# Earthquakes

## Earth Science - Middle School

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focus seismic waves epicenter epicenter hanging wall direction of wave direction of wave direction of wave footwall footwall hanging wall earthquake damage What are Earthquakes? The theory of plate tectonics describes the movement of the plates of the lithosphere relative to each other. This movement creates forces that push and pull on the crust. Crustal rock can absorb and store energy, but only so much. There is a point at which the stress is more than the rock can hold, and the rock breaks. When the rock breaks, the stored energy is released and this energy travels through the Earth. This sudden release of energy created when rocks break is called an earthquake. Types of Seismic Waves Seismic waves are the vibrations from earthquakes that travel through the Earth and are recorded on instruments called seismographs. There are three different types of seismic waves released by an earthquake. Kinds of Faults A fault is created when tension in Earth’s crust pulls the rock layers apart. Normal Fault A normal fault occurs when a hanging block moves down relative to the footwall. Reverse Fault When the hanging wall moves up relative to the footwall, a reverse fault is formed. Reverse faults are the result of compression (forces that push rocks together). Strike-slip Fault A strike-slip fault forms when rocks move past each other horizontally. The San Andreas Fault is an example of a strike-slip fault. P-waves or primary waves are the first energy waves released. They are the fastest of the seismic waves and can travel through solids, liquids and gases. Because they are the fastest seismic waves, they are the first waves to be measured by a seismograph. P-waves are pulse waves that move rock back and forth, first compressing then stretching the rock. S-waves or secondary waves are the second type of waves created by an earthquake. S-waves are slower than P-waves and therefore arrive at a seismograph after the P-waves. S-waves are also called shear waves because they travel in an S pattern through the rock, creating shear stresses. S-waves can travel through solids, but they cannot travel through liquids. The slowest and most destructive seismic waves created by an earthquake are called surface waves. Surface waves travel along the surface of the Earth’s crust in a circular motion and shake the surface of the Earth up and down. Measuring Earthquakes An earthquake’s seismic waves are measured and recorded by an instrument called a seismograph. The actual recording of the seismic waves is a seismogram. Using seismograms, seismologists can determine an earthquake’s start time and its epicenter. The magnitude of most earthquakes is measured on the Richter scale, invented by Charles F. Richter in 1934. Earthquakes are classified in categories ranging from minor (3 on the Richter scale) to major (7 or more on the Richter scale). Photographs courtesy of USGS. Time P wave S-P interval S wave surface wave Earthquakes © Copyright NewPath Learning. All Rights Reserved. 94-4106 www.newpathlearning.com
\|xiBAHBDy01786ozX direction of wave direction of wave direction of wave P wave S-P interval S wave surface wave What Are Earthquakes? Describe: Earthquake: ______________________________________________ __________________________________________________________ Epicenter: _________________________________________________ __________________________________________________________ Focus: ____________________________________________________ __________________________________________________________ Types of Seismic Waves _____________________________ are the vibrations from earthquakes that travel through the Earth and recorded on instruments called _____________________________ . There are three different types of _____________________________ released by a(n) _____________________________ . Kinds of Faults A ________________________ is created when tension in Earth’s crust pulls the rock layers apart. Normal Fault Describe: ______________________________ ______________________________________ Reverse Fault Describe: ______________________________ ______________________________________ Strike-slip Fault Describe: ______________________________ ______________________________________ P-waves or primary waves Describe: _____________________________ ______________________________________ ______________________________________ S-waves or secondary waves Describe: _____________________________ ______________________________________ ______________________________________ Surface waves Describe: ______________________________ ______________________________________ ______________________________________ Measuring Earthquakes An earthquake’s seismic waves are measured and recorded by an instrument called a ________________________. The actual recording of the seismic waves is a ________________________. Using seismograms, seismologists can determine an earthquake’s start time and its ________________________. What is the Richter scale? __________________________________________ __________________________________________ __________________________________________ Photographs courtesy of USGS. Key Vocabulary Terms crust earthquake epicenter fault focus normal fault plate tectonics P-waves reverse fault Richter scale seismic waves seismogram seismograph seismologist strike-slip fault surface waves S-waves Time Earthquakes © Copyright NewPath Learning. All Rights Reserved. 94-4106 www.newpathlearning.com