SO461 Geological Oceanography

Fall 2014 Test 2


There are 8 questions, each of which refers to a figure. Four are worth 15 points, and 4 are worth 10 points. Your answers should clearly indicate understanding of the procedures and concepts involved, and should be well thought out with appropriate terms and details to support your answer.


Authorized reference:

  1. You may use one 3x 5 card, personally hand written on both sides, during the exam. You will turn it in with the exam.
  2. Calculators will not be allowed. For the calculations, you may set it up and leave the answer in terms of a sum, product, or quotient, or you can make reason simplifications to get a rough idea of the answer.


Budget your time. Do not leave anything blank, and note that I am looking for an understanding of the important concepts and appropriate use of terminology.


Do not talk about the exam to any students in the course who have not completed the exam.


All work on this exam is individual. You may not any materials (books, notes, computers) other than your personal 3x5 note cared, and you may not use IM, texting, talking, or any other means to communicate with other individuals. If you are caught with an electronic device that is not turned off and inaccessible you will get a 0 on the exam.


(15 points) Figure 1. Map showing four locations (A-D) where three earthquakes have been located. One location has no earthquakes. The four focal mechanisms below the map (i-iv) correspond to the earthquakes; each might occur 0, 1, 2, or 3 times. Complete the table below.


Table 1. Location analysis


Focal mechanism (i-iv) and reason (3 locations)

Reason for no earthquakes here (1 location)



























(10 points) Figure 2. Eltanin 19 profile.



What exactly is depicted on this graph?


How exactly is it measured and computed?


What physical process creates the features that we measure?



(10 points) Figure 3. Earthquake focal mechanism.  (There were multiple versions, but all were the same kind of fault).



What is actually measured to create this diagram?



What can this diagram tell you about the orientation of the fault that caused the earthquake? Be specific.



What kind of fault is this? Can you predict which way it will move?






(15 points) Figure 4. Vectors showing the motion of Plate A if Plate B is held constant.


What defines an Euler pole, and why is it important for plate tectonics?



What can you say about the Euler pole for this pair of plates?



What kind of plate boundary boundary is this?


Could the earth focal mechanism on the right go with the boundary? Draw a diagram showing the distribution of forces responsible for the earthquake, clearly indicating if you are drawing a map or cross sections view and which way we are looking in your diagram.






(15 points) Figure 5 . Map (left) and velocity diagram for three plates (P1, P2, and P3).


If the P1-P2 boundary is a trench, plate P1 subducts.

If the P3-P2 boundary is a trench, plate P2 subducts.

If the P3-P1 boundary is a trench, plate P3 subducts.


Determine the type of triple junction, its stability, and how it will migrate with respect to the three plates.




(10 points) Figure 6. Two velocity diagrams for the triple junction in the previous question. Note very different numbers on the axes.




Could both of these diagrams be correct? Why or why not?







If you redid your computations in the previous problem (you used the diagram on the left for it), would your conclusions change much?




(15 points) Figure 7. Four magnetic profiles measured in the same region. Scales show distance, age, and anomaly names.  SOLUTION


Identify at least three anomalies on each profile, using either the name or the age. If there is not a ridge on aprofile, indicate in which direction it would be.


Why dont the anomalies line up vertically on the page? Include at least two reasons.


How would you compute half spreading rates? Show how you would compute on one profile (you do not need very accurate numbers), and indicate which of these profiles would have the fastest and slowest spreading rates.



(10 points) Figure 8. Seismogram recorded in Guam. The earthquake was in the Philippines.




How is a seismogram like the one above converted into a number to record the size of the earthquake, and what scale is used?



Does distance from the epicenter affect the record at this station, and does that affect the size of the earthquake on the scale you just discussed?



There is a second scale used for earthquakes. What is it, and how does it differ from the first one, both in how it is computed and values obtained for each earthquake?





There were three versions of the profiles. Two were mirror images, and another changed the top to bottom order. Anomaly picks are given for one.



Beach Ball Forces



Focal mechanism, for a thrust (reverse) fault.

Best location for a cross section is perpendicular to strike of the fault, shown by the red line.  This is the map view.

One nodal plane dips gently down to the NE (diagram in red).  Max normal stress is horizontal, minimum is vertical.  This is the cross section (side view).  This is the more likely solution at a subduction zone.

The other nodal plane dips steeply down to the SW (diagram in green).  Max normal stress is horizontal, minimum is vertical.  This is the cross section (side view).