What are seismographs?

Around the globe, 1,000 or so seismology stations record about 1,000 earthquakes a day, most of them too slight to be felt by the average citizen. Their major instruments are batteries of seismographs, designed to report the assorted shock waves sent zinging through the planet. Seismographs come in different styles to cope with the various vibrations of different seismic waves.

One style is the pendulum seismograph. Its pendulum is a heavy weight, suspended to stay still while the rest of the equipment shudders. Another is the strain seismograph. Its two sturdy posts sway together and apart as vibrating waves pass by in the ground below. All seismo¬graphs are fitted with electric gear to record their sensitive changes on rolls of revolving graph paper or on magnetic tape.

You might suppose that such an earthquake station runs like clock¬work    until you grasp the complexity of seismic waves. These vibrating shock waves come in three major types with numerous variations. The first to arrive are primary or "P" waves. These roar forth from the quake, traveling through the rigid crust at perhaps 3'11 miles per second. At deeper levels, through more elastic materials, they speed up to perhaps 8; 1 miles per second. "P" waves cause back and forth vibrations in the direction in which they are traveling.

The next arrivals are secondary or "S" waves. These vibrate sideways as they go. Their speeds range from two miles per second to4'11 miles per second through more rigid materials. "S" waves cannot shiver through the earth's molten core, which is somewhat bigger than the planet Mars. The last arrivals are the "L" waves that dawdle through the crustal surface. Some of their leisurely waves are 500 miles long.

As a rule, a seismograph specializes in one type of seismic wave. Some report vertical and others horizontal displacements in the earth. Some record the "P" waves that penetrate the entire planet; others, the surface "L" waves and the "S" waves that curve down to the core and up again. The speeds and arrival times of the various shock waves give clues to the location and intensity of a major quake. But masses of data must be computed and compared to verify the facts.

Data from three separated stations is compared to pinpoint the epi¬center, where the earthquake occurred in the earth's crust. Masses of details from many seismographs must be analyzed to estimate the strength or intensity of an earthquake. This takes time. Measuring the changes that a major quake can make in the earth's crust takes even longer.