About This Investigation
This laboratory investigation is an online version of the hands-on investigation "Determining and Measuring Earth's Layered Interior" The flow of these activities are very similar. However, in response to educator requests, we have created digital versions of the models so students can complete the activity using their computers either in the classroom or from home during virtual instruction.
Time: ~60 minutes
Learning Objectives:
By the end of the exercise, students should be able to:- Demonstrate that Earth is not homogenous.
- Explain how the internal structure of Earth (concentric layers of different density and composition) is inferred through the analysis of seismic data.
- Explain the role models play in the scientific process, especially when used in combination with observational data.
- Explain how models are refined through the collection of additional data.
Prior Knowledge:
Students should know what earthquakes are, understand causes of earthquakes, that earthquakes are a source of seismic waves, that seismic waves propagate outwards in all directions, how seismic waves a recorded, and information a seismogram conveys.
Welcome to the Layered Earth lab! In this lab you will explore Earth's interior! upon completion of the lab you will receive a personalized certificate of completion, and a printout of all your responses to the questions asked throughout the lab
What is inside of our Earth?
Exploring how we know, what we know, about Earth’s interior structure
What is your name?
Brainstorm #1 from video: What is beneath your feet?
Brainstorm #2 from video: What is deep in Earth below the dirt and foundations of buildings?
Brainstorm #3 What evidence do you have for what is deep inside of the Earth?
Which of the following may be true about Earth’s interior based on the direct evidence discussed in the videos above?
Seismologists
Collecting and analyzing when seismic waves arrive around the Earth
Background: The simplest solution to the question: “What is beneath our feet” is a homogeneous Earth, or one comprised entirely of the rock we see at the surface. Since seismic waves travel through Earth, they make a useful tool to “probe” the inside of Earth to discover what might actually be inside.
Hypothesis: The Earth is comprised entirely of rock similar to what we see at the surface.
Task: Your task is to help test this hypothesis by analyzing a set of seismograms from a single earthquake to determine how long it actually takes for the seismic waves released from an earthquake to arrive at various points on Earth’s surface.
Step 1: Obtain a seismic data from a recent earthquake
On the map below, please select an earthquake to use for this exploration of Earth’s interior. Choose one of the earthquakes shown, or selected the “Plot Latest” button to use the latest earthquake recorded.
Step 2: Exploring your data
As you can see above, your request returned many seismograms. This plot is called a record section and it displays a collection of seismograms from the same earthquake, but each is the recording from a different seismic station. We will use the record section to answer two questions about each seismic station.
- How far away was each station from the epicenter of the earthquake?
- How long did it take the seismic energy from the earthquake arrive at each station?
On a record section, each seismogram is plotted according to the distance from the seismograph to the epicenter, on the x-axis. This distance, as measured by the geocentric angle , is provided in degrees, where 1 degree is ~111km on Earth surface. The time since the earthquake is shown on the y-axis and is displayed in seconds.
Use the information on the record section to answer the following questions.
What is the magnitude of the event?
Step 3: Analyze the record section
- Click “Show Record Section” below to reveal the record section you selected previously.
- Starting with the first seismogram on the left, move your cursor up or down the seismogram until it is over the point the seismic energy first arrived. An information bubble will display the travel time, or the time it took the seismic energy to travel from the epicenter to the station, and the number of degrees the station is away from the epicenter.
- Record the travel time and station distance in the appropriate windows below.
- Click “Add Data Point” to add this information into the data table and associated graph.
- Repeat these previous steps for each seismogram in the record section to complete the plot.
Repeat this process for each seismogram in the record section to complete the plot.
Data Table
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Step 4: Examine your data
What information did we collect from the record section?
Working as a Theoretician
Building a model to predict when seismic waves should arrive
Background:The simplest solution to the question: “What is beneath our feet” is a homogeneous Earth, or one comprised entirely of the rock we see at the surface. Since seismic waves travel through Earth, they make a useful tool to “probe” the inside of Earth to discover what might actually be inside.
Hypothesis: The Earth is comprised entirely of rock similar to what we see at the surface.
Task: Your task is to help test this hypothesis by creating a model of a homogeneous Earth, using the known velocity of seismic waves in rock ~ 11km/s. From this model you will predict how long it should take seismic waves to reach various distances around Earth.
Step 1: Building Your Model
We want to build a model that will determine how long it should take seismic waves to travel from an earthquake to various distances around Earth if it is comprised entirely of the rock we see at the surface.
Which of the following information should be included as a parameter in the construction of our model that will help answer this question? (Select all that apply)
Step 2: Using the model to predict travel times
Now that you have built your model let’s use it to determine how long it should take seismic waves to travel from an earthquake to various distances around Earth if it is homogeneous.
- Select a point on model’s surface by clicking on it to add a station. Note that you can adjust the position of a station you have added by clicking on it to select and then dragging it around.
- Once your station is added to the model, its position, in degrees from the epicenter, will be recorded in the table below.
- The distance of the travel path from the epicenter to the station (red line) will also be measured and reported in the data table.
- Use the "Calculate" buttons in the data table to convert the distance in the model to an Earth distance, and to calculate the time it would take seismic energy to travel this path assuming an 11km/s velocity. Your results will be plotted on the graph.
- Repeat this process for a total of at least 10 stations, that are evenly distributed across the surface of the model.
Scale Model of One Quarter of EarthClick on the Earth model to add at least 10 stations. Click and drag a station to reposition it. | |
 Seismic stations Selected seismic stations Earthquake Path seismic waves travel | |
Measure at least 10 more stations to proceed.
Distance Conversion Explained:
The Earth has a diameter of 12,742km, while our scale model has a diameter of 450px*. This gives us an Earth to model ratio of 12,742km:450px. We can use this ratio to convert the distances seismic waves would travel in our model to distances in the Earth, where x = Earth distance and m = the distance we measure on the model in px.
12,742km:450px = x:0.0px
x = 12,742km × (0.0px / 450px)
In the above step, units of px and px are canceled and we are left with km.
x = 12,742km × 0
x = 0km
*Note that the abbreviation px stands for pixels which is a basic unit for a computer image. For example, an image might be 238 pixels wide and 671 pixels tall.
Travel Time Calculation Explained:
To predict how long (travel time) it should take the seismic waves to arrive at a given point, if the Earth is made entirely of rock, we need to preform a simple calculation. We will divide the scaled distance to each station, by the velocity we are assuming the waves are traveling in our model. In this case, that velocity is 11km/s. This will give us a travel time measured in seconds (s).
Travel time = Distance / Velocity
In our case, or Distance is 0km and our velocity is 11km/s
Travel Time = 0km / 11km/s
Travel Time = 0s
| Station Location Δ [°] | Model Distance [px] | Earth Distance [km] | Predicted Travel Time [s] | |
|---|---|---|---|---|
What would the Earth distance be if the distance we measured on the model was 265px?
Compare Observed & Modeled Data
Background: To test our hypothesis, described below, you have already collected observations of when seismic waves arrive at stations around the earth as a seismologist. You have also worked as a Theoretician and used a model to predict when seismic waves should arrive at various points around the Earth.
Hypothesis: The Earth is comprised entirely of rock similar to what we see at the surface
Task: Your task is to compare the model data and the observed data to see if they match. If the two data sets match, then we can conclude that Earth is homogeneous or all rock throughout. However, if your observations do not match the seismologists’ findings, than we can reasonably assume that the Earth is not homogenous or made entirely of rock and will need to develop a new model.
Step 1: Plot both data sets on the same graph
Use the button below to plot the travel times you observed from the seismograms above.
Then plot the predicted travel times that you generated from your model
Step 2: Compare the data
How does our model data match the observed data?
What does your answer above imply about our hypothesis that Earth’s interior might consist of a homogenous material (e.g. rock) with a constant velocity of 11 km/s?
Examine your observed data again. After what distance, in degrees, do you see the noticeable jump in the arrival of seismic waves where they suddenly arrive much later than expected?
Step 3: Interpret the Results
What might be causing the seismic waves to arrive late at this distance and beyond? One possibility is that something inside of the Earth is blocking the direct seismic waves, but some seismic waves still arrive just later than expected. This is similar to the way a tree blocks the direct light from the sun, but some light still falls behind the tree after bending around it and reflecting off other things.
We can test this idea by using another scale model Earth. This time we will plot the regions where the seismic waves arrive “on time” and “late” relative to one another.
Scale Model of One Half of Earth
Click anywhere on the sphere to add earthquake to that location.
Drag orange circle to measure the length.
Do we have enough information to define the shape of what might be blocking the seismic waves?
Add at least 20 additional earthquakes distributed around the model to further constrain the object we see.
As you add additional earthquakes what shape is being defined as you add data from multiple earthquakes
Since we used a scale model, we can measure the size of the core you have discovered from the seismic data. To measure the size of the core turn “on” the ruler which will appear in the center of the model above. Drag the orange circle to measure the radius of the core (e.g. from Earth’s center to the edge of the circle you have defined with seismic waves. Once completed… compare your finding to the accepted value of Earth’s core.
What is the radius of object you just measured in Earth?
Congratulations !
You have used seismic waves from the M earthquake that occurred on near to discover and measure Earth’s core!
You observed changes in seismic wave propagation at ~° from the earthquake. Using this you calculated that the radius for the core was km.The currently accepted radius of Earth's core is 3486km. That means that your response was within % of this!
Discovery of Earth’s Core
Although Earth’s core had been previously inferred from the Earth’s gravity, Irish geologist Richard Oldham, provided the first direct evidence that the Earth had a central core in 1906. Using a process similar to the lab you just completed, he examined the arrivals of the P waves from a number of earthquakes that occurred in different locations on Earth. Oldham saw a change in seismic arrivals at ~120° and concluded that the radius of the core was 40% of the radius of Earth… or ~2548km. While this measurement differs from what you calculated (and todays accepted value)… you were using seismograms from seismic stations that are far more sensitive than those available when Oldham was alive.
Earthquakes create seismic waves that travel through the Earth. By analyzing these seismic waves, seismologists can explore the Earthʼs deep interior. On January 17, 1994 a magnitude 6.9 earthquake near Northridge, California released energy equivalent to almost 2 billion kilograms of high explosive. The earthquake killed 51 people, caused over $20 billion in damage, and raised the Santa Susana Mountains north of Los Angeles by 70 centimeters. It also created seismic waves that ricocheted throughout the Earthʼs interior and were recorded at geophysical observatories around the world.
The paths of some of those seismic waves and the ground motion that they caused are shown below. On the right, the horizontal traces of ground motion (seismograms recorded at various locations around the world) show the arrival of the different seismic waves. Although the seismic waves are generated together, they travel at different speeds. Shear waves (S waves), for example, travel through the Earth at approximately one-half the speed of compressional waves (P waves). Stations close to the earthquake record strong P, S and Surface waves in quick succession just after the earthquake occurred. Stations farther away record the arrival of these waves after a few minutes, and the times between the arrivals are greater. At about 100 degrees distance from the earthquake, the travel paths of the P and S waves start to touch the edge of the Earth's outer core. Beyond this distance, the first arriving wave — the P wave — decreases in size and then disappears. P waves that travel through the outer core are called the PKP waves. They start to appear beyond 140 degrees. The distance between 100 and 140 degrees is often referred to as the “shadow zone”. We do not see shear (S) waves passing through the outer core. Because liquids cannot be sheared, we infer that the outer core is molten. We do, however, see waves that travel through the outer core as P waves, and then transform into S waves as they go through the inner core. Because the inner core does transmit shear energy, we assume it is solid.
You used seismic waves to detect and measure Earth's core. What else do seismic waves tell us about Earth's core?
Download your certificate of completion and answer sheet below.
For successfully discovering Earths core!
You have used seismic waves from the M earthquake that occurred on near to discover and measure Earth’s core!
You observed changes in seismic wave propagation at ~° from the earthquake. Using this you calculated that the radius for the core was km.The currently accepted radius of Earth's core is 3486km. That means that your response was within % of this!