1/15/2013

1

Measuring Earthquakes

Seismograph

• An instrument that detects and records

waves produced by earthquakes

• Record different types of wave motion

• Up-and-down motion

• Side-to-side in a north-south direction

• Side-to-side in an east-west direction

Seismogram

• A record sheet is placed on a rotating drum and attached to a base

• A pen that rests on the drum is attached to a heavy weight

• When the bedrock shakes the drum shakes producing a zigzag pattern

• The distance the pen moves from the center line is related to the energy released in the earthquake

Interpreting a Seismogram

• P waves which travel faster than S

waves arrive at a seismograph station

first

• The farther a seismograph station is

from the epicenter the greater the

difference in the arrival times of the P

and S waves

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Locating an Epicenter

• Seismograms interpret the distance from that station and the earthquake’s epicenter

• Need at least three different stations to plot an epicenter’s location

• Use each station’s distance as the radius of a circle on a map and the point where all these circles meet is the epicenter location

Magnitude

• Is the measure of the amount of energy

released in an earthquake

• Richter scale measures the magnitude

of an earthquake

• An increase by one whole number on

this scale represents a 31-fold increase

in energy

Magnitude

• Richter scale measures the intensity of

ground movements

• Richter scale can not accurately

indicate the amount of energy released

in very large earthquakes

Magnitude

• Moment Magnitude is another scale used in measuring earthquakes

• This measures energy released at an earthquake’s source

• Not as easy to measure as the Richter magnitude it more accurately indicates the total energy of an erathquake