GEOL 344: EARTHQUAKES AND SEISMIC HAZARDS (SPRING 2008) MIDTERM EXAM NAME: ___________________________________
GRADE: ______ / 136
TIME AVAILABLE: 75 MINUTES Instructions: There are NINE pages of questions. Take a minute or so now to page through the entire exam to get a feeling for the length. Answer the easier questions first. Do not waste time struggling through any one question. Move on to another question and come back to the ones with which you were having difficulty. READ THE QUESTIONS CAREFULLY. Think carefully about your wording, trying to avoid ambiguity and unclear answers. PART 1: True or False
(20 points)
For each of the following statements, circle whether the statement is true or false. IF THE STATEMENT IS FALSE, provide a corrected statement.
Hint: FOUR of the statements are false. 1. Due to motion along a normal fault during an earthquake, a moletrack is often produced along the surface. TRUE / FALSE 2. Earthquakes occur in the lithosphere, which is made up of the crust and the upper mantle.
TRUE / FALSE
3. The strength of rocks increases with depth without exception.
TRUE / FALSE
4. Earthquake sequences typically involve a mainshock that occurs at the base of the zone of later aftershocks. TRUE / FALSE 5. The combined effect of principal stresses and fluid pressure is referred to as the effective stress.
TRUE / FALSE
6. The dynamic frictional strength of a fault is greater than the static frictional strength.
TRUE / FALSE
7. The Richter magnitude of an earthquake can be calculated using a nomograph.
TRUE / FALSE
8. In a focal mechanism solution, the shaded portions represent dilatational first-arrivals.
TRUE / FALSE
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PART 2: Multiple Choice
(28 points)
Circle the correct answer from those provided for each question. 1. Which one of these countries is not on the list of the top ten historic death tolls due to earthquakes? (2) ( Iran / Japan / Indonesia / China ) 2. For every decrease of 1 in magnitude, the number of earthquakes that occurs increases by a factor of about: (2) ( 2 / 10 / 1000 ) 3. The largest earthquake ever recorded (M9.5) occurred in this country in 1960: (2) ( Indonesia / Chile / United States ) 4. The largest earthquake ever recorded in the United States occurred in this state: (2) ( Alaska / California / Missouri / Washington ) 5. The New Madrid earthquakes of 1811-1812 were examples of _____________________ earthquakes. (2) ( plate boundary / volcanic / intraplate ) 6. Another term that has the same meaning as hypocenter is: (2) ( focus / epicenter / meizoseismal region ) 7. The deepest earthquakes recorded anywhere in the Earth are no deeper than: (2) ( 33 km / 300 km / 680 km ) 8. The force per unit area applied to rocks inside the Earth is also referred to as the: (2) ( strain / stress / elasticity ) 9. If fluid pressure is added to rocks that were initially dry, the Mohr circle may touch the failure line by: (2) ( shifting to the right / shifting to the left / increasing in size ) 10. Attenuation refers to the __________________________ as the seismic waves move away from the focus. (2) ( decrease in seismic energy / increase in seismic energy / interference between different seismic waves ) 11. If two earthquakes differ in magnitude by 1, then the wave amplitudes differ by a factor of: (2) ( 1 / 10 / 32 )
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12. The magnitude scale used to measure the world’s biggest earthquakes is abbreviated as: (2) ( ML / Mb / Ms / MW ) 13. The maximum possible intensity during an earthquake is: (2) ( 9.5 / 10.2 / VIII / XII / XX ) 14. In a lower hemisphere stereographic projection, a vertical fault plane would be represented by a: (2) ( straight line / curved line within the circle / curved line around the circle’s edge )
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PART 3: Fill in the blanks
(36 points)
1. The US Army Corps of Engineers and the US Nuclear Regulatory Commission have both adopted the definition of an active fault as being one that has ruptured at least once in the past ____________________ years.
(2)
2. List three different types of earthquake hazards: (1) ____________________________________________ (2) ________________________________________ (3) ________________________________________ (6) 3. The three different types of plate boundaries where earthquakes occur are: (1) ________________________________________ (2) ________________________________________ (3) ________________________________________
(6)
4. The portion of the brittle lithosphere capable of producing earthquakes is called the ______________________________ zone.
(2)
5. In 1989, the Loma Prieta earthquake occurred along a small portion of the San Andreas fault that had not produced earthquakes for a long time, resulting in a _____________________________ along the fault.
(2)
6. The outer boundary of a fault plane, where the slip becomes zero, is called the _______________________. (2) 7. Lithostatic stress is equal in magnitude from all directions. It can thus be referred to as being ________________________________.
(2)
8. The type of normal stress that acts perpendicular to a plane that contains zero shear stress is called a ____________________________ stress, and there are always three of them.
(2)
9. The frictional type of motion of faults that results in cyclical slip events (i.e., earthquakes) separated by periods of no slip at all is called ____________________________ sliding. Faults that do not behave this way may undergo a constant creeping motion, also called ________________________ sliding. 10. Name any one geodetic measuring technique: ________________________________________
(4) (2)
11. The distance to an earthquake from a seismograph station can be determined by measuring the _______________________________________ on the seismogram.
(2)
12. A lower hemisphere stereographic projection graph is more commonly called a _______________________. (2) 13. Another term having the same meaning as focal mechanism solution: ______________________________. (2)
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PART 4: Topical Questions A. FOCAL MECHANISMS AND EARTHQUAKE GEOGRAPHY
(20 points)
1. The map above shows focal mechanism solutions for moderate to large magnitude earthquakes during FebMar 2008. Circle the focal mechanism solution that corresponds to the questions below. Label each circled answer with the corresponding question number. 1. 2. 3. 4.
A strike-slip focal mechanism solution A normal fault (or oblique normal) focal mechanism solution in oceanic crust A reverse fault focal mechanism solution along a subduction zone A solution containing two normal faults that strike approximately NE / SW
(2) (2) (2) (2)
2. All focal mechanism solutions contain two possible fault planes to explain the observed pattern of first-arrivals. These two solutions represent the _________________________________ assumption.
(2)
3. Create an artificial focal mechanism diagram in the circle below. The first fault has a strike and dip of 135º/80º and the second has a strike and dip of 315º/10º (both measurements use the right-hand rule). Shade in the compressional zone to show that the earthquake occurred in response to a reverse faulting event. The lower hemisphere stereographic projection graph on the right is for reference only. (10)
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B. EARTHQUAKE GEOLOGY IN PICTURES
(32 points)
From the eight questions shown below, choose FOUR and answer the questions posed. Make sure that you answer ALL aspects of the question! 1. This map shows the distribution of M≥7 earthquakes that happened in the world in 2007. (a) Discuss why these earthquakes occurred where they did (i.e., describe the tectonic environments). (b) Comment on the depths of these earthquakes (see color bar) and how they compare to our expectations about earthquake depths. (Note, the yellow star simply represents the last earthquake that happened in 2007 so is no more important than the rest)(8)
2. (a) What information is being illustrated by these plots of earthquakes in the western Pacific? (b) What specific term is used to describe this pattern of earthquakes? (c) What type of tectonic environment is implied by these earthquakes? (d) Sketch the outlines of the tectonic plates on these figures. (8)
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3. (a) What process is being illustrated in the figure below? Describe it in a few sentences. (b) What feature is labeled by the letter B? (c) What feature is labeled by the letter C?
(8)
4. In the space below, construct 2 graphs. (a) The first graph should be a plot of stress vs strain for an elastic material up to the point of failure. Be sure to label important components of the graph and indicate what the slope of the line represents. (b) The second graph should show the Mohr failure line on a shear stress vs normal stress plot (do not include a Mohr circle). Again, label any relevant components of the graph and indicate what the slope of the line represents. (8) (a)
(b)
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5. (a) Name the process being illustrated in the sequence of figures below. (b) For each step, describe and explain what happens, also indicating how the amount of displacement varies as a function of perpendicular distance away from the fault. (8)
6. The figure below shows the four types of seismic waves produced during an earthquake. Label all four types of waves and indicate whether they are body or surface waves. (8)
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7. The figure below shows an artificial seismogram representing the record of an earthquake. (a) Label each component, A-D. (b) Use an arrow to indicate on the seismogram the point in time when the first seismic waves from the earthquake arrived. (c) Based on first-arrival data, was the seismograph station in the compressional or dilatational zone relative to the fault that produced the earthquake? (8)
(a) A: ________________________________________
B: _________________________________________
C: ________________________________________
D: _________________________________________
(b) Indicate on figure with an arrow. (c) The first-arrival indicates a ________________________________ zone caused by motion along the fault. 8. The figure below shows the focal mechanism solution for the M6.0 earthquake that struck Wells, Nevada, on 21 February, 2008. Fill in the information below regarding the solutions for Fault Plane 1 (FP1) and Fault Plane 2 (FP2). LABEL which fault is which on the figure. Also answer the question posed below the table. (8) FP1
FP2
STRIKE:
______________
______________
DIP:
______________
______________
DIP DIRECTION:
______________
______________
Question: Are the two possible faults that define this solution pure dip-slip, pure strike-slip, or oblique-slip faults? Explain your answer. Answer: _________________________________________________ ________________________________________________________ _ 9. BONUS QUESTION:
(8)
For those of you locked in a seismic frenzy, answer one more of the above questions for up to 8 bonus points! [144 possible points]
