Unit 7-2:Locating an Earthquake

Seismographs

Seismograph: The instrument used to record earthquake waves. There are different kinds of seismographs

Due to the different types of earthquake waves. There are seismographs to record the horizontal

movement. There are seismographs to record the vertical

movement.

Seismographs

Seismograph construction: A heavy weight is attached to a base anchored in

the bedrock. The weight stays perfectly still (due to its

incredibly high amount of inertia) when the earth moves.

At the end of the weight is a pen. The pen is placed against a rotating cylinder,

called a seismogram.

Seismograph

Seismogram: The rotating drum is on the surface of the earth.

It moves up/down, back/forth when an earthquake occurs.

On the rotating drum, is placed a graduated sheet of paper. The pen records data on the paper. When the drum moves, the pen does not, so the change

in position is recorded.

Seismograph

Seismographs show three zigzags for each earthquake. One for P waves. One for S waves. One for L waves.

P waves arrive first, then S waves, and L waves arrive last.

Seismographs are also calibrated to record earthquakes within a certain range.

Determining the distance to the epicenter. How can a seismograph help determine the

location of an earthquake? By looking at the time difference!

P waves arrive earlier than S waves, so the larger the gap of time between their arrivals, the farther away the epicenter is.

By using the difference in time between P and S waves, we can very accurately determine how far away the epicenter is.

Determining the distance to the epicenter. We use a time-travel graph to determine the

distance. No, it doesn’t actually go back in time. Find the difference in time between the arrival of P

and S waves Go to the graph and find where that difference

lines up The distance from the epicenter is on the bottom

axis.

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are needed to see this picture.

Locating the Epicenter

Knowing the distance from the epicenter is great, but we don’t know the direction of the earthquake. To determine the epicenter, we need three

seismic stations’ data. By knowing the distance from each seismic

station, we can draw circles on a map and where all three cross is the epicenter.

Locating the Epicenter

How do we determine the depth of the focus? We look at the L waves. The longer the gap between L waves and S

waves, the deeper the focus is. Remember, the epicenter is on the earth’s surface

directly above the focus.