What is an Epicenter?• A point on Earth’s surface where EQ

originates.

Focus: A point of an EQ within the Earth.

Always need three stations in order to find an epicenter

• Primary wave- travels phastest so it arrives at seismic stations phirst.

• Compressional wave• Pass through solids and liquids

(magma).

• Secondary wave- arrives at a seismic station second.

• Vibrates side-to-side.• Solids wave- only travels through

solids.• Shear wave

Notice the vertical axis

This represents TIME (in minutes!)

Let’s enlarge a small section

Between each minute, we notice that there are 3 segments.

If there are 60 seconds in a minute, each one of these segments must be ________seconds.

What would this time be?

Notice the horizontal axis

This represents DISTANCE (in thousands

of Kilometers!)

Let’s enlarge a small section

There are also two curves on this chart that display the way different earthquake waves behave.

This chart can be used for answering many different questions!

How long does it take an S-Wave to travel 5,000km?

The recording station tells us it took 6 minutes and 20 seconds for the P-Wave to reach them. How far away from the epicenter of the earthquake must they be?

Use the data from the recording stations:

• Station A: San Francisco, California

P-Wave arrival 3:02:20 S-Wave arrival 3:06:30

What is the time difference between P and S wave arrivals?

TAKE A PIECE OF PAPER, AND MARK OFF THE DIFFERENCE IN ARRIVAL TIME

2800Km

4:10

Use the data from the recording stations:

• Station B: Denver, Colorado

P-Wave arrival 3:01:40 S-Wave arrival 3:04:10

What is the time difference between P and S wave arrivals?

TAKE A PIECE OF PAPER, AND MARK OFF THE DIFFERENCE IN ARRIVAL TIME

1600Km

2:302:302:302:30

Use the data from the recording stations:

• Station C: Missoula, Montana

P-Wave arrival 3:01:20 S-Wave arrival 3:03:00

What is the time difference between P and S wave arrivals?

TAKE A PIECE OF PAPER, AND MARK OFF THE DIFFERENCE IN ARRIVAL TIME

1000Km

2:302:302:30

1:40

Recording BoardDifference in arrival times:

San Francisco: 4:102,800km

1,00

0

2,00

0

3,00

0

4,00

0

5,00

0

Slide your compass to the EXACT distance on the scale.

. .

.

Compare your answer to the historical record

Need a closer view to be sure? Go to http://earthquake.usgs.gov/regional/states.php and retrieve the seismicity map (like the one on the left) for the state containing your possible solution.

USGS

Based on this map, earthquakes have previously occurred in the region where our epicenter fell. Thus our answer seems likely

What if our epicenter had fallen here instead? Does this mean our solution is wrong or less likely? What could help us find out?

Credits: This file was originally created by Robert M. DeMarco of Marlboro Central High School

Edits have been added by:

Chris Visco - Sachem High School

Eric Cohen- Westhampton Beach High School

Michael Hubenthal - IRIS Consortium