A seismogram is a record of the
seismic waves from an earthquake. A
seismograph or
seismometer is the measuring instrument that
creates the seismogram. Almost all seismometers are
based on the principle of inertia: a suspended mass
tends to remain still when the ground moves. The
relative motion between the suspended mass and the
ground will then be a measure of the ground’s
motion.
On a seismogram from an earthquake, the P-wave is the
first signal to arrive, followed by the slower
S-wave, then the surface waves. The arrival times of
the P- and S-waves at different seismographs are used
to determine the location of the earthquake. Given
that we know the relative speed of P- and S-waves,
the time difference between the arrivals of the P-
and S-waves determines the distance the earthquake is
from the seismograph.
