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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
1/15/2013
2
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