# Unit 7-2: Locating an Earthquake. Seismographs Seismograph:  The instrument used to record earthquake waves.  There are different kinds of seismographs.

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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.

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.

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