GEOL/GEOE 344: EARTHQUAKES AND SEISMIC HAZARDS (FALL 2001) FINAL EXAM NAME: TIME AVAILABLE: 120 MINUTES TOTAL POINTS: 110 (yep, that s 10 bonus points, just for showing up!) Instructions: There are several pages of questions. Take a minute or so now to page through the entire exam and 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. Avoid ambiguous and unclear answers. 1
PART 1: Fill in the blanks (15 points) 1. The spacing between normal faults is generally thought to be controlled by: (1) 2. Reverse faults that do not pierce the Earth s surface are called (1) 3. Do the surface geometries of thrust faults more resemble strike-slip or normal faults? (1) 4. The 1971 Sylmar earthquake (M6.7) occurred below which city? (1) 5. The 1964 Gulf of Alaska earthquake had a moment magnitude of M w = (1) 6. An example of a seismic gap along the Aleutian subduction zone is the (1) 7. The last subduction zone megathrust earthquake along the Cascadia subduction zone was a M~9 event in the year, which produced a that reached Japan the next day. (2) 8. What was the biggest seismic hazard associated with the M7.2 volcanic earthquake on the flanks of Kilauea, Hawaii, in 1975? (1) 9. Paleoseismological studies are usually applicable to faults that have been active during the past years. (1) 10. Define the following terms: coseismic (1) aseismic (1) nonseismic (1) 11. Give an example of a secondary, off-fault, instantaneous, geomorphic feature: (1) 12. Not all buildings are designed to withstand the maximum credible earthquake. Instead, each structure is designed in accordance with the risk. (1) 2
PART 2: Topical Questions (75 points) 1. THE 1983 M7.3 BORAH PEAK EARTHQUAKE (19 points) i) Examine the surface rupture trace map for the 1983 Borah Peak earthquake along the Lost River Fault system on left. Label the approximate locations of the following features using numbers 1 through 6: 1: Epicenter location 2: Chilly Buttes liquefaction/sand blow site 3: A segment boundary along the fault zone 4: The Double Spring Road trench site 5. The point of maximum slip along the fault trace 6: The site of a landslide that occurred after the earthquake (6) ii) Indicate on the map how much of the fault system is thought to have ruptured during the large earthquake event that occurred in this region 10,000-15,000 years ago. (2) iii) Describe the 3 scales of segmentation along the Lost River fault system. (3) iv) What was the focal depth of this earthquake? What was the probable reason for the earthquake hypocenter being at this depth? (2) iii) List THREE seismic hazards that may be associated with this region of Idaho as well as the associated seismic risks (low, moderate or high). (6) SEISMIC HAZARD SEISMIC RISK 3
2. SUBDUCTION ZONE EARTHQUAKES (21 points) i) The diagram below shows a cross section through a typical subduction zone. Label the elements of the subduction zone indicated by empty boxes. (5) ii) Indicate on the above diagram any FOUR locations where earthquakes may occur in a subduction zone setting, and explain below why earthquakes occur in those portions of the subduction zone. (8) iii) Explain how both convergence rates and the age of the subducting oceanic crust affect earthquake magnitudes along subduction zone megathrusts. (4) iv) Subduction zone earthquakes may result in both uplift and subsidence features. Describe evidence for each of these along the Aleutian subduction zone. (4) 4
3. PALEOSEISMOLOGY (17 points) Examine the figure below showing stratigraphic units that were mapped along the walls of a trench across the San Andreas fault. i) Is there any current geomorphic evidence of a paleoearthquake along the fault in this location? (2) ii) Is there any stratigraphic evidence of a paleoearthquake in this location? (2) iii) Is the regional erosion rate rapid or slow in this location? Explain your answer. (3) iv) What is meant by the term event horizon? Label an event horizon on the above figure. (3) v) If two event horizons could be distinguished in a trench cross-section, how would a paleoseismologist determine the slip rate along the fault? (4) vi) How would a paleoseismologist then determine how long it will be before the next earthquake may be expected to occur at this location? (3) 5
4. SEISMIC HAZARD ASSESSMENTS (18 points) The diagram below shows a probability density function that was created to describe the probability that an earthquake of M>7 will occur along a highly active fault in the Smallville area in the 60 year period subsequent to 1990. It was developed after a trenching study across the Smallville Fault zone. Dt Probability Density 0 30 60 Time (years) i) What information about the active fault would be needed in order to create such a probability density function for the region? (3) ii) Based on this curve, should you be overly concerned about a M>7 earthquake occurring in Smallville today (i.e., in the year 2001)? Explain your reasoning. (3) iii) Looking into my crystal ball, I see that LexLuther Corp. will begin building a new nuclear power plant in Smallville in the year 2020. At that time, you will be working for the Smallville geotechnical consultant firm, Homer Simpson and Son Inc. You will be hired to determine the seismic hazard potential of the nuclear power plant site. a) How would you calculate the conditional earthquake probability over the construction time period Dt if an earthquake had not occurred since 1990? (3) 6
iv) LexLuthor Corp. will decide to build their new plant to the building codes associated with a maximum credible earthquake of M6.5. The nuclear plant has a projected life span of 35 years. Why would you want to be the hell out of Smallville by that time? (geologic reasons only please!) (2) v) Being highly suspicious of probability density functions, and having learned that the geological expert who produced it went to Boise State, you decide to embark on your own paleoseismologic investigation of the Smallville Fault. In your infinite wisdom, you discover that one out of three earthquakes along the Smallville fault does not rupture the surface. What does this imply about the earlier calculated recurrence interval for the fault? (2) vi) If this information had been known in 1990, how would it have affected the probability density function diagram? (2) vii) If it turned out that the Smallville fault ruptured like clockwork once every 60 years (so the recurrence interval is known with absolute precision), always producing exactly the same magnitude earthquake, draw the resultant probability density function that would have been developed in 1990. (Draw it directly on the existing figure, and assume that the last earthquake occurred at time = 0) (3) 7
PART 3: General Questions (20 points) From the following list of questions, choose FIVE to answer. Do not answer more than five. Only the first five answers will be graded. Dig it? i) Describe the main faults associated with the Himalayan collision zone, and explain why the city of New Delhi in India is at particular risk from a potential M>8 earthquake. (4) ii) The occurrence of thrust faults that produced the California Coast Ranges parallel to the San Andreas fault in central California is somewhat enigmatic, but the thrust faults that define the Transverse Ranges along the Big Bend of the San Andreas fault are to be expected. Explain why this statement is true. (4) iii) Explain how a stepping sequence of uplifted marine terraces near a subduction zone can be used to determine the slip rate on the causal fault, and why this method may be somewhat inaccurate for the determination of subduction zone megathrust recurrence intervals. (4) iv) Los Angeles is experiencing a deficit in moderate magnitude earthquakes based on an examination of recurrence intervals of thrust faults in the Los Angeles Basin. Explain what is meant by this statement and briefly describe the reasoning behind it. (4) 8
v) Considering the magnitude of the 1994 Northridge earthquake, damage and loss of life in Los Angeles was relatively minor. Describe at least TWO geologic reasons why this was the case. (4) vi) On the figure below, label the locations of the following: 1. Andes Cordillera back-arc reverse faulting. 2. Aleutian subduction zone. 3. Hellenic subduction zone. 4. The epicenter of the largest earthquake during this semester (M 8.1). (4) vii) What are the fundamental differences between Chilean type and Mariana type subduction zones, and what are the implications of these differences for earthquake magnitudes?. (4) 9
viii) The Puget Sound region of western Washington experienced large magnitude earthquakes in 1949 (M7.1), 1965 (M6.5), and 2001 (M6.8). Why is this an earthquake-prone region, what was the cause of these specific earthquakes, and why was damage minimal in each case despite the large magnitudes? (4) ix) Explain the differences between seismic hazard and seismic risk. Provide examples of two locations that have a similar seismic hazard and evaluate the associated relative seismic risk in each location. (4) x) Explain the major differences between deterministic and probabilistic seismic hazard assessments. (4) xi) A rectangular shaped hospital is being planned in a city that is prone to earthquakes. Is it important to give serious consideration to the orientation and height of this building? Explain your answer. (4) 10