PART 2: Analysis of an Earthquake

Step by Step
Most earthquakes are caused by sudden movements on faults or by volcanic eruptions.  Of course plate boundaries are characterized by faults of different types. Reverse faults occur at convergent plate boundaries, normal faults occur at divergent plate boundaries, and strike-slip faults occur at transform plate boundaries. All three types of plate boundaries can be found around the edges of the Caribbean plate, although the divergent type of boundary is restricted to a very small region west of Jamaica, just south of the Cayman Islands. Near the eastern margin of the Caribbean plate there are recently active volcanoes in the lesser Antilles.  These are associated with a convergent plate boundary and caused by subduction of the North American plate beneath the eastern edge of the Caribbean plate.  For more details, visit the seismic hazard website for the Caribbean Disaster Mitigation Project (CDMP).  

In this section of the exercise you will analyses three seismic records of an earthquake.  The purpose is to determine (1) when the earthquake occured, (2) where the epicenter is located, (3) the magnitude of the earthquake, and (4) the specific cause of the earthquake.  Below you will find three seismograms for the earthquake.  The first seismic station is in Kingston, Jamaica, the second station is in Mayaguez, Puerto Rico, and the third station is in Port of Spain, Trinidad.  The arrival time of the P-wave is given on each of the seismograms, and is also given on your answer sheet.

For each seismogram measure the S-P interval and the amplitude.  Enter the values on your answer sheet. 

Seismogram #1

On your answer sheet record the following measurements:

Arrival time of the P-wave.
S-P interval (seconds).
Amplitude (mm).
Seismogram #2

Again, on your answer sheet record the following measurements:
Arrival time of the P-wave.
S-P interval (seconds).
Amplitude (mm).
Seismogram #3

Record the following measurements:
Arrival time of the P-wave.
S-P interval (seconds).
Amplitude (mm).

Now, for each seismic station determine the distance to the epicenter using the following graph.  The red line shows how the S-P interval relates to the distance to the epicenter.  For each seismic station, find the S-P interval time on the vertical scale, move across horizontally to the red line, then vertically down to the distance scale, and record the distance on your answer sheet.  Now, for the same distance move vertically up to the line labelled "Travel time P-wave," then horizontally to the left, back to the time scale, and record the P-wave travel time on your answer sheet.

When did the earthquake occur?  You can determine this by subtracting the travel time of the P-wave from the arrival time of the P-wave. Do this for each seismic record, and enter the times on your answer sheet.

T(0) = (P-wave arrival time) - (P-wave travel time)


The next step is to determine the Richter magnitude.  This can only be done for seismograms that did not go off-scale.  For each seismogram, use the following nomogram to estimate the magnitude.  Do your work on the copy of the nomogram on the answer sheet.  For each seismogram mark the distance to the epicenter on the left scale and mark the amplitude on the right scale.  Connect the distance and the amplitude with a straight line.  Where the line intersect the middle scale gives the magnitude. Do this for each of the seismograms, for three estimates of the magnitude.  Enter the values on your answer sheet.

Next, you can determine the location of the epicenter. Using a compass on your copy of the map shown below, around each seismic station (Port of Spain, Trinidad; Mayaguez, Puerto Rico; Kingston, Jamaica), draw a circle having a radius of the distance to the epicenter.  The epicenter is where the three circles intersect.  Show your work on your answer sheet, and mark the epicenter with a small filled circle.

2.1 What's your best estimate for T(0), the time that the earthquake occured at the epicenter?

2.2 What's your best estimate for the magnitude of the earthquake?

2.3 Where is the epicenter?  Describe the location.

2.4 Is the epicenter in an area with a history of earthquakes?

2.5 What do you suppose caused the earthquake? (Be specific.)

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