PLANET X. Module 1.    THE INTERIOR OF THE EARTH

 

A teaching exercise prepared by Gail Ballard in partial fulfillment of the requirements of EAS 4900

 

Target level   Year 8 - 12

 

Rationale

 

Seismologists use the time it takes waves from earthquakes to travel through the earth to develop models of the composition of the Earth.  A plot of these travel times is called a Travel Time Curve.  Because the seismic waves reflect (echo) off the mantle, core and inner core and are bent as they pass through the layers of the Earth, the travel time curves can be complex.  An example of their interpretation is given at the end of the exercise.  The planet X exploration exercise takes students through the process of creating theoretical Travel Time Curves for two simplified planet models, one with a core and one without. Using seismic data from Planet X., a fictional colony, and actual Earthquake seismograms students will choose the best model for each planet.  By reducing the complexity and retaining the essential features of actual Travel Time Curves, these simple models give students an understanding of the use of seismology as a tool for exploring the interior of the Earth and any other planet.  

 

Focus question 

 

Does the Earth have a core?

 

Objectives

 

Students will: 

 

  1. Create theoretical Travel Time Curves for two simple Planet Models, one with a core and one without, using measurement and computational skills.
  2. Use generated and actual seismograms to choose a Planet Model of best fit for a fictional earth colony, Planet X. and for Earth.
  3. Demonstrate an understanding of the composition of the Earth and how seismology was used to develop the present model.

 

Materials Needed

 

  1. Printed copies of Travel Time Curve Activity for each individual or group.
  2. Paper, pencils, metric ruler, protractor and calculator for data manipulation.

 

Organization

 

1.      Begin with a general discussion of the interior of the Earth with a focus on how this model was developed using the tool of seismology.  Refer to Journey To The Center Of The Earth by Jules Verne for a fictional means of obtaining the model.

2.      Introduce the Planet X scenario and divide into smaller groups.  Each group should complete the Travel Time Curve Activity.  Difficulty level can be modified by inserting or deleting more given information in the Tables.  For example, Surface distance (S) data can be deleted and students can use the equation S = Rq, where R is the radius and q is the angle D in radians, to compute this distance. There is repetition of data from Table 1. to Table 2. that some students may discern.  Attention could be drawn to this if time is limited.

3.      As an extension students may examine the Iris Travel Time Curve provided at the end of the exercise and relate the various lines to the actual paths of seismic waves through the Earth.

4.      Complete activity with a discussion of other ways scientists have developed the present Earth model (for example, meteorite composition, rock density studies, magnetic striping) and ways this theoretical model could be tested (Moho discontinuity drilling.)

 

 

 

This is a link to Travel Time Activity

 

 

Reference Data

 

 

 

Table 1.  Seismic Data for Planet Model 1.

 

 

Station #

Surface Distance, S

 

 

Angle Delta, D

 

 

 

Linear Distance, L

 

 

P-Wave Travel Time

 

 

S-Wave Travel Time

 

 

Surface Wave Travel Time

 

 

(Km)

(Degrees)

(Km)

(Minutes)

(Minutes)

(Minutes)

Meas.

Calc.

1.

2,780

25

2,765

2,758

4.2

7.2

8.1

2.

5,560

50

5,447

5,385

8.2

14.0

16.3

3.

9,452

85

8,631

8,608

13.0

22.4

27.6

4.

11,675

105

10,894

10,109

15.3

26.3

34.1

5.

14,455

130

11,815

11,772

17.8

30.7

43.9

6.

16,679

150

12,319

12,308

18.6

32.1

48.8

7.

20,015

180

12,738

12,742

19.3

33.2

58.5

Figure 1 Completed table of data for Planet Model 1.

 

 

Table 2.  Seismic Data for Planet Model 2.

 

 

Station #

Surface Distance, S

 

 

Angle Delta, D

 

 

 

Linear Distance, L

 

 

P-Wave Travel Time

 

 

S-Wave Travel Time

 

 

Surface Wave Travel Time

 

 

(Km)

(Degrees)

(Km)

(Minutes)

(Minutes)

(Minutes)

Meas.

Calc.

1.

2,780

25

2765

2758

4.2

7.2

8.1

2.

5,560

50

5,447

5,385

8.2

14.0

16.3

3.

9,452

85

8,631

8,608

13.0

22.4

27.6

4.

11,675

105

10,894

10,109

15.3

26.3

34.1

5.

14,455

130

11,815

11,772

 

 

43.9

6.

16,679

150

12,319

12,308

22.1

 

48.8

7.

20,015

180

12,738

12,742

23.2

 

58.5

Figure 2 Completed table of data for Planet Model 2.

 

 

Table 3. Actual Seismic Data

 

Station Name
Station Location

(Degrees)

First Arrival Time

(UTC  hr:min:sec)

Travel Time

(Minutes)

LVC.IU

7

20:34:42

1.3

TRQA.IU

24

20:37:46

4.5

HKT.IU

50

20:42:11

8.95

POHA.IU

88

20:45:00

11.76

KONO.IU

100

20:46:28

13.23

GNI.IU

121

20:52:23

19.15

GUMO.IU

143

20:52:26

19.2

TATO.IU

163

20:52:46

19.53

Figure 3 Table of Actual Seismic data from traces recorded worldwide of the 6/23/01 Peru earthquake which occurred at 20:33:14 UTC.  Seismogram traces in the Activity were downloaded from WILBUR II, a program on the IRIS website.  These traces were read using WINQUAKE to determine the times given above. 

 

Figure 4 Graph of Travel Time Curves for Planet Model 1 and Planet X.

 

 

Figure 3 Graph of Travel Time Curves for Planet with Core.  Note the data value at 120 degrees for the Actual Seismic data.  This station is in the P-shadow and should not show a direct P wave.  The seismogram from this station shows a very small amplitude first arrival then a larger amplitude refracted P (see Figure 13).  If students use the time of first arrival they should appreciate that this is a diffracted p-wave, similar to the small amount of light that still exists in shadow.  In this case the data plot for 120 degrees will be at 14.8 minutes.

 

 

References

 

Bolt, Bruce A., EARTHQUAKES, W.H. Freeman and Company, New York, NY, 1999

 

 

Links to relevant sites

 

www.iris.washington.edu/cgi-bin/wilberll

This is the direct URL to WILBER ll.  The IRIS home site has earthquake education information and links to other good sites.

 

psn.quake.net/software.html

Download Winquake 2.6 from Larry Cochrane from this site.

 

www.indiana.edu/~pepp/

This site gives PEPP real-time data and has good links for teachers.

 

 

http://geology.usgs.gov/pdf/planet.pdf

Large file containing This Dynamic Planet, an excellent resource for earthquake science.

 

http://pubs.usgs.gov/publications/text/dynamic.html

This Dynamic Earth: the Story of Plate Tectonics.  A companion text to the above booklet.

 

www.eas.purdue.edu/~braile/educindex/educindex.htm

Larry Braile’s collection of hands-on activities for earthquake science.

 

 

Core Curriculum Standards Met

 

Earth Sciences:

 

Topic 1.             Inquiry, Process and Problem Solving, - Field and Lab.

Topic 8.             Composition of the Earth.

Topic 8.1           Ways in which data is collected about Earth's layers.

Topic 18.5         P & S waves.