seismometer is defined as any instrument that measures the time dependence of displacement, velocity or acceleration of the ground; used to detect seismic waves caused by earthquakes, nuclear explosions, volcanic eruptions, etc., and in prospecting. The quantity under study is measured using a mass that provides sufficient inertia. If the displacement of the ground on which the instrument rests is sufficiently fast, the suspended mass will remain stationary and provide a fixed point in space against which to measure the motion of the ground.

Seismometers are usually combined with a timing device and a recording device to form a seismograph. The seismograph is the measuring instrument that is used to record seismic phenomena. It differs from the seismometer, an instrument that performs only the measurement and not the recording of the same. The seismograph consists of a series of elements that allow the graphical representation of the trend of the seismometric signal over time in the form of a seismogram (which is a graph representing the dependence of the quantity in question on time).

In its simplest form, it contains a horizontal bar, pivoted at one end and with a recording pen at the other. The bar, supported by a spring, bears a heavyweight. As the ground moves, the bar remains roughly stationary owing to the inertia of the weight, while the rest of the equipment moves. The pen traces the vibrations on a moving belt of paper. Most modern seismographs are based on the type developed by John Milne and his colleagues in the 1880s.

By analyzing the seismogram it is possible to know the magnitude, the nature (with a single station only partially), and the distance of the earthquake from the point where the recording of the seismogram itself took place. The seismograph must therefore faithfully represent the movement of the ground or the magnitudes (acceleration or velocity) with which you can later extrapolate the absolute movement of the ground. The seismograph is very heavy in order to detect only the tremors.

Basic operating principles

Seismometers consist of a few basic elements:

  • A frame integral with the ground. In order to reduce the noise generated by anthropic causes (passage of cars, etc.) it is generally fixed at a certain depth in the ground.
  • A suspended inertial mass.
  • An attenuation system to prevent long-term oscillations following an event.
  • A mechanism for recording ground motion relative to the mass (considered fixed).
  • Modern seismometers also use methods of signal amplification by mechanical or electronic devices. However, these mechanisms do not equally amplify the signals at every frequency, but the amplification curve is poked at specific frequencies.

Modern seismographic instruments use analog and digital sensors, amplifiers, and recording instruments. Analog instruments are divided into three main groups: short-period instruments, with measured periods of less than 6 seconds (typically around 1 sec), long-period instruments that measure signals with periods longer than 6 s (with characteristic periods typically around 15 s), and wide-band instruments. The first two are characterized by a characteristic frequency, around which they can amplify the signal optimally, while parts of the signal with different frequencies are attenuated and cannot be measured accurately. Wide-band seismometers, on the other hand, use a feedback mechanism that consists of keeping the inertial mass stationary with sophisticated electromagnetic mechanisms. The forces that were required to hold the mass stationary are then measured and the ground motion is derived, thus avoiding saturating the measurements. Because the inelasticity of the ground tends to attenuate high frequencies more, short-period seismometers are used more in measurements of local earthquakes, while telesms are measured more efficiently by long-period seismometers. Recently, the distributed computing project Quake-catcher network has begun investigating the possibility of using low-cost accelerometers – such as those contained in computers – as low-sensitivity seismometers.

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