Bernice Coltman, age 15, of Oakland, CA for her question:
HOW DOES A SEISMOGRAPH WORK?
A seismograph is an instrument that records small movements of the ground. From these reports, scientists called seismologists can tell the exact location and size of earthquakes.
Seismographs can also be used to locate oil, to study the earth's interior and to find the thickness of glaciers.
A seismograph is made with a weight that is suspended from a frame by a delicate spring. The frame moves with the ground. But the weight, due to inertia, tends to stay stationary. The relative motion between the weight and the frame is magnified by using an electromagnetic transducer and an electronic amplifier.
The transducer, a coil attached to a weight, moves in the magnetic field created by a magnet attached to the frame. This movement indicates an electric voltage in the coil which passes to the amplifier.
The amplified voltage controls a recording device that marks the ground motion on a moving sheet of paper. The seismograph can detect ground motion as small as 39 ten billionths of an inch.
More simply stated, any movement of the earth causes the seismograph frame to move up and down, but the weight does not move. A writing device attached to the weight marks a record of the earth's motion on the recording instrument.
Different types of seismographs are used to measure short and long seismic waves. A Press Ewing seismograph records long waves at least 500 miles in length. The Benioff linear strain seismograph measures changes in the distance between two piers attached to the ground.
Apollo astronauts placed five special seismographs called seismometers on the moon. These instruments detect seismic waves caused by tiny moonquakes and send information about the waves back to earth by radio. Seismometers show that the moon has a thick, strong crust.
Seismographs are used in groups of three to measure separately three types of ground motion: up down, north south and east west.
There are more than 500 seismograph stations throughout the world.
The seismographs used for scientific studies and prospecting are tiny and rugged. Hundreds are placed around the site being studied. Then, the scientists set off explosives to create seismic waves, which travel to underground rock layers and are reflected.
The seismographs measure the reflected waves and indicate what lies beneath the surface.
Seismology, the study of seismic waves, has provided evidence that scientists believe supports the plate tectonics theory. According to this theory, the earth's crust is made up of about 20 plates that are in continual motion. The collision or separation of the plates creates volcanic islands, volcanoes and oceanic ridges and trenches.
The plate movements also cause earthquakes. Seismologists have shown that most major earthquakes occur along the edges of the plates.