An Earthquake sends both types of waves zinging through the Earth. At great distances the small, repaid vibrations cannot be felt, but the seismograph detects them.

In principle, the seismograph has the pendulum hanging like a gate from a gatepost. The post is tilted a little towards the pendulum so the weight will hang at rest in one position.

The post is mounted on bedrock in an underground vault. Facing the pendulum is a rotating drum covered with photographic paper.

The vault is dark, except for a narrow beam of light aimed at a mirror on the pendulum. The mirror reflects the beam to the drum.

As the drum turns, the beam makes a line on the photographic paper. Even when the bedrock vibrates, the pendulum hangs particularly motionless, so the light beam reflected by its mirror is steady.

But the drum vibrates with the rock, and this makes the line on the photographic paper zigzag. The wiggly line is a seismogram the portrait of an Earthquake.

Speed and Distance: The waves from a distant Earthquake arrive in groups. First come compression waves, the kind set off by striking the end of a steel rod.

Seismologists call them P-waves. ‘P’ stands for primary, meaning first to arrive. But it might as well mean ‘push’, since the vibration is a pushing motion.

These waves are very fast. They travel at speed of thousand 250 miles a minute. Later come vibrations called S-waves.

‘S’ stands for secondary, meaning second to arrive. But it might as well mean ‘shake’, since these waves have a shaking motion, like the ones set off by striking the side of a steel rod.

Seismologists have prepared a table showing how long the second group lags behind the first at various distances. Here is a sample of figures from the table:

Distance from Earthquake                                Time Lag between

Centre to Station                                         P-waves and S-waves

Suppose a seismogram shows a group of P-waves, then a group of S-waves that arrived 6 minutes and 30 seconds later.

The seismologist knows that this means: an Earthquake has occurred 3,000 miles away.

Recording Earthquake Waves