LOCATING AN EARTHQUAKE

Suggested by Cheryl Dodes and adapted by Michael J. Passow

Objectives:

To locate the position of the epicenter and to determine the magnitude of an earthquake.

Materials:

a)      4 seismograms of the same earthquake at different localities.

b)    Travel-time graph.

c)      Earthquake-magnitude chart.

d)      Sketch map showing location of recording stations.

e)      Drawing compass.

f)        Pencil

Procedure:

a) Using a piece of paper and the scale provided, determine the time span between the arrival of the P-wave (T_p) and the arrival of the S-wave (T_s) on the seismograms from each station --  Record in Table 1 (T_s - T_p).

b) Measure the amplitude (amount of deflection) of the S-wave at each station. (Vertical scale) - Record on Table 1.

c) Using the Travel-Time graph and the Earthquake Magnitude Chart, complete Table 1.

Recording Station     Ts – Tp                    Distance               Amplitude                 Apparent Magnitude

Santa Barbara          _________sec    ____________km       ___________mm           ____________________

Cal Tech/JPL           _________sec    ____________km       ___________mm           ____________________

CSUDH                   _________sec               ____________km       ___________mm           ____________________

    Lancaster                 _________sec    ____________km       ___________mm           ____________________

 The plot below is a graph of travel time versus distance for P-waves and S-waves.  For example, after 5 seconds of travel time, an S-wave travels about 20 km, while a P-wave travels about 41 km.  It should be noted that the graph is an approximation of more precisely measured travel times.

     Once you know the difference between the P-wave and S-wave arrival times, (t_s – t_p) you can estimate the distance to the epicenter simply by scaling the appropriate time interval on the graph.  For example, let us say that (t_s – t_p) is determined to be 10.25 seconds.  Take a piece of scrap paper and measure 10.25 seconds onto the scrap from the vertical scale of the graph above.  Move the scrap of paper along the graph until the vertical distance between the S-wave curve and the P-wave curve is exactly 10.25 seconds.  Now, look along the horizontal axis to read the distance to the epicenter that corresponds to a travel time difference of 10.25 seconds.  The distance to the epicenter is approximately 80 km.

     Determine the distance to the epicenter for each of the four seismographs above and record your data in Table 1.

EXAMPLE OF THE CALCULATION OF THE RICHTER MAGNITUDE OF AN EARTHQUAKE

(from Bruce Boil, Earthquakes – A Primer: W.H. Freeman & Co. San Francisco. p. 105)

     To use the chart below, measure the amplitude of the S-wave above the baseline in each seismograph -- see the example. Record these values in Table 1. 

    From the data in Table 1, now use a straight-edge to connect the distance (left-hand scale below) with the amplitude (right-hand scale.) The magnitude of the quake is where a straight line connecting these two values crosses the magnitude line.
Record these values in Table 1.

The chart below provides more information about what Richter Scale magnitude means in terms 
of common and uncommon events. 

THE RICHTER SCALE OF EARTHQUAKE MAGNITUDE

Richter        TNT for Seismic    Example

Magnitude      Energy Yield      (approximate)

-1.5                6 ounces   Breaking a rock on a lab table

 1.0               30 pounds   Large Blast at a Construction Site

 1.5              320 pounds

 2.0                1 ton      Large Quarry or Mine Blast

 2.5              4.6 tons

 3.0               29 tons

 3.5               73 tons   

 4.0            1,000 tons     Small Nuclear Weapon

 4.5            5,100 tons     Average Tornado (total energy)

 5.0           32,000 tons

 5.5           80,000 tons     Little Skull Mtn., NV Quake, 1992

 6.0        1 million tons     Double Spring Flat, NV Quake, 1994

 6.5        5 million tons     Northridge, CA Quake, 1994

 7.0       32 million tons     Hyogo-Ken Nanbu, Japan Quake, 1995; Largest 

                               Thermonuclear Weapon

 7.5      160 million tons     Landers, CA Quake, 1992

 8.0        1 billion tons     San Francisco, CA Quake, 1906

 8.5        5 billion tons     Anchorage, AK Quake, 1964

 9.0       32 billion tons     Chilean Quake, 1960

10.0       1 trillion tons     (San-Andreas type fault circling Earth)

12.0     160 trillion tons     (Fault Earth in half through center,

                               OR Earth's daily receipt of solar energy)

      The map  below shows the location of the various recording stations listed in Table 1 and also shows the approximate position of known faults in the area. 

     On the map, draw an arc with a compass from the position of each recording station.  The radius of the arc should be equal to whatever distance that particular station is away from wherever the earthquake occurred.  The intersection of these arcs indicates the approximate location of the epicenter of the earthquake.

QUESTIONS (to be answered on an attached piece of paper, if not done on-line:

1)  List the steps used to help you determine the actual location (epicenter) of the earthquake.

 2)  List the steps you used to determine the magnitude of the earthquake.

3) What value should be the same for all of the seismographs? Why?

4) Along which fault did the earthquake occur?