Lesson 3: Recording Earthquake Waves
Recording Vibrations
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Seismic waves travel in concentric circles and lose their energy as they travel further
away from the origin
Seismometers can detect vibrations from the smallest earthquake
o When it is connected to a system that produces a permanent record, it is called a
seismograph
There are different kinds of seismographs that can tell many things:
o The general direction of the source of the earthquake
o The magnitude of the earthquake
o The distance to the earthquake source (measured by the time the waves are
registered on the seismogram)
Reading a Seismogram
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Seismograms are the recordings made by the seismograph.
o Originally recorded on paper
o Now recorded electronically and data transferred directly to computers
o Each mark on a seismogram is called a minute mark
o There are 30 minutes per line, and time is recorded in military time
The 1964 Alaska Earthquake in Prince William Sound had a magnitude 9.2, the largest in
US history
It is the length of time the ground shakes, not the violence of the shake,that causes the
most damage.
Aftershock is an earthquake that follows a large earthquake, or main shock, on the same
fault
Locating the Epicenter of an Earthquake
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Focus – the location within the earth where the earthquake rupture occurs and the
energy is released
o Most occur at less than 100 km into the earth (shallow focus earthquakes) and
are the most destructive
Epicenter – the point on the earth’s surface that is directly above the focus
Earthquake waves are strongest near the epicenter
Seismologists use a special time – distance graph and the data from three seismograph
stations to locate the epicenter of an earthquake