Seismographs and Seismograms

What Is a Seismograph?

Seismographs are instruments used to record and measure earthquakes. During an earthquake, vibrations initiated by the breakage of rock along the fault zone radiate outward from the point of fracture. Seismographs detect, amplify, and record these vibrations. The visual record produced by a seismograph is called a seismogram.

Seismograph diagram

How Do Seismographs Work?

The motion of the earth during an earthquake is measured in terms of its movement relative to some object that remains independent of the ground motion. In a seismograph this object consists of a mass suspended on springs within a case. The unit is called a seismometer. During an earthquake the mass remains still, while the case around it moves with the ground motion.

Most modern seismographs work electromagnetically. A large magnet is used for the mass and the outside case contains numerous windings of fine wire. Movements of the case relative to the magnet generate small electric signals in the wire coil. These signals are then amplified electronically and stored digitally on a computer or played out on a recording drum called a seismogram.

 
Sample Seismogram

One seismograph station, having three different pendulums sensitive to the north-south, east-west, and vertical motions of the ground, will record seismograms that allow scientists to estimate the distance, direction, Richter Magnitude, and type of faulting of the earthquake. Seismologists use networks of seismograph stations to determine the location of an earthquake, and better estimate its other parameters.

Modern Seismographs