Seismograph or seismometer is called the sensitive instrument which detects the seismic waves generate by earthquakes.
Basic principles of seismographs working are quite simple:
- A weight, which is also known as “internal mass” , can move relative to the instrument frame and it is attached to the system
- A recording system of the movements of the frame.
The recording system used in the seismographs is usually optical levers or mechanical linkages to amplify the small motions of the earth, recording them on soot-covered paper or photographic paper. The most ultimate machines use electronic devices equipped with electronic sensors or amplifiers.
The most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection. Most systems record continuously, but some record only when a signal is detected, as shown by a short-term increase in the variation of the signal, compared to its long-term average (which can vary slowly because of changes in seismic noise).
Seismographs can’t be placed everywhere. The foundation is the most important and all professional seismographs are mounted on bedrock or boreholes the last one being considered the best mounting due to the fact that this avoids thermal effects, ground noises or tides of the weather.
When we refer to a seismograph, we must also talk about Intensity and Magnitude Scales.
- Magnitude measures the energy released at the source of the earthquake.
- Intensity measures the strength of shaking produced by the earthquake at a certain location.
Magnitude Scale was measured by Charles Richter, a seismologist at the Institute of Technology from California, in 1935 and he developed what we call now Richter Magnitude Scale. On his scale the magnitude is expressed in whole numbers and decimal fractions.
The intensity was measured for the first time by Michele de Rossi from Italy (1874) and Francois Forel from Switzerland (1881), who both independently published similar intensity scales. They both joined their works afterwards and produced the Rossi-Forel Scale in 1883. During the time, a lot of studies about scales of intensity were published; the only one used currently within US is Mercalli Intensity Scale, developed in 1931 by Harry Wood and Frank Neumann, an American seismologists. On Mercalli scale are 12 increasing levels of intensity of the earthquakes.