Wilson Central School District

Earth Science Journal!
Finding the Epicenter

How can you tell how far away lightning is from you?

Similar to lightning and thunder, earthquakes create P and S-waves.

The earthquake happened at one specific time, but the P and S waves travel at different speeds, so they would hit locations away from the earthquake at different times, even though they both started out at the same time.

It is kind of like runners starting a race. Even though they start at the same time, the faster runner will travel farther and finish first. 
The greater the distance to the finish line, the greater the time difference between the fastest and slowest runners, even though they began the race at the same time.

When the Earth breaks, or quakes, waves of energy are sent out in all directions! These seismic waves can cause a tremendous amount of death and destruction.

• • Primary waves: P-waves , pressure, push-pull, travel fastest of the seismic waves and travel through all materials .
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• • Secondary waves: S-waves, shaking, shear, side-to-side, travel through solids only (no liquids or gasses)
    
    

Surface Waves: seismic waves that only travel on the earth's surface. Slowest of the seismic waves!

This is similar to Station #2 in your Regents Laboratory Performance Test!
You need a flat surface in order to draw circles with your safety compass.

Let's see if you can find the epicenter of this earthquake!
This is a seismogram (record of the earthquake) from San Francisco.

By finding the time difference between the arrival of the Primary waves (P-wave) and
Secondary waves (S-wave), we can determine how far San Francisco is from the earthquake epicenter.

Finding Epicenter Distance

First, always figure out what each grid line is worth.

What is each grid line worth?

Using scrap paper!
 
Mark the difference in time between the S and P waves on your scrap paper.
Don't use the corner of the paper!
It is much less precise!

Slide your scrap paper until the two marks align with the P and S wave curves.
Be sure to keep your paper vertical!

Read directly down from the edge of your scrap paper to find the epicenter distance!

How far is San Francisco from the Epicenter?

In your journal, write down in your own words how you use a piece of
scrap paper to find the distance to the epicenter.

How did you find the distance to the epicenter?

1.  Find the difference in the arrival times between the P and S waves.
2.  Use scrap paper and reference tables (page 11) to mark this time difference.
3.  Slide your scrap paper until your marks match the P and S wave lines.
4.  Read down to the x-axis to find the distance to the epicenter.

Now to map the distance to the Epicenter for this earthquake!
Use your drawing compass to make a 1600 km circle around San Francisco.
Please take care of our compasses.

Draw a circle around San Francisco at this distance!

Label the radius of the circle so you can double-check later.
The earthquake happened somewhere on or near this circle.
But we don't know in which direction from San Francisco.

For that, we need more circles!

Now for Salt Lake City

Be careful!

Always check the units.
Then double-check!

Notice how each mark is now worth 10 seconds?

Use your scrap paper and reference tables to find the distance to the Epicenter from Salt Lake City.
Draw a circle around Salt Lake City at your Epicenter distance.

These two circles should cross at two points.
One of these points is where the earthquake happened!

We need a third seismograph station to find which location the epicenter is!

Now for Houston!

Check the units!

Double-check your subtraction.
Use your scrap paper and reference tables to find the epicenter distance from Houston!
 

Draw your third circle from Houston at the distance you found; you should have found a location where all three circles come close together.

This is the Epicenter!

Label the epicenter!

Congratulations if you found the epicenter to be near Denver!

Earthquakes help us understand the Earth's interior!

Seismic waves give us a picture of the inside of the Earth, as a sonogram gives us a picture of a baby in the womb.

But the inside of the Earth looks more like this.

Constructive Response Regents Questions

Base your answer to the question on the diagram below, which shows two seismogram tracings, at stations A and B, for the same earthquake. The arrival times of the P-waves and S-waves are indicated on each tracing.

1. Explain how the seismographs at station A and station B indicate that station A is farther from the earthquake epicenter than station B.
Acceptable responses include, but are not limited to:
Station A is farther from the earthquake epicenter than station B because...
*1. The time difference between the P-wave and S-wave is greater at station A
*2. The  P-wave arrival time at station A is later than station B.
*3. The amplitudes of the earthquake waves are smaller at station A.
2. What is the distance to the epicenter for station A? 5,200 - 5,600 km
3. Explain why you could not accurately identify the epicenter of this earthquake.
You need at least three seismic stations to triangulate the position of the epicenter.

Finding the Origin Time of the Earthquake

Practice subtracting time!
Please write down these time subtractions.

Now, subtract these times and see what you get.
Remember, do NOT use a calculator!

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What is interesting about the answers?

We may come up with the same earthquake origin time during this activity, but due to statistical error, usually, that is not the case. You can check your answers against one another to see if the times are close, but don't change your data to make it look good.

You may end up losing points for not following the correct procedure.

When did this earthquake happen?
(Times are in 24-hour Military Time)
12:18:00 = 12:18:00 PM

The travel times must be found using your reference tables!

If San Francisco is 1600 km from the epicenter, it takes time for the earthquake waves (P + S) to travel that distance.
This is called the P or S wave, "travel time."

If you subtract the travel time from when that earthquake wave was felt, you have the time the earthquake happened (origin time of the earthquake).

Sometimes you need to borrow a minute or hour in order to subtract time.

Remember that when you borrow 1 minute, you must add 60 seconds to the number of seconds already there.

Now, let's try it using the S-wave times.

Does this make sense?

Again, usually, the numbers don't come out exactly the same due to the many places errors can occur.  Double-check your subtraction if your times are close, but don't change the data!

The earthquake happened at one specific time, but the P and S waves travel at different speeds, so they would hit locations away from the earthquake at different times, even though they both started out at the same time.

Try to find the origin time for all the cities.

Putting it all together!

Reflection: Explain how to find the epicenter of earthquakes.

Bonus
(Read this article and answer the questions in your journal)
Which travels faster, sound or a seismic wave?
http://www.planetseed.com/posted_faq/50809

Seismic waves can be a type of sound wave, so both travel through the same medium at the same speed. However, the speeds would be different if we compared sound waves traveling through the air with seismic waves traveling through the Earth. Seismic waves travel through the earth at a much higher speed than sound waves travel through the air.

Before going further, let’s clarify what sound and seismic waves mean in this context. Here, a sound wave would be a compression wave traveling through the air and created at the epicenter of an earthquake, which is the point on the earth's surface from which the earthquake seems to radiate. A seismic wave would be a disturbance traveling through the earth and created at the hypocenter or focus of an earthquake, which is the point within the earth where the slippage causing the earthquake occurs.

At first, it seems impossible to hear an earthquake before feeling it. After all, seismic waves travel much faster through the solid earth than sound travels through the air. However, hearing an earthquake before feeling it might be possible under certain circumstances.

From the focus of an earthquake, two different waves travel outward through the earth. One is called the P-wave, and the other is called the S-wave. The P-wave is the faster of the two. A seismograph on the earth's surface records the P-wave of an earthquake first, with the S-wave arriving several seconds later.

A person standing on the ground is generally unaware of the P-wave. However, the S-wave can create enough disturbance to be felt.

Moreover, it is possible for a P-wave to generate a sound in the air when it reaches the surface at the epicenter. S-waves cannot travel through the air.

Therefore, it may be possible for an earthquake at a focus several kilometers beneath the surface of the earth to generate a P-wave that travels to the surface and creates a sound that reaches a person's ear before the S-wave reaches the person's location.

In 1976, researchers found that a sound was recorded with a microphone that seemed to be related to the P-wave of an earthquake, while the shaking associated with the S-wave began about two seconds later.

This would be consistent with a person hearing a rumbling noise, followed shortly by the sensation of the ground moving. In such a case, this person could honestly say, "I heard it before it hit."

Now, this does not mean that sound waves travel faster through the air than seismic waves travel through the earth. P-waves and S-waves travel through the earth many times faster than sound travels through the air. Hearing a sound before feeling the earth shake means that the faster P-wave generated a sound that was heard by the person before the ground movement of the slower S-wave was felt by the person.

1. What is the "hypocenter" of an earthquake?
2. According to the article, what travels faster: the speed of sound in the air or earthquake P or S-waves traveling through the earth? 
3. How is it possible for a human to hear an earthquake before they feel the earthquake?

Seven Steps to Earthquake Safety
http://earthquakecountry.org/sevensteps/

When it comes to disaster, there are simple things you can do to make yourself safer. The information on this page is designed as a step-by-step guide to give details on what to do before, during, and after an earthquake. Start with the simple tips within each step to build on your accomplishments.

An example of this in Step 1 is moving heavy, unsecured objects from top shelves onto lower ones. This will only take minutes to complete, and you are safer from that hazard!

The information in the steps below will help you learn to prepare better to survive and recover wherever you live, work, or travel.

Before the next big earthquake, we recommend these four steps that will make you, your family, or your workplace better prepared to survive and recover quickly:

Prepare

Step 1: Secure your space by identifying hazards and securing moveable items.
Step 2: Plan to be safe by creating a disaster plan and deciding how you will communicate in an emergency.
Step 3: Organize disaster supplies in convenient locations.
Step 4: Minimize financial hardship by organizing important documents, strengthening your property, and considering insurance.

Survive and Recover

During the next big earthquake and immediately after, is when your level of preparedness will make a difference in how you and others survive and can respond to emergencies:

Step 5: Drop, Cover, and Hold On when the earth shakes.
Step 6: Improve safety after earthquakes by evacuating, helping the injured, and preventing further injuries or damage.

After the immediate threat of an earthquake has passed, your level of preparedness will determine your quality of life in the weeks and months that follow: