© 2007 Encyclopædia Britannica, Inc.

Intermediate Science EXPLORE

Hunting

Have you ever wondered what it would be like to be in an earthquake? Have you ever been in an earthquake? Do you know what causes them? Do they occur in certain areas of the world more often than other areas? In this activity, you will learn all about earthquakes in general and study one infamous earthquake in detail.

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© 2007 Encyclopædia Britannica, Inc.

Student Activity Page–copy and distribute as needed EXPLORE

First you need to find out the causes of earthquakes around the world. Visit the following websites and then explain the causes of earthquakes.

Plate Tectonics: The Cause of Earthquakes http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html

Why Do Earthquakes Happen? (PDF file) http://www.iris.edu/edu/onepagers/no2.pdf

What causes earthquakes?

The following websites are designed to show the most recent earthquakes as they have happened all around the world. Visit the sites and answer the questions.

National Earthquake Information Center http://wwwneic.cr.usgs.gov/neis/current/world.html

Pre-Assembled Event Information http://www.iris.edu/seismon/

Seismicity Maps of Recent Earthquake Activity in the USA http://earthquake.usgs.gov/eqcenter/recenteqsus

Where in the world did the most recent earthquake occur?

What was the magnitude (a number from the Richter scale) of the most recent earthquake?

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Student Activity Page–copy and distribute as needed EXPLORE

Write the location and magnitude of five other recent earth-quakes in the United States.

1)

2)

3)

4)

5)

Does there seem to be any pattern to the distribution of the earthquakes? If so, why might that be?

Click on the “Near Real Time Earthquake List” on this site: http://earthquake.usgs.gov/eqcenter/recenteqsww/index.php?old=

world.html

Have any earthquakes occurred in the last hour? If so, how many? ____________________________ Identify at least three regions in which an earthquake occurred in the past day: ____________________________

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What was the magnitude of the largest earthquake? _____________________________

© 2007 Encyclopædia Britannica, Inc.

Student Activity Page–copy and distribute as needed EXPLORE

Describe in general what happens during an earthquake at each of the following magnitudes:

1-3

3

4

5

6

7 and above

Now that you have learned something about the most recent earthquakes around the world, you will need to choose an earthquake from the past to research. There are many ways to find an earthquake to study. Search through some of these sites until you find a historic earthquake that is interesting to you.

Earthquake “Top 10” Lists and Maps http://earthquake.usgs.gov/eqcenter/top10.php

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© 2007 Encyclopædia Britannica, Inc.

Student Activity Page–copy and distribute as needed EXPLORE

Recent Significant Earthquakes http://earthquake.usgs.gov/eqcenter/eqarchives/year/byyear.php

Mark Twain and the October 8, 1865, San Francisco Earthquake http://projects.crustal.ucsb.edu/understanding/accounts/twain.html

Earthquakes in History http://pubs.usgs.gov/gip/earthq1/history.html

Provide the following information about the earthquake you chose:

Effects on the structures of the nearest cities or towns:

Date of the earthquake

Where was the epicenter?

Name of faults or plates involved: What major population areas were affected?

Estimated cost of damage: How many people died because of the earthquake?

Describe any aftereffects (aftershocks, landslides, tsunamis).

How many were injured?

Magnitude on the Richter scale or Intensity on the Mercalli scale:

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TEACHER MATERIAL

Earthquake Hunting

accumulated to a point where they exceed the strength of the rocks, leading to rapid fracture. These rock fractures usually tend in the same direction and may extend over many kilometers along the zone of weakness. In the 1906 earth-quake the San Andreas Fault slipped for 430 kilometers, with a maximum hori-zontal fault offset of about six meters… .

Earthquake magnitude Because the size of earthquakes varies enormously, it is necessary for purposes of relative comparison to compress the range of wave amplitudes measured on seismograms by means of a mathematical device. In 1935 the American seis-mologist Charles F. Richter set up a “magnitude scale of earthquakes” as the logarithm to base 10 of the maximum seismic wave amplitude (in thousandths of a millimeter) recorded on a standard seismograph (the Wood-Anderson tor-sion pendulum seismograph) at a dis-tance of 100 kilometers from the earth-quake epicenter… .

Intensity scales The level of violence of seismic shaking varies considerably over the affected area. This intensity is not capable of simple quantitative definition and, particularly before seismographs capable of accurate measurement of ground motion were developed, the shaking was estimated by reference to intensity scales that describe the effects in qualitative terms. Subsequently, the divisions in these scales have been associated with accelera-tions of the local ground shaking. Intensity depends, however, in a compli-cated way not only on ground accelera-tions but also on the periods and other features of seismic waves, the distance of the point from the source, and the local

geological structure. Furthermore, it is distinct from magnitude, which is a measure of earthquake size specified by a seismograph reading.

A number of different intensity scales have been set up during the past century and applied to both current and ancient destructive earthquakes. For many years the most widely used was the 10-point scale devised by Michele Stefano de Rossi and François-Alphonse Forel in 1878. The scale now generally employed in North America is the Mercalli scale, as modified by Harry O. Wood and Frank Neumann in 1931, in which intensity is considered to be more uni-formly graded. Alternative scales have been developed in both Japan and Europe for local conditions. The European (MSK) scale of 12 grades is similar to the abridged version of the Mercalli.

Additional Websites

U.S. Geological Survey: Earthquake Hazards Program – About Earthquakes

http://earthquake.usgs.gov/learning/kids/

The California Institute of Technology: Documenting Earthquakes – A Virtual Exhibit in Six Parts

http://archives.caltech.edu/exhibits/earthquake/index.html

The Exploratorium: Faultline – Seismic Science at the Epicenter

http://www.exploratorium.edu/faultline/credits.html

From the Encyclopædia Britannica

Earthquake … any sudden disturbance within the Earth manifested at the surface by a shaking of the ground. This shaking, which accounts for the destructiveness of an earthquake, is caused by the passage of elastic waves through the Earth’s rocks. These seismic waves are produced when some form of stored energy, such as elastic strain, chemical energy, or gravitational energy, is released suddenly.

Few natural phenomena can wreak as much havoc as earthquakes. Over the centuries they have been responsible for millions of deaths and an incalculable amount of damage to property. While earthquakes have inspired dread and superstitious awe since ancient times, little was understood about them until the emergence of seismology at the beginning of the 20th century. Seismology, which involves the scientific study of all aspects of earthquakes, has yielded answers to such long-standing questions as why and how earthquakes occur… .

Principal mechanisms in nature Earthquakes are caused by the sudden release of energy within some limited region of the rocks of the Earth. The form of energy involved is produced by elastic strain, gravitational potential, chemical reactions, or motion of bodies. Of these, the release of elastic strain energy is the most important, since this form of energy is the only kind that is stored in sufficient quantity in the Earth to produce major earthquakes. Earthquakes associated with this type of energy release are called tectonic earthquakes.

Measurements of triangulation lines across the San Andreas Fault before and after its rupture in the 1906 San Francisco earthquake led to the so-called elastic rebound theory for tectonic earthquakes. As formulated by the American geologist Harry Fielding Reid, the theory explains that a tectonic earthquake occurs when stresses in rock masses have

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