To measure the intensity of shaking during an earthquake, geologists use seismographs, which display earth movements by means of an ink-filled stylus on a continuously turning roll of graph paper. Modern seismographs have digital versions of the same design but record the data directly to computer systems for analysis. 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 surface waves.
seismographs are built according to 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. since building perpetually hovering platforms is impractical, engineers have designed an instrument known as an inertial seismograph, which makes use of the principle of inertia, 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, recording the zigzag trace of the earthquake motion on the graph paper.
seismographs are used in series, some set up as pendulums and others as springs, to measure ground motion in many directions. Engineers have made seismographs that can record motions as small as one hundred millionth of an inch, about equivalent to being able to detect the ground motion caused by a car 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. Many seismologists are employed to monitor earthquakes around the world and to verify that countries are not testing nuclear weapons.
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