MYSTERIOUS EARTHQUAKES, UNEXPECTED
EPISODIC TREMOR AND SLIP:
Congratulations! You have just been hired as a geologist to study a strange
phenomenon in northern Washington State. Your job is almost like detective
work. You must determine what’s happening by looking at a variety of pieces of
evidence, some of which are over 30 years old. You will compare your findings
with other geologists and once you’ve learned what’s happening, you can predict
what’s going to happen in the future (and get paid big bucks!).
About the Cascadia Region
1. What type of boundary exists between the
Juan de Fuca plate and the North American plate?
2. What types of tectonic plates are they?
Juan de Fuca Plate: __________
North American Plate: __________
3. What’s happening to the edge of the continental plate during the “Growth of a Continent”
4. Compare the animation to the Cascadia region. What overall direction would you expect
point A (on the map above) to be moving due to the deformation? How do you know that?
ETS Student Work, modified on 12/18/11 at 2:20 PM Page 1
Looking at the data…
5. Calculate the horizontal and vertical movement of these stations and draw the horizontal
vectors on the map…
ALBH NEAH SEAT PABH
Note: Data from the UNAVCO Plate Boundary Observatory uses a reference frame where the
interior of the North America Plate is not moving
ETS Student Work, modified on 12/18/11 at 2:20 PM Page 2
Evidence #1: What was that???
6. According to this GPS receiver, what
overall direction was the North
American Plate moving before the
location of the arrow?
7. Take a look at the data collected
between Oct 1 and Oct 15, 1994. The
arrow points to this time period.
a) What direction was the North
American Plate moving during this
brief time in October? Hint: in
order to connect the green lines
what direction would the plate had
to have moved?
b) Approximately how far did it move?
Be sure to include units!
8. Think about what you have already learned about long-term motion of the Cascadia
convergent boundary. Would you expect the reversing movement to the southwest during
October 1994 to occur along this subduction zone? Why or why not?
9. What do you think scientists did to be sure that the data they collected weren’t caused by
ETS Student Work, modified on 12/18/11 at 2:20 PM Page 3
A Pattern Seems to Emerge . . .
10. From looking at the data, describe the
direction of movement of this station between
1996 and 1999.
11. About how often is the reverse motion
12. Approximately how many millimeters of reverse
motion occurs during each slip event?
13. Work with your team mate to explain what was happening. And why? (Hint: this is a
converging subduction zone . . .)
Evidence #2: Putting the Pieces Together
ETS Student Work, modified on 12/18/11 at 2:20 PM Page 4
14. What do you notice about the seismograms?
15. What are some observations you can make about these seismograms compared to a ‘normal’
16. What do you think is the next step the scientists took?
Do we see this everywhere in the Pacific Northwest?
17. After watching the animation, draw what the time series plots look like for the 3 areas:
18. What is the PABH data showing here? Does this station experience a slow slip event
movement to the southeast? Take a look at the longer time series plot; does it exhibit any
slow slip movement?
19. Explain how the data from PABH are different from observations at ALBH.
The BIG Picture:
Why You NEED to Know This!
20. Draw a quick sketch of the setup of the blocks demonstration.
21. Describe what happens as to the blocks as the rubber band is pulled.
ETS Student Work, modified on 12/18/11 at 2:20 PM Page 5
22. Why are there higher stress levels during the ETS events? What could happen?
23. Using conservative estimates, if 3 centimeters of movement is added to the locked plate
every year, In 500 years, how much ‘slip deficit’ would build up on the locked portion of the
24. Why would predicting when a tsunami would happen be so important?
ETS Student Work, modified on 12/18/11 at 2:20 PM Page 6