DEFINITION OF MEASURING EARTHQUAKES

How are the vibrations of an earthquake measured? Geologists

use seismographs, which display Earth movements by

means of an ink-filled stylus on a continuously turning roll of

graph paper. When the ground shakes, the needle wiggles and

leaves a characteristic zigzag line on the paper. Many seismograph

records clearly show the arrival of P and S body waves,

followed by the surface waves.

Seismographs are built using a few simple principles. To

measure the shaking of the Earth during a quake, the point of

reference must be free from shaking, ideally on a hovering

platform. However, since building perpetually hovering platforms

is impractical, engineers have designed an instrument

known as an inertial seismograph. These make use of the

principle of inertia, which is the resistance of a large mass to

sudden movement. When a heavy weight is hung from a string

or thin spring, the string can be shaken and the big heavy

weight will remain stationary. Using an inertial seismograph,

the ink-filled stylus is attached to the heavy weight, and

remains stationary during an earthquake. The continuously

turning graph paper is attached to the ground, and moves

back and forth during the quake, resulting in the zigzag trace

of the record of the earthquake motion on the graph paper.

Seismographs are used in series—some set up as pendulums

and some others as springs, to measure ground motion

in many directions. Engineers have made seismographs that

can record motions as small as 1 hundred-millionth of an

inch, about equivalent to being able to detect the ground

motion caused by a car driving by several blocks away. The

ground motions recorded by seismographs are very distinctive,

and geologists who study them have methods of

distinguishing between earthquakes produced along faults,

earthquake swarms associated with magma moving into volcanoes,

and even between explosions from different types of

construction and nuclear blasts. Interpreting seismograph

traces has therefore become an important aspect of nuclear

test ban treaty verification.

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