Objectives:  To determine graphically certain boundaries within the earth.

Materials:

 

Protractor—4 inch or larger

12” scale divided into 10ths

Compass capable of drawing a 10” circle

Calculator

 

Background:

When an earthquake occurs, vibrations called seismic waves are produced.  These waves travel outward from an earthquake’s focus, its point of origin, in all directions.  Two kinds of seismic waves travel through the Earth:  P-waves and S-waves.  The third kind, surface waves, results when P- and S-waves reach the surface and travel outward along the surface from an earthquake’s epicenter, the point on the surface directly above its focus.  By studying P- and S-waves, scientists have been able to develop a model for the Earth’s interior.

P-waves can travel through solids, liquids, and gases, and they move faster through dense material than through less dense material.  Studies of earthquake waves have shown the P-wave velocity increases or decreases at certain depths inside the Earth.  A change in velocity suggests the presence of a boundary between materials of different densities.

The outer layer of the Earth is known as the crust.  As P-waves travel through the crust, they suddenly increase in speed at a depth of about 50km.  The change in speed indicates they are entering a material of higher density than the crust.  This layer is called the mantle.  This boundary between the crust and the mantle is called the Moho or Mohorovocic discontinuity.  At a depth of about 2,900km, the P-waves sharply decrease in speed, indicating they are entering the core.  This boundary between the mantle and the outer core is called the Guttenburg discontinuity.  At about 5100km, P-waves speed up, indicating another boundary, this time between the outer core and the inner core.  This boundary is called the Lehmann discontinuity.

S-waves can travel through solid material only, not through liquids or gases.  Like the P-waves, their speed changes according to the density of the material through which they are traveling, speeding up in dense material and slowing down in less dense material.

Scientists have found the S-waves cannot be detected on the side of the Earth opposite an earthquake; therefore S-waves were not able to travel through the Earth’s core. Since S-waves cannot travel through liquids, and the P-waves velocity

suggested that the outer core was less dense than the inner core, scientists think that the outer core may be somewhat fluid and the inner core solid.

Shadow zone:  P- and S-waves are not detected at seismograph stations that are at an angular distance greater than 103^o from the epicenter of an earthquake.  P-waves are detected at an angular distance of about 143^o or more.  These findings indicate a zone on the Earth’s surface between 103^o and 143^o from an epicenter in which no P- or S-waves can be detected.  This region is called an earthquake’s shadow zone.  Its existence can be explained by the way seismic waves bend as they travel through the Earth and by the inability of S-waves to travel through liquids.

When seismic waves travel through material that changes density gradually, they change direction gradually.  When they travel from one material to another material of different density, they change direction sharply.  As P- and S-waves travel through the mantle, they may bend slightly because of slightly different densities.  When P-waves enter the boundary between the mantle and the core, the difference in densities causes them to bend sharply.  They are reflected away from the core and reach the surface about 103^o from an epicenter, or they are bent inward and travel through the core.  When they leave the core, they are bent sharply again and reach the surface at about 143^o from an epicenter.  When S-waves reach the boundary between the mantle and the core, they are reflected back because they cannot travel through the dense fluid in the outer core.

Modified from:

What Earthquake Waves Tell Us about the Earth’s Interior

Donald G. Korba

From Earth Science Investigations (1990)

American Geologic Institute

Alexandria, Virginia

Pages 59-63.

 

Procedure:

 

PART 1.

 

1. On Sheet 1, draw the 103^o angles as shown in Figure 1.  The area between the 103^o angles on the side of the earth opposite the epicenter is called the ___________   ___________ of the ___-waves.  Label it.

 

2. On Sheet 1, draw the 143^o angles following the same procedure as before.  The areas between the 103^o angle and the 143^o angle on each side of the earth are called the ____________  __________ of the ___-waves.  Label them.

 

3. Label the center of the earth, the surface, the focus, and the epicenter.

 

4. At each seismic station, indicate which body waves reach that station.  Your choices are:  P- and S-waves, P-waves only, S-waves only, none

 

PART 2.

 

1. On Sheet 2(the 10 inch diameter earth) draw one set of 103^o angles following the procedure in Part 1.  Make the solid lines red.

 

2. On  Sheet 2(the 10 inch diameter earth) draw one set of 143^o angles following the procedure in Part 1.  Make the solid lines blue.

 

3. Transfer the 103^o angles to Sheet 3 for all the earthquake foci shown.

 

4. What geometric shape have you outlined?  Draw it.

 

5. This is the boundary between the ____________ and the ___________

______________.  It is called the __________________ discontinuity.

 

6. Complete the following ratio:

Depth to center of the earth in km                  Depth to boundary in km
-----------------------------------      =         -------------------------
Depth to center of earth in inches                Depth to boundary in inches

  6320      Depth to boundary in km
-------  =  -----------------------
   5              you measure

Calculated depth to boundary is ________________ km.

Depth to boundary from Table 1 is _____________ km.

Why the difference?____________________________________________

7.  Transfer the 143^o angles to Sheet 4 for all earthquake foci shown.

8.  What geometric shape have you outlined?  Draw it.

9.  This is the boundary between the __________  ___________ and the

_____________  ____________.  It is called the ____________ discontinuity.

10.  Complete the following ratio:

Depth to center of the earth in km                  Depth to boundary in km
-----------------------------------      =         -------------------------
Depth to center of earth in inches                Depth to boundary in inches

  6320      Depth to boundary in km
-------  =  -----------------------
   5              you measure

Calculated depth to boundary is ________________ km.

Depth to boundary from Table 1 is ______________ km.

Why the difference? __________________________________________

 

 

PART 3

Table 1.

 

Boundary	Depth	Name_____________
Surface	0	——–
Crust-mantle	~50 km	Moho(Mohorovicic discontinuity)
Mantle-outer core	2900 km	Guttenburg discontinuity
Outer core-inner core	5100 km	Lehmann discontinuity
Center of Earth	6320 km	———

1.  On Sheet 5(the pie shape figure), draw to scale the interior of the earth using the data in Table 1.

2.  Place appropriate depths on the left side.  Place appropriate boundary names on the right side.  Indicate the surface and the center of the earth.  Label the 4 zones within the earth.

PART 4.

1.  Assume the earth is a perfect sphere.  Using the data in Table 1. and the formula for the volume of a sphere (V= 4/3  p r³) , calculate what percent of the volume of the earth is crust, mantle, and core.  Round off to first decimal.

% volume of earth is crust _______________ %

% volume of earth is mantle ______________ %

% volume of earth is core ________________ %

TOTAL _________100%_______