A Slice in Time (The IslandWood Fault, earthquakes, and geologic principles)

Key goals and objectives: SWBAT explain the events of the IslandWood fault, and relate the events to the mega-thrust earthquake 1100 ybp (an estimated 7.4 shallow earthquake). SWBAT recognize simple geologic principles, such as crosscutting, original horizontality, and superposition.

Introduction and Activity: 1100 years ago, there was a large, shallow earthquake in the Seattle region that uplifted the south end of Bainbridge Island, and we have evidence of that quake here at IslandWood.
We already saw how glaciers dramatically modified the landscape of the Puget Sound Lowland, so, by extension, if an earthquake breaks the surface in this area, we know that it is younger than the retreat of the glaciers (i.e. it happened after the glaciers left the area).
[Show BI LIDAR; note several things: the north-south trending hills (evidence of glacial retreat), the Toe Jam Hill Fault (east-west trending scarp south of Blakely Harbor), and small scarps south of Mac’s Pond (see figure below and IW LIDAR for more detail).]

The core lesson: We already know that before the glaciers came into the Puget Lowlands that rivers from the Cascades and the Olympics flowed into our region. Those deposits comprise the bedrock of the region. The glacier left its own deposits, mostly till, the stuff that gets bulldozed in front and continually ground, and the erratics we have already seen, on top of this bedrock.
Which one is older the bedrock of the glacial deposits? In geology this the law of superposition — the younger rock is always on top of the older rock. In our case, the glacial deposit are younger then the bedrock, so in an undisturbed location the glacial deposits should always be on top.
Here at the trench we have something interesting going on, can you see what is going on? In this wall, you can see that the older bedrock [point to the rock] has been lifted over the younger glacial till [point to the rock(s)]. Draw what you see. [Have students sketch what they see, and do not worry about accuracy, you will explain the events later.]
What did you think happened here? Do you see anything that seems out of place in this wall? How would you know there was an earthquake? What would you look for? In geology the term crosscutting is used to describe this wall, and indicates that a disturbance (or (fault) is present.
This is what happened here, as we mentioned already, 1100 years ago there was a large earthquake that deformed the ground surface along an existing fault. When this happened the rocks were deformed and moved, and are now facing in different directions. In geology this is called original horizontality because they were deposited in horizontal planes. In this event the ground has been uplifted 23 feet (7 meters) above the mean sea level [Show IW_LIDAR_Faults.pdf, IW_Trench_Profile.pdf, IW_Trench_Photo.pdf (Sherrod 2003)]

Puget Sound stratigraphy, showing both before and after the Vashon glaciation (Booth, Haugerud, and Troost 2003:22).

Conclusion: How big was this earthquake? How would it have affected people living the area? How would we know that it did affect the people in area, and how would we look for that? What geologic principles are present, and how do we know? Make sure that you draw or record those relationships. What geologic principles are present, and how do we know? Make sure that you draw those relationships.

I already said that this earthquake uplifted the entire south end of Bainbridge Island, and now we are going to continue looking for evidence of this massive earthquake.

Assessment: Built-in to the experience or based on a follow-up in the next lesson. You may also ask them to share their drawings to see if they accurately depicted the older sediments overriding the younger sediments.
There are many concepts imbedded into this lesson, and the most important is to recognize the principles of superposition, crosscutting, and original horizontality.

Extension: Technology Innovations with LIDAR and Aerial photo comparisons. With the former it is easy to see the North-South orientation of glacial landscaping, faults that were previously undocumented, and most strikingly virtual deforestation. Because LIDAR uses lasers to collect data, with computer modeling the trees can be removed for more topographic accuracy [compare the images on learn.IslandWood.org].

Science Notebooks or Journals: In addition to the images above to illustrate the geologic principles herein, students should draw the typical three-step sequence to illustrate faulting:

Alternative Conceptions

Background Information: see the earthquake page.

References: see the citations page.

Created by Matthew John Brewer on November 23, 2003, modified on March 8, 2004.

These pages were created by Matthew John Brewer on March 23, 2004 as part of the Graduate Program at IslandWood and fulfillment of the Independent Study Project. Last updated on May 18, 2004.

To contact the webmaster or to find out more about IslandWood you should go to the contact us web page.

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