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Chapter 6, section 2
Seismograph →
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Seismic waves →
Recording Earthquakes
Seismograph- an instrument that detects and records seismic waves
Seismogram- chart made by a seismograph in response to a earthquake
Earthquakes produce three types of waves. Each waves travels at a
different speed and causes the Earth’s crust to move in different ways:
1) P waves (primary waves) Moves the fastest and are first to be recorded by a seismograph
 Moves through solids and liquids
 Moves by squeezing and stretching (like a slinky)
2) S waves (secondary waves) Moves more slowly and are second to be recorded by a seismograph
 Can only move through solids
 Moves up and down (like a rope)
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3) Surface waves (long waves or L waves) When P and S waves reach Earth’s surface their energy is
converted into surface waves
 Moves the slowest
 Only travels on Earth’s surface
 Causes the greatest damage
 Can move back and forth or up and down
Locating an
earthquake →
In order to determine the epicenter of an earthquake scientists need
a seismogram from at least 3 seismograph stations
1) Determine the elapsed time between the arrival of the P wave and the
arrival of the S wave
2) Use a ‘time-distance graph’ to determine the distance from the station
to the earthquake (P and S waves always travel at the same speed)
3) On a map, draw a circle around the epicenter. Radius= distance to the
epicenter
4) Repeat for the two other seismic stations
5) The point where the three circles meet is the location of the epicenter
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Measuring
earthquakes →
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1) Richter Scale- measures the magnitude of an earthquake (the amount
of energy released by an earthquake)
o Magnitude is determined by measuring the amplitude of the
largest wave
o Each whole number on the Richter scale is ten times greater
in wave amplitude than the number below (and releases about
32 times more energy)
2) Mercalli Scale- measures the intensity of an earthquake (the amount of
damage it causes)
o The scale ranges for I (felt by few people) to XII (total
destruction)
o Less scientific because the intensity changes depending on the
distance to the epicenter, building quality, and ground material
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