North Seattle Community College's
PHYSICAL GEOLOGY 101
Instructor:  Tom Braziunas

 

quake1.jpg (4094 bytes)
Boat Beached by Tsunami, 1964 Alaska Earthquake
NOAA Public Domain Photograph*

Review Lab:
Earthquakes, Geologic Dating & Cross-Sections

@2002 -- The information contained in this document is copyrighted.
No reproduction may be made without prior approval from the author (Dr. Tom Braziunas).

 

I. Introduction:

This week we expand on our geologic skills developed over the past few weeks.  The Internet offers some excellent learning tools developed by educators and public agencies.  Our textbook also provides many examples of real geologic structures on which to practice observational and analytic techniques.   The lab exercise this week has 20 questions which are worth 1 to 4 points each for a possible 30 POINTS TOTAL.

*All photographs shown in this lab exercise are online at:
  http://www.ngdc.noaa.gov/seg/hazard/slideset/earthquakes/

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Damage to Railway by 1965 Seattle Earthquake
NOAA Public Domain Photograph

II. Strikes and Dips:

Please visit the Lower Ugab valley in Namibia.  If you can't make it in person, then open your textbook to Figure 9-1 on page 152, or click on the image below.  

ugab.jpg (3879671 bytes)

Beautiful spot to park our land rover and explore!  We see such folding and faulting -- and it doesn't take a geologist to appreciate the earth stresses involved!  Of course, our friends and/or family (along for the ride) expect us to provide some interpretation of exactly what we are seeing.

Question 1 (1 point). What type of fold is this?  Look at the types of folds shown in Figure 9-8 of our textbook.   Which one(s) of these might best describe this fold? _________________

Question 2 (1 point). The folding is so extreme that it is difficult to decide which rock layers are the younger ones and which are older. What sedimentary structures could we look for to help us decide which layers are younger and which are older? (See page 139 of your textbook). _____________________

Question 3 (1 point).If the rock layers get younger as you go from right to left across the outcrop (so the most recently formed layers are to the left), is this fold an anticline or syncline?  Why? (Remember about the distinction based on the relative age of the innermost rocks.) _________________

Question 4 (1 point).  Walk directly across the road from your rover.  At that location on the outcrop, measure the dip of that very thick light-color sedimentary layer near the bottom of the cliff.   What is the dip direction and angle? _____________________

Question 5 (1 point).Follow this same layer to where it curves and reaches the top of the outcrop.  What is its dip direction and angle now? ______________________

Question 6 (1 point).  Notice the fault that runs through the structure just behind the jeep.  It is pretty obvious where it cuts through the thickest light-color unit about one-third the way up the outcrop.  You can also follow this fault all the way to the top of the outcrop (use a ruler and follow the straight line of this fault).  What type of fault is this (normal, reverse, thrust)? Why?  _____________________

Question 7 (1 point).  One centimeter on this photograph is equivalent to about 2 meters (6 feet) on the real outcrop.  For example, we can measure that the land rover is 2 cm long.  This is equivalent to 4 meters (12 feet) for the actual vehicle.  So now we can use our ruler to determine how much of a shift has occurred along this fault.  Measure the distance (in meters) based on the observed offset for that lower thick light-color sedimentary layer.  How much offset is there? __________

Question 8 (1 point).  Carefully follow the fault to the top of the outcrop.  Look for the patterns in the layers on each side of the fault to see which ones match up.  What is the approximate offset (in meters) along this fault line at the top of the outcrop?  What layers did you base this estimate on? _____________________

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quake3.jpg (3707 bytes)
Aerial view of San Andreas fault
NOAA Public Domain Photograph

III. Interpretation of Geologic Cross-Sections:

To review our principles of relative dating as applied to geologic cross-sections, we will make use of a neat learning tool available on the Internet.  "Athro Limited" is a private company which provides education modules on the Internet.  Click here to access the activities related to the interpretation of geologic sequences.

Question 9 (4 points). Find the list of hypothetical geologic examples and click on "fault."  We are asked to determine the correct sequence of geologic events shown by the cross-section.    In order to do this, we need to apply the principles of relative dating which we have learned.   As your answer to this question, complete the sequence correctly and explain the logic and principle behind your choice for each event.  Your explanations are as important as the correct sequence in earning the points for this exercise.

Question 10 (4 points). Return to the list of hypothetical geologic examples and click on "folds and an intrusion."   We are again asked to determine the correct sequence of geologic events shown by the cross-section.  Again, complete the sequence correctly and explain the logic and principle behind your choice for each event.

Question 11 (4 points).  Finally, return to the list of hypothetical geologic examples and click on "two intrusions."  Again, complete the sequence correctly and explain the logic and principle behind your choice for each event.  This is a much more difficult exercise than the previous two because we will find several possibilities for the sequence of geologic events .  You do not need to complete the second half of this particular exercise (about resolving these ambiguities in the relative dating).

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Damage from the Armenian SSR Earthquake, December 7, 1988
NOAA Public Domain Photograph

IV. Earthquakes:

An excellent Internet educational tool has been developed to step students through the process of locating the epicenter of an earthquake and determining its magnitude on the Richter scale.  This interactive "Virtual Earthquake" activity is part of a series of "Geology Labs On-Line." These labs were designed by Gary A. Novak of the California University at Los Angeles and supported by grants from the US National Science Foundation and the California State University System. 

The learning module nicely steps us through the analysis and completes the calculations.  Please write down your answers along the way so that you can copy them to the submission form for this lab.  Click here to begin.  Read through and complete each step.  Choose any one of the four options for generating an earthquake. 

Question 12 (1 point). Which earthquake region did you choose?   _______________________

Question 13 (1 point). What are the three seismic stations and what are the three S-P interval times for these three seismic stations? _______________________

Question 14 (1 points). Convert the three S-P interval times to distances using the travel-time graph provided.  (NOTE:   The S-P curve is simply the difference in the S curve and P curve that is shown in your lab manual's travel-time graph for earthquake waves.)  These three distances are: ____________

Question 15 (1 point).  Compare your calculated epicenter location with the true epicenter location.  Where is the true epicenter in latitude and longitude?  Click here to link to an Internet program which will provide latitude and longitude information on any city in the world.  Select the "Astrodienst Atlas" option and input the city closest to the epicenter location.  This city and its geographic coordinates are: ______________________ 

Question 16 (2 points).  Now we will proceed to calculate the magnitude of this earthquake.  Follow the instructions in the online module.  What are the three S wave amplitudes you measured? _____________

Question 17 (1 point).  What is your estimate for the Richter magnitude of the earthquake and how does it compare to the true magnitude? _________________

Question 18 (1 point).  Complete the final form and receive your Certificate of Completion.  Copy and paste it into the space provided for this answer on your lab submission form. 

Question 19 (1 point).  Pick a city in the world where a seismic station would only receive P waves but not the S waves from this earthquake.  You will need to use a globe or world map in order to make your choice.   Name the city and its country and explain why it would not receive the S waves from this earthquake. ______________

Question 20 (1 point).  Pick a city in the world where a seismic station would receive neither the P waves nor the S waves from this earthquake.  Name the city and its country and explain why it would not receive the P or the S waves from this earthquake. ______________

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Izmit (Kocaeli) Turkey Earthquake, August 17, 1999
NOAA Public Domain Photograph