A seismograph is a scientific instrument which records information about the duration, intensity, and direction of a seismic disturbance, classically an earthquake. In addition to registering earthquakes, seismographs can also track large explosions, tidal waves, and other events which cause the ground to shake. These devices are used to gather more information about seismic disturbances, and to monitor the earth for early warning signs of seismic events.
Properly, a seismograph should really be known as a “seismometer,” or a “measurer of seismic events,” rather than a “writer of seismic events,” but the two terms are used interchangeably by many people. A true seismograph provides a scrolling printout that records seismic events with the use of a weighted pendulum that vibrates when the Earth does, causing wavering lines to appear on the paper. Modern seismometers can record information in other ways, however, with many operating remotely and beaming information back to a recording station.
The earliest version of the seismograph was developed in China around the second century. Various versions of the device were developed over the following centuries, with models similar to modern incarnations appearing around the 1880s. Later testing of some of these devices showed that they were about as accurate as modern seismic devices, although some were calibrated slightly differently.
In order for a seismograph to be effective, it must be designed in such a way that it only reflects true earth tremors, rather than surface disturbances. Seismographs are usually anchored to the bedrock to ensure that their readings are accurate, and they are often very heavy, with the weight acting to counter minor surface disturbances like passing walkers. Researchers also routinely calibrate their seismographs so that they can ensure that the readings are accurate.
Information from a seismograph can be used in all sorts of ways. After a seismic event, the device can be used to determine the source of the event and its duration, and that information can be cross-referenced with the amount of damage caused to learn more about how seismic events impact human life. By studying decades of seismograph data, researchers can also learn more about the geologic composition of an area, and the processes involved in seismic disturbances. Many intelligence agencies also use seismographs, as they can be used to monitor the testing of artillery and other military explosives.