3.3 Seismograph requirements for the registration of distant earthquakes
We will concern ourselves with the details of constructing a seismograph in chapter 4. For the registration of teleseisms in general a seismograph needs to fulfil the following requirements:
As in all seismographs the pendulum component is damped significantly, as otherwise it would - once disturbed - oscillate with its own frequency and therefore overshadow or falsify the incoming signals. If the pendulum is sufficiently damped it quickly returns to its initial position after each movement and is ready to receive new signals. So the damping functions similarly to the dashpot of a car: If the damped pendulum swings freely after an initial displacement the second 'swinging' would only amount to 1/23 of the initial displacement (this damping does not allow the pendulum to achieve displacement-resonance and the seismographic coil always remains within the magnetic field of the permanent magnet).
The pendulum must be very sensitive, so that it can perceive the weak signals of distant earthquakes.
During their journey through the earth, the earthquake waves are expanded in time and space. Earthquake signals that last about 10 seconds in the vicinity of the epicentre may excite a seismograph at a distance of 10,000km for an hour or more. Therefore the signals from distant quakes have a very large period of oscillation. This long period of oscillation, in our case being circa 20 seconds, can only be recorded with a seismograph that has a long eigenperiod (one therefore often speaks of 'long-periodical seismographs'). If one wanted to use a string-pendulum for a seismograph pendulum, the string would have to be extremely long: for a period of 20 seconds it would have to measure about 115 meters. How to realize such a long-period pendulum within a housing of reasonable size will be explained in chapter 4.