”Seismographs” or ”Seismometers” are instruments, which detect and record earthquake waves. A seismograph contains a heavyweight suspended from a support, which is attached to bedrock. When seismic waves from a distant earthquake reach the instrument the inertia of the weight keeps it stationary, while the earth and support vibrate. The movement of the earth in relation to the stationary weight is recorded on a rotating drum.
Some seismographs detect horizontal motion while others detect vertical motion. The trace of the earthquake waves is usually recorded on a traveling paper as a series of zig-zag lines. These records are called ”seismogram”. From the seismograms, the time interval between the arrival of P and S-waves can be calculated and with the help of the travel-time graph, the distance between the recording station and the epicenter is determined.
Seismographs are usually used to determine:
- Magnitude: the size of the earthquake
- Depth: how deep the earthquake was
- Location: where the earthquake occurred
The difference between Seismograph, Seismometer, and Seismogram:
- Seismometer: Seismometer is the internal working part of the Seismograph, that is in the form of a pendulum or simply a mass hanging with a support. The pendulum has a marking nib beneath it, which marks the movement of shaking or trembling on a paper that is wrapped on a rotating drum. A seismometer is often used synonymously with “seismograph”, but a seismometer is merely a part of a seismograph.
- Seismographs: The word seismograph is used against the whole instruments used to record the motion of the ground during an earthquake. They are installed in the ground throughout the world and operated as part of a seismographic network. The earliest “seismoscope” was invented by the Chinese philosopher Chang Heng in A.D. 132. This did not, however, record earthquakes; it only indicated that an earthquake was occurring. The first seismograph was developed in 1890. A seismograph is securely mounted onto the surface of the earth so that when the earth shakes, the entire unit shakes with it EXCEPT for the mass on the spring, which has inertia and remains in the same place. As the seismograph shakes under the mass, the recording device on the mass records the relative motion between itself and the rest of the instrument, thus recording the ground motion. In reality, these mechanisms are no longer manual, but instead work by measuring electronic changes produced by the motion of the ground with respect to the mass.
- Seismogram: A seismogram is the recording of ground shaking in the form of zig-zag line on the specific location of the instrument. The horizontal axis on a seismogram shows the time in seconds, while the vertical axis expresses the ground displacement in millimeters. In case of no earthquake, the line drawn by the seismometer is quite straight with very minor wiggles, which are caused by the local noise and trembling. The modern seismographs record the shakings in digital form, thus there are no more paper recordings.