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Geology 15 Fall 2013 Mid Term II 200 Points
Name: _________________________________ Date: ____________________
1. (1 pt.) Interseismic strain in a subduction zone looks like the drawing above.
a. true b. false
2. (1 pt.) During large subduction zone earthquakes, some areas go up and some areas go down.
a. true b. false.
3. (4 pts.) The largest earthquake ever recorded in historic times was:
a) Alaska, March 27th, 1964.
b) Sonora, Mexico, May 3rd, 1887.
c) Mexico City, September 9th, 1985.
d) none of the above.
4. (5 pts.) What is a focal mechanism, or first motion mechanism?
The following five questions refer to the three focal mechanisms (beach balls, first-motion plots) shown
below. North is indicated by the letter N above each mechanism.
N
N
N
A
B
C
5. (4 pts.) Focal mechanism A is a
a. reverse mechanism b. normal mechanism c. strike-slip mechanism d. none of these
6. (4 pts.) Mechanism B is a
a. reverse mechanism b. normal mechanism c. strike-slip mechanism d. none of these
7. (4 pts.) Mechanism C is a
a. reverse mechanism b. normal mechanism c. strike-slip mechanism d. none of these
8. (4 pts.) You'd expect to find mechanism A along:
a. an East-West trending strike-slip boundary
b. a North-South trending strike-slip fault
c. a North-South trending transform fault
d. all of the above
e. none of the above
9. (4 pts.) You'd expect to find mechanism B along plates that were
a. melting
b. converging
c. sliding past each other
d. diverging
The following questions refer to the plate map of the Pacific Northwest shown below: the Cascadia
Subduction Zone. Double lines indicate oceanic ridges, a single line indicates strike-slip boundary, and a
dashed line indicates a subduction zone.
At each plate boundary, draw the focal mechanism that would be expected to occur at that boundary.
Assume upper hemisphere projections for the “beach balls”.
10. (4 pts.) Which boundary is convergent?
a. A b. B c. C d. D
e. E
11. (4 pts.) Which boundary is a transform fault? a. A b. B c. C
12. (4 pts.) Which are divergent boundaries? a. D, C, E
d. F
e. E
b. A, G, D c. A, B
13. (4 pts.) Oceanic ridges are compressive boundaries. a. True b. False
14. (4 pts.) Where would you expect to see new oceanic crust being formed?
a. C b. E c. G d. all of these locations
e. none of these locations.
15. (4 pts.) Where would you expect to see oceanic crust being "consumed"?
a. A b. F c. E d. none of these locations.
16. (4 pts.) The motion on the fault labeled C is:
a.
b.
17. (4 pts.) In addition to the information gained from a focal mechanism, we must use other
information to determine the orientation of the causative fault. This information includes:
a. orientation of faults, aftershock patterns, presence of surface scarps
b. s-wave arrival times, alignment of planets
c. distribution of lost animal reports
d. none of the above
18. (1 pt.) We now use the moment magnitude scale because the others saturated for different
earthquakes, whereas the moment magnitude scale, Mw, is linear for all types of earthquake (and other
source) processes. a. true b. false
19. (20 pts.) Show by a drawing how a subduction zone behaves both during and between earthquakes.
Explain in a few sentences what your drawing depicts.
20. (4 pts.) Using the principals of paleoseismology, how many earthquakes are shown in the trench
exposing the fault to the right?
a. one event b. two events
c. three events d. four events
21. (4 pts.) What type of fault is this?
b. reverse fault c. strike slip
a. normal fault
d. blind thrust
22. (4 pts.) Different estimates of earthquake magnitude are derived from different methods based on
local shaking (Richter scale), body waves (mb), surface waves (MS), or seismic moment (MW).
TRUE/FALSE
23. (4 pts.) Earthquake moment is calculated by multiplying all but which of the following quantities
together?
a. the shear strength of the rocks
b. the rupture area of the fault
c. the average displacement (slip) on the fault
d. the Modified Mercalli Intensity at the epicenter
24. (20 pts.) Use the following nomogram and seismological data to estimate the magnitude of this
earthquake.
25. (15 pts.) What is the study of geodesy? What measurements are made? What do we hope to learn
from geodetic measurements?
GPS stations collect data in 3 parts which are shown in Time Series Plots (Time Series or TSP), where
time is always on X-axis:
 North/South movement over time (abbreviated N/S)
 East/West movement over time (abbreviated E/W)
 Height (up/down) movement over time (not shown in this activity)
Pacific Beach, Washington GPS Data
2004
2005
2006
2007
2008
2009
26. (2.5 pts.) What are the units of measurement for these time series? Circle the best choice.
a) centimeters and months
b) meters and years
c) millimeters and years
d) centimeters and years
27. (2.5 pts.) How long of a time period is shown in the time series plots?
a) 3 years
b) 4 years
c) 5 years
d) 5 1/2 years
28. (2.5 pts.) How far North did the Pacific Beach station move on the time series?
a) 30 millimeters
b) 40 millimeters
c) 60 millimeters
d) 90 millimeters
29. (2.5 pts.) Did the station move South in the same time period?
a) No, because trend line only moves up.
b) Yes, because trend line moves down.
c) Can’t tell from time plots given.
30. (2.5 pts.) How far East did the station (and therefore the Earth below it) move on the TSP?
Remember to use a straightedge to help.
a) -35 millimeters
b) 40 millimeters
c) 60 millimeters
d) 73 millimeters
31. (2.5 pts.) What overall direction was this station moving?
a) North only
b) Northwest
c) Northeast
d) Southwest
32. (2.5 pts.) What was the annual movement in the North direction?
a) 60 mm/yr
b) 3 years
c) 10.9 mm/yr
d) 16 mm/yr
33. (2.5 pts.) Calculate the annual Eastward movement:
a) 73 mm/yr
b) 13.3 mm/yr
d) 0 mm/yr
c) 401.5 mm/year
34. (40 pts.) A single 600 km long strike-slip fault has offset three stream channels which are spaced 200
km from each other. Trenching across these channels and fault systems, paleoseismologists have made
estimates for the timing of past earthquakes.
The following is a table that lists the timing of these earthquakes at the 3 different sites. Calculate the
Recurrence Interval between each earthquake at each site. Determine the average Recurrence Interval
at each site. Answer some questions.
Is the average Recurrence Interval about the same for each site?
What can the Recurrence Interval say about this fault?
Which sites (A, B, C, or D) are more “due” for an earthquake?