Old Earth Ministries Online Geology Curriculum
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Geology - Chapter 10: Earthquakes
When the earth shakes, it is direct evidence that the planet is a living, moving body. Most earthquakes are a direct result of plate tectonics. The study of earthquakes is called seismology. Every year seismic stations record more than 150,000 earthquakes, many of which are too small to be felt by people. Through the use of these stations seismologists can calculate the exact location, depth, and magnitude of the earthquake.
Monday - Read Text
Tuesday - Research
Wednesday - Quiz
Thursday - Review
Friday - Test
This lesson plan is designed so that your child can complete the chapter in five days. The only decisions you will need to make will be concerning the research task for Tuesday. It is up to you to determine if the student will simply fill in the answers, or provide a short essay answer. You will also need to determine the percentage that this research will play in the overall chapter grade, if any.
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Earthquakes are the vibrations in the crust caused by the sudden movement of two rock bodies relative to each other. The intersection of the two rock bodies is called a fault. A fault may start in a single rock body, as tectonic forces put stress upon the rock until it reaches its elastic limit, which is the point at which rock breaks. This can be understood by looking at the figure at right, which explains the elastic rebound theory. In this overview of a road, the first figure (Time 1) shows the road intersected by a reference line. As the earth moves beneath the road, it is stretched in two directions, indicated in the second figure (Time 2). When the rock reaches the elastic limit, it breaks, and causes a fault line. The two sides move laterally in relation to each other, and this movement causes the earthquake. For an animation of elastic rebound theory, click here.
Faults do not always move laterally in relation to each other. The movement of the rock body is commonly referred to by the term "slip." Slip is usually referred to by a vector, or direction in which the movement occurs. Faults can be characterized into three groups based on the slip.
In a strike-slip fault, the two rock bodies are moving laterally in relation to each other, as can be seen in the image at right. Examples of strike-slip faults include the San Andreas Fault in California.
The second type of fault type is the normal fault. This type of fault results from the extension of the crust. The hanging wall move downward in relation to the footwall.
The thrust fault results when the hanging wall moves up relative to the footwall.
A fourth type of fault called an oblique-slip fault, combines lateral movement with vertical movement. For an animation of these fault types, see fault movement animation.
The easiest way to understand fault movement is to examine strike-slip faults. As forces push the two sides of the fault, two things can happen. First, movements along the fault can occur frequently, in small increments. For example, the San Andreas Fault is moving at an average rate of 4 cm per year, or about 1.6 inches. This slow movement is known as tectonic creep. This slow movement prevents stress from building up. However, in some cases, the stress upon the fault does not result in slow, steady movement. In this case, stress builds up over time, eventually leading to a rapid release of stress, resulting in an earthquake. Typically, movement during a earthquake rarely exceeds a few meters. The largest recorded movement of one side of a fault relative to another is only 15 meters, or about 49 feet. In the San Francisco earthquake of 1906, movement was about 7 meters (23 feet).
When an earthquake occurs, it produces three types of seismic waves that can be detected by instruments. These waves travel through the earth's crust at different velocities, all arriving at a measuring station (seismometer) at different times. The first to arrive is the primary wave, or P-wave (at right). These are longitudinal waves identical to sound waves passing through a liquid or gas. The particles move forward and backward in the direction of wave travel.
The next wave to arrive is called the secondary waves, or S-wave (at right). S-wave particles oscillate back and forth at right angles to the direction of wave travel. S-waves cause strong movements on the seismograph. For an animation of p- and s-waves, see Seismic Wave Motion.
The final wave to arrive is called a surface wave. Surface waves travel relatively slowly over the earth's surface, and are similar in motion pattern to the orbit of particles in a water wave, and producing the same effect. If visible, the land's surface would look similar to water waves at a beach.
The device that measures the earthquake is called a seismometer (or seismograph). The first seismometer was invented in China in 132 A.D. The instruments that we use today trace their origin to the invention of the horizontal pendulum seismograph, invented in 1880. The output of a seismograph is called a seismogram. It is a record of the ground motion at the seismometer. Around the world there are thousands of seismometers monitoring for earth motion. Many are run by governments, and many more are operated by colleges and universities.
You can view several excellent online seismometers that provide near real-time information. Here are links to several for you to examine.
For an excellent animation of how a seismograph works, click this link.
The Richter magnitude scale assigns a single number to indicate the size of an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismometer output. The chart below describes the scale.
Major Earthquake Frequency and Predictions
Different faults move at different rates. Scientists can examine the past history of earthquakes on a fault by examining the rocks in the area of the fault. Once a major earthquake occurs, they can give an estimate of when the next earthquake can occur. For example, a study of earthquakes in the New Madrid Seismic Zone shows that major quakes occur on average every 300 to 500 years. The last major earthquake was the 1812 New Madrid Earthquake, therefore we are only about 200 years since the last major earthquake. We should not see another earthquake in this region for at least another 100 years. However, earthquakes are unpredictable. For all we know, the next one could be occurring right now! All scientists can do is offer a prediction based on historical earthquake data.
In California, the San Andreas Fault actually lies on the surface, and stress buildup can actually be measured. Scientists can use these measurements to enhance their predictions. Other technologies show promise in aiding earthquake prediction. For more, see earthquake prediction.
Major Earthquakes in History
There are many earthquakes in modern history that we could examine. I will only highlight a few here. Click the earthquake name for more information.
Indian Ocean Earthquake, 26 Dec 2004 - This earthquake created a tsunami, a series of waves created when a body of water is rapidly displaced on a massive scale. It occurred after an earthquake in the Indian Ocean (magnitude 9.3), and killed about 230,000 people.
San Francisco Earthquake, 18 Apr 1906 - A earthquake of magnitute 7.8 struck this major city, killing about 3,000 people and starting a fire that destroyed a large portion of the city.
New Madrid Earthquake, 7 Feb 1812 - The highest magnitude earthquake to ever strike the continental United States (magnitude 8.0 estimated), this earthquake occurred before the region was heavily populated.
Shaanxi Earthquake, 23 Jan 1556 - This magnitude 8 quake is listed as causing the most fatalities, with an estimated 830,000 people killed in one event.
For more information and additional historical earthquakes, see List of earthquakes at Wikipedia.
Earthquakes and Plate Tectonics
The majority of earthquakes are directly related to the movement of our tectonic plates, as can be seen in the map below. This map shows 358,214 earthquakes over a 35 year period. Most are concentrated around the margins of our tectonic plates.
You may have heard the term "Pacific ring of fire," referring to the zone of earthquakes and volcanic eruptions encircling the Pacific Ocean basin. Ninety percent of the world's earthquakes occur along the ring of fire. A simple map of the ring of fire is at right.
Research the answers to the following questions about plate tectonics. Your parents may have you simply answer the questions, or they may have you put it in essay form. Please follow your parents instructions.
To answer these questions, utilize a search engine to locate the best webpages, or consult a textbook/encyclopedia. You may also use the links at the bottom of this page.
Today you will complete a 7 question practice quiz. The link to the quiz will open a new window. You can come back here and check your answers. Do not click the Back button on your browser during the quiz. After the quiz, continue your research project, if necessary.
Please review the terms in bold in the text, and ensure you have completed your research work from Tuesday.
Today you will take the end of chapter test. Please close all other browser windows, and click on the link below. During the test, do not click on the Back button on your browser.
After you have completed the test, you may proceed to Chapter 11 on your next school day. Please return to the introduction page for the link to the next chapter.
Return to the Old Earth Ministries Online Geology Curriculum homepage.
Seismic Building Zone Map - Shows building zone codes for the United States. Red areas are most likely to experience earthquakes...green the least likely.