Earthquakes! (Kevin Seegan)

Title: Earthquakes: model and interpreting data.

Principle(s) Investigated: Plate tectonics, continental drift, plate boundaries, convection currents, slab pull, divergent boundaries, convergent boundaries, transform boundaries, seismic gap, earthquakes.

Standards :

All Earth processes are the result of energy flowing and matter cycling within and among the planet’s systems. This energy is derived from the sun and Earth’s hot interior. The energy that flows and matter that cycles produce chemical and physical changes in Earth’s materials and living organisms. (MS-ESS2-1)
- 1. Plate tectonics accounts for important features of Earth’s surface and major geologic events. As a basis for understanding this concept:
- a. Students know evidence of plate tectonics is derived from the fit of the continents; the location of earthquakes, volcanoes, and midocean ridges; and the distribution of fossils, rock types, and ancient climatic zones.
- b. Students know Earth is composed of several layers: a cold, brittle lithosphere; a hot, convecting mantle; and a dense, metallic core.
- c. Students know lithospheric plates the size of continents and oceans move at rates of centimeters per year in response to movements in the mantle.
- d. Students know that earthquakes are sudden motions along breaks in the crust called faults and that volcanoes and fissures are locations where magma reaches the surface.
- e. Students know major geologic events, such as earthquakes, volcanic eruptions, and mountain building, result from plate motions.

Materials:

From any hardware store:

1 1/2 inch thick plywood (approximately 3 feet by 4 feet)

wood screws

pulley

rubber spacer

washers

mounting hardware for winch and pulley

countersink drill bit for mounting the winch to the plywood

metal ribbon

bricks of varying sizes

optional: wood screws and industrial wheels for easy transportation.

From Harbor Freight:

25 foot bungee cord

boat winch

sheets of 50 grit and 160 grit sandpaper

spray adhesive

carabiner

Other:

phone and tripod for recording

projector and screen

Procedure:

This lab is designed to provide a model for students to observe and record data about earthquakes. In order for the data collection to be meaningful, it is best to use a single example of the demonstration. For this reason, prior to class I recorded a run-through of the demo itself.

First, I explain my use of safety equipment of leather gloves and impact resistant eye protection. Since I am dealing with moving objects and high tension in the bungee cord, it is important to wear eye protection and to have the students seated in such a way that they cannot be injured if something does go wrong.

Then I give a demonstration with the model, showing each of its components and what they do. At this point I am not referencing how they directly relate to earthquakes or plate movements, but the students do realize that this is an earthquake model. When executed correctly, the movement of the bricks on the sandpaper will be sudden and incremental- thus modeling the movement of Earth's tectonic plates along a transform boundary.

For the sake of meaningful data collection, I have the students watch the pre-recorded demo I had recorded on the projector. This allows for multiple viewings. For the first viewing, I have the students watch the video with no sound and count the amount of times the bricks move. The second time, I have them watch it with sound. There is usually a contrast between how many times they think it moves the first time and the second time, because the sound indicates another movement that is not easily caught.

I then have students fill out a lab form:

I repeat the video a final time, and pause the video after each movement so that students can record the distance of each movement. We then take a look at the data provided.

Student prior knowledge:

1. Plate tectonics accounts for important features of Earth’s surface and major geologic events. As a basis for understanding this concept:

a. Students know evidence of plate tectonics is derived from the fit of the continents; the location of earthquakes, volcanoes, and midocean ridges; and the distribution of fossils, rock types, and ancient climatic zones.

b. Students know Earth is composed of several layers: a cold, brittle lithosphere; a hot, convecting mantle; and a dense, metallic core.

c. Students know lithospheric plates the size of continents and oceans move at rates of centimeters per year in response to movements in the mantle.

d. Students know that earthquakes are sudden motions along breaks in the crust called faults and that volcanoes and fissures are locations where magma reaches the surface.

e. Students know major geologic events, such as earthquakes, volcanic eruptions, and mountain building, result from plate motions.

Explanation:

The point of this lab is to demonstrate the constant force applied on tectonic plates at all times. Since faults are not always easily accessible or easy to visualize, I think a good method of driving the concept home is via a model. The following processes and more are covered:

Tension and force in the Earth's crust: the winch and bungee cord

Seismic Gap: time between movements of the brick. Students will find that this is inconsistent, much like the nature of earthquakes. Students can also hear the brick start to move before a rupture, representing a foreshock.

Different types of faults: The finer grit sandpaper models faults which creep along much like the San Andreas. The rougher grit sandpaper can represent an area that has more convergence.

One thing that I would advise for any others who carry out this demo: Use an inelastic cable and spring instead of just a bungee cord, this way it is easier to visualize the stress on the bricks.

Attaching a second set of bricks in tandem fastened with another spring could show how earthquakes "talk" to each other along a fault line.

Also, if one has a strain gauge, this demo can be used with a weight scale to calculate the coefficient of friction on the board, and predict what force is necessary to cause an "earthquake." This adds insight because it contrasts with the prior learned knowledge that the rupture length and time between ruptures are difficult to calculate.

$F_\mathrm{f} \leq \mu F_\mathrm{n}$

$F_\mathrm{f}\,$

is the force of friction exerted by each surface on the other. It is parallel to the surface, in a direction opposite to the net applied force.

$\mu\,$

is the coefficient of friction, which is an empirical property of the contacting materials,

$F_\mathrm{n}\,$

is the normal force exerted by each surface on the other, directed perpendicular (normal) to the surface.

Questions & Answers: Give three thought-provoking questions and provide detailed answers.

Applications to Everyday Life:

If the students find the friction coefficient, they can apply this thought to how tires are designed and what shapes are best for creating the highest friction coefficient on a given surface. Students could design tires for remote control cars and have them drive across different materials to see which design is best.

Students can hypothesize where the larger earthquakes take place in the world, and why. They can then evaluate where hazardous areas are due to data available.

Students can create earthquake preparedness kits, as earthquakes are still largely unpredictable as demonstrated in the experiment. Better to be safe than sorry!

Photographs:

Videos: