past century, scientists have developed several ways of measuring earthquake intensity. The
currently accepted method is the moment magnitude scale, which measures the total amount
of energy released by the earthquake. At this time, seismologists have not found a reliable
method for predicting earthquakes.
Measuring Magnitude
Aseismometeris a machine that records seismic waves. In the past, all seismometers were
seismographsbecause they produced a graph-like representation of the seismic waves they
received. The paper record is called aseismogram. Modern seismometers record ground
motions using electronic motions detectors. The data are then kept digitally on a computer.
Seismographs have a pen suspended from a stationary frame, while a drum of paper rotates
beneath it. The pen is weighted so that it is suspended and not attached to the ground.
The drum is attached to the ground. As the earth shakes in an earthquake, the pen remains
stationary but the drum moves beneath it. This creates the squiggly lines that make up a
seismogram (Figure7.39).
Figure 7.39: A seismograph that had recorded an earthquake is beginning to record another
earthquake, likely an aftershock. ( 16 )
Seismograms contain information on how strong an earthquake was, how long it lasted, and
how far away it was. The wiggly lines that are produced in a seismogram clearly show
the different arrival times of P- and S-waves (Figure7.40). As with words on a page, the
seismogram record goes from left to right. First, there is a flat line, where there was no
ground shaking. The first waves to be recorded by the seismogram are P-waves since they
are the fastest. S-waves come in next and are usually larger than P-waves. The surface