A teaching exercise prepared by Gail Ballard in partial fulfillment of the requirements of EAS 4900
Target
level Year 8  12
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?
Students will:
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
Table 1. Seismic Data for Planet Model 1.
Station # 
Surface
Distance, S 
Angle Delta, D 
Linear
Distance, L 
PWave Travel
Time 
SWave 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 
PWave Travel
Time 
SWave 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.
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 Pshadow 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 pwave, 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.
Bolt, Bruce A., EARTHQUAKES, W.H. Freeman and Company, New
York, NY, 1999
www.iris.washington.edu/cgibin/wilberll
This is the direct URL to WILBER ll. The IRIS home site has earthquake education
information and links to other good sites.
Download Winquake
2.6 from Larry Cochrane from this site.
This site gives
PEPP realtime 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 handson 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.