Seismographs

Seismographs generally consist of two parts, a sensor of ground motion which we call a seismometer, and a seismic recording system. Modern seismometers are sensitive electromechanical devices but the basic idea behind measuring ground movement can be illustrated using a simpler physical system that is actually quite similar to some of the earliest seismograph systems.

The physics behind the sensor is Newton's Law of Inertia:

"A body in motion tends to stay in motion unless acted upon by a force, and a body at rest tends to remain at rest unless acted upon by another force."

The idea is to have some "mass" that is at least partly decoupled (by the spring) from the ground so that when the ground moves, there is a differential motion between the mass and the earth. If you measure the differential motion and known something about the mass and the coupling system (the spring), you can recover the ground motion from the differential motion. The damping agent (fluid in container) keeps the spring from vibrating too long.

The sensor and the recorder do not have to be located next to one another. Seismometers are spread throughout the world, but are usually concentrated in regions of intense earthquake activity or research. These days, the recording system is invariably a computer, custom designed for seismic data collection and harsh weather. Often they are also connected to a satellite communication system. Such systems enable us to receive seismic signals from all over the world, soon after an earthquake.

 

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.