Earthquakes

Introduction

Earthquakes are one of the most powerful natural forces that can disrupt our daily lives. Through careful study, geologists are slowly learning more about such questions as these:

Why do earthquakes occur?

Why do some locations such as Califonia and Japan receive so many earthquakes?

Can earthquakes be predicted?

Can we design a city to better withstand an earthquake?

Can we stop earthquakes before they occur? Should we try?

This lesson will guide you through several activities to help you think like a geologist. You are free to move around from one part of the lesson to another by clicking on the special hypertext. However, you will understand each activity better if you complete them in this order:

What is an earthquake?

Perhaps you remember being in an earthquake--the ground rumbles, hanging lamps begin to sway back and forth, shelves begin to rattle or spill their contents, the floor and walls shake.... Even if you do not remember seeing or feeling an earthquake, you have probably lived through thousands of tiny earthquakes during your lifetime. The earth is constantly creating earthquakes. An earthquake is the shaking of the earth caused by pieces of the crust of the Earth that suddenly shift. The crust, the thin outer layer, is mostly cold and brittle rock compared to the hot rock deeper inside. This crust is full of large and small cracks called faults. Although these faults can be hundreds of miles long, usually you cannot see the cracks because they are buried deep underground and because the pieces of crust are compressed together very tightly. The powerful forces that compress these crustal pieces also cause them to move very slowly. When two pieces that are next to each other get pushed in different directions, they will stick together for a long time (many years), but eventually the forces pushing on them will force them to break apart and move. This sudden shift in the rock shakes all of the rock around it. These vibrations, called seismic waves, travel outward in all directions and are called an earthquake. The underground location where the rock first broke apart or shifted is called the focus of the earthquake.

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Definitions and Glossary

Focus:

Focus is the location within the earth where underground rock moves and sends out earthquake waves.

Epicenter:

The epicenter is the location on the surface of the earth directly above the focus of an earthquake.

Earthquake waves (seismic waves):

Earthquake waves are the shock waves created at the focus of an earthquake and sent out in all directions through the earth.

Earthquake:

An earthquake is the rapid vibration of the earth created by a sudden movement of large sections of rock.

Seismograph:

A seismograph is a device that records earthquake waves.

Seismogram:

A seismogram is the picture drawn by a seismograph.

Plate tectonics:

According to the theory of Plate Tectonics, the outer layer of the earth

is broken up into large, brittle plates of rock that float on warmer soft rock below.

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Big Quake or Small Quake? The Seismograph

How would you measure the strength of an earthquake?
Chinese Seismograph vase courtesy of National Geographic magazine
For centuries different societies have designed many creative ways to measure the shaking of the earth. Nearly 2000 years ago, for example, the ancient Chinese made a special vase that had several sculpted dragons mounted all around the sides of the vase. Each dragon held in its mouth a metal ball. When the ground shook, some of the balls would fall from the mouths of the dragons into the waiting mouths of the sculpted frogs to show how the ground had moved. Modern Seismograph
Today geologists measure earthquake waves with a seismograph. A typical seismograph works in a very simple way:
A heavy weight is fastened to a horizontal rod as shown in the diagram. This rod hangs from a pole ( and is free to swing from side to side when the ground shakes. At the other end of the rod (away from the pole) is an ink pen, and directly underneath the pen is a piece of paper rolled around a cylinder . This cylinder rotates so that the pen continuously draws an ink line along the moving paper. If the ground does not move, the rod does not swing, and the pen stays in place, so the ink line is smooth and straight. If the ground shakes, however, the row swings and so the pen draws a zig-zag line as the paper turns. The stronger the shaking, the sharper the zig-zags. This zig-zag picture made on the paper roll is called a seismogram.
Do you know how easy it is to build a seismograph? (Click on the highlighted word to give it a try.)
Since every earthquake is a little different, each quake makes its own unique zig-zag pattern (seismogram) on paper. Here are some sample seismograms from an earthquake:



Most seismograms look similar to the ones above -- maybe a bit confusing at first. Reading a seismogram does not have to be complicated, however. If you know what to look for, you can pick out the important parts in a seismogram and get some very valuable information.

Different Ways to Shake: Types of seismic waves

When you drop a rock into a calm lake, you see (water) waves traveling outward in all directions through the water. When you pop a balloon, sound waves travel outward in all directions through the air. When rock deep inside the earth suddenly moves, it sends seismic waves outward in all directions through the earth.

Moving Plates Seismic waves start where a large section of rock suddenly shifts. This rock movement creates at least three types of waves:

Primary (P)waves : Pressure waves caused when rock is pushed or pulled forward or backward. Primary waves, the fastest wave sent out by an earthquake, travel down into the earth rather than along the surface.(Remember: P waves - primary,p ressure, push-and-pull

Secondary (S) waves : Shear waves caused when rock is shaken or whipped from side-to-side, like the wavy motion of a slithery snake. Secondary waves, the second-fastest wave sent out by an earthquake, travel down into the earth rather than along the surface. (Remember: S waves - second, shear, side-to-side)

Surface (L) waves : Up-and-down (rolling) or side-to-side motion of the earth surface. Surface waves, the slowest earthquake waves, travel along the surface of the earth rather than down into the earth. Although they are the slowest of all earthquake waves, L waves usually cause more damage to society than P or S waves. L waves were named after a geologist who studied them. ( Remember: L waves are always the last to arrive.
Every earthquake creates these three kinds of waves. P and S travel downward into the earth, and L waves travel along the top of the crust.

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How Far Away?

How far away can earthquake waves reach? Would you feel an earthquake that struck the other side of your city? the other side of your country? the other side of the earth? At the instant an earthquake occurs, P, S and L waves immediately begin racing outward in all directions, losing energy as they spread out. If they encounter no interference, P and S waves for a large earthquake should quickly travel all of the way through the middle of the earth and faintly arrive on the opposite side of the globe. An earthquake at the south pole, for eample, would shake the north pole in less than half an hour (though the vibrations would be very weak). This is what P and S wavesshould do. However, the P and S waves do not always make it to the opposite side. Why not? You can find out later in the activity Shadows from the Core.
Suppose you owned a seismograph. One day it suddenly starts measuring earthquake waves that came from somewhere else. Using only your own seismograph, can you determine where that "somewhere else" was located? You cannot determine exactly where it was located, but you can determine how far away from you it was. How?
Because S waves travel slower than P waves, they always arrive at locations around the world after P waves arrive. This may sound simple, but it is also very useful. How much does the S wave fall behind the P wave? If your seismograph can tell you how many minutes the S wave is behind, you can tell how far they had to travel to get that many minutes behind. Try this in the activity Race of the Waves.

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Where Did the Quake Hit?

"If a tree falls in the woods but noone is there to hear it, does it make noise as it falls?" People have debated this question for years. Here is another good question: Can you locate the exact eipcenter of an earthquake even if it hits in the middle of nowhere and nobody is around to feel it? Some quakes hit busy cities or small villages where somebody can tell you about it, but even if a quake hits far away from people, it sends out waves to all parts of the globe. You can use your seismograph to record these waves and determine how far away the epicenter is from your seismogram. To pinpoint the exact location of a quake, you need at least three seismographs. Why do you think two seismographs are not enough?
Be a geologist again and try locating the epicenter of an earthquake. The activity Can You Read a Quake? has seven different seismograms that all came from the same earthquake. This activity did not tell you where the earthquake actually hit. Can you use these seismograms to pinpoint the actual epicenter? Try this in the activity Where did it hit?

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