Morris Buchanan, age 12, of Clayton, Ind., for his questions
How does a 'seismograph work?
Seismology is the study of earthquakes and a seismograph is the instrument used to record the size, time and place of earthquakes. It works by responding to the smallest shakes and trembles in the earth's crust. The earthquake itself takes place maybe miles below the surface in the rocky layers of the earths crust. It is belibved that the earths crust is fitted around the surface in a series of blocks. Sometimes these crystal blocks move and rub against each other. This may be the cause of earthquakes.
When the quake occurs, the solid earth is shaken up in all directions. The shaken area shakes the next area and so on sometimes clear around the earth. At the same time the upheaval bangs down on the rocks below the crust, sometimes clear through the earth. These shakes and quakes produce vibrations far, far from the region in which the earthquake occurred. One set of vibrations travels around the crystal rocks which make the shell of the earth. Another set travels down, down towards the center of the earth.
The seismograph must be sensitive enough to feel and record these vibrations. Naturally, such a sensitive instrument would respond to every passing truck and this would make it useless for detecting earthquakes. For this reason, the instrument is fastened onto and into a firm and solid bed of rock or concrete. The instrument itself is a heavy weight hung from a support. When it is struck by vibrations from the ground, the weight tends to resist and, in so doing, it causes its support to vibrate.
Vibrations from an earthquake reach the seismograph from the horizontal and from the vertical.
In a seismograph center, there are usually three instruments. One records the vibrations from the north south horizontal, one records those from the east west horizontal and one records the up and down vibrations. All of them are able to record the size of the earthquake from the strength of the vibrations.
One station, however, cannot record the distance of the earthquake. They can tell the direction it came from but not how far. For this reason, earthquake stations all over the world keep in touch with one another. When data come in from two or three far distant seismographs, it is possible to angle in on the exact spot and maybe pinpoint it.
At one time, the vibrating support of the seismograph recorded its data onto a turning drum. In a more modern instrument, the vibration is made to generate an electromagnetic current which is measured in an instrument called a galvameter. It reflects a beam of light onto a photographic plate. This photographic method is far more accurate and sensitive than the old style instrument which wrote its recordings onto a revolving drum.
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