click here.  The vibrations recorded on the page represent the arrival of the P- and S-waves, along with the surface waves.  The paper record from the seismograph is called a seismogram.  When viewing a seismogram, there is a relatively straight line before the arrival of the P-waves.  This is called the base line and it records the many microseisms in the environment (vibrations produced by air pressure changes, trains, waves crashing against the coast, etc.).  The seismic waves visibly “jump” off the baseline, heralding the arrival of an earthquake.

 

The key to identifying the epicenter of an earthquake such as the one you will work with in lab, is to identify the difference in arrival times between the P- and S-waves.  Recall that P-waves always travel faster than S-waves.  In reality, P-waves are almost twice as fast as S-waves.  Therefore, the greater the difference in their arrival times at the seismograph, the farther the waves have traveled from the earthquake’s epicenter.  A good analogy would be to imagine two runners.  One runner is twice as fast as the other runner.  If these two runners compete in a 100-meter race, the faster runner will win.  But how far ahead will s/he be from the second runner?  ______ meters.  If they compete again, but in a 10,000-meter race, how far ahead will the faster runner be at the finish line from the other runner?  _____ meters.  By this example, the farther they’ve traveled (10,000 meters), the farther ahead the fastest runner gets.  The same is true for P- and S-waves.  The farther away from the epicenter a seismograph is located, the greater the difference in arrival times between the two sets of vibrations. 

 

Look at the following seismographs and determine the difference in arrival times between the P- and S- waves.  Note the dots on the seismogram represent minutes.  Place your answers on the pre-lab activity.

 

 

RICHTER SCALE

The Richter Scale measures the strength of an earthquake’s magnitude.  Magnitude represents the amount of energy released from an earthquake’s focus.  Developed in 1935 by Charles Richter, the Richter Scale uses the amplitude of the seismic waves to measure how much the ground is shaking.  The amplitude of a seismic wave is measured by how high the wave peak is above the baseline. 

 

 

The Richter Scale is a logarithmic scale.  This means that, from one number to the next, there is a 10-fold increase in wave amplitude which represents the ground shaking 10 times greater than for the previous number.  For example, a change from a 3 to a 4 on the Richter Scale means that the ground holding the seismograph is shaking 10 times more for a 4 than a 3.  The amount of energy needed to shake the ground that much is roughly 32 times greater than the previous number.  So the amount of energy needed to go from a 3 to a 4 on the Richter Scale is 32 times as much energy.  Now, figure out how much energy a reading of 5 from the Richter Scale represents than a 3.  Recall that to jump from a 3 to a 4 requires 32 times the energy.  Likewise to jump from a 4 to a 5 requires 32 times more energy.  So a 5.0 on the Richter Scale requires 1024 times more energy than a 3.0 (32 times 32).    That’s a significant increase in energy released from the focus of a 5.0 earthquake!

 

Here are the typical effects of earthquakes in various magnitude ranges:

 

Richter Magnitude Richter Effects

< 3.5

generally not felt, but recorded

3.5 - 5.4

often felt, rarely causes damage

< 6.0

at most - slight damage to well designed buildings, can cause major damage to poorly constructed buildings over small area

6.1 - 6.9

can be destructive in areas where up to a 100k or more people live

7.0 - 7.9

Major EQ - can cause serious damage over large areas

8 or greater

Great EQ - can cause serious damage in areas several hundred km acres

 

EARTHQUAKE FACTS AND STATISTICS

Click on this link to the National Earthquake Information Center of the USGS:  USGS Earthquake Statistics.
Using the charts given for the “Number of Earthquakes Worldwide” and the “Number of Earthquakes for the United States” from 2000-2010, fill in the table for Question 5 on the pre-lab activity.  For each year, write in the magnitude of the most frequent earthquakes experienced worldwide and in the United States.   Ignore the category of "No Magnitude" on the chart.