Earthquake recorders are called seismographs or seismometers if they track the complete history of the complete history of the shaking throughout the earthquake. Seismographs are essential for accurately defining the location of remote earthquakes, measuring earthquake size, and determining the mechanism of the fault rupture.
The English engineering professor, John Milne, an early pioneer of seismographs surmised in 1883 that "it is not unlikely that every large earthquake might with proper appliances be recordered at any point of the globe". This prediction was fulfilled as early as 1889 when a German physicist E. Von Rebeur Pashwitz determined that an earthquake he rcorded in Germany was that of a damaging earthquake that had shaken Tokyo, Japan. The significance of this discovery can be seen as the earliest example of "remote sensing"- which is the surveillance of earthquakes in inhabited and uninhabited parts of the world which can be monitored uniformly.
Seismographs are constructed by freely suspending a mass from a frame attached to the ground; the mass is therefore reasonably independant of the frames motion. When the frame is shaken by earthquake waves, the inertia of the mass causes it to lag behind the motion of the frame. The classical seismograph used before the coming of digital technology required the relative motion cause by earthquakes to be recorded by pen and ink on paper wrapped around a rotating drum, or by a light spot on film, producing the familar record known as a seismogram.
During an earthquake, the ground moves simultaneously in three dimensions: for example, up-down, east-west, and north-south. A single seismograph records only one of these three components of motion. In most seismograph recordings, the relative motion between the mass and the frame is not the true motion of the ground. The actual motion must be calculated by taking into the account the physics of the pendulums motion.
Displacements of the rock in earthquakes vary in direction throughout the motion. From the three components of a seismograph (, up-down, east-west, and north-south), the complete wave motion as it evolves with time can be reconstructed. By combining the records of an earthquake by a series of seismographs, the seismologist can create a representation of the ground shaking by identifying the predominantly P (Primary) and S (Secondary or Surface) waves on an earthquake, enabling the total energy of the shaking to be calculated (Magnitude). Three component digital seismographs and becoming more commonly used.
In modern seismographs the relative motion between the pendulum and frame produces an electrical signal that is magnified electronically thousands and even hundreds and thousands of times before it is used to drive an electric stylus, or to be recorded to a computer hardrive producing the resulting seismogram. These electrical (voltage) signals produced by the responses of the inertial mass to the ground motion are passed through low-noise electronic circuits that act as filters. These filters are set to pass through only those waves in the frequency range of scientific interest.
Modern seismographs record the ground motion uninterruptedly, and they contain clock devices which continuously provide the time. It is possible to measure, with high fidelity, the wave lengths and amplitudes of the waves. Like astronomy which which depends on a variety of sophisticated optical and radio telescopes, a well equipped earthquake observatory has a wide variety of seismographs. Essentially these instruments are able to record, over a very wide spectrum of frequencies, the vertical and two horizontal components of the earthquake motion that arrives at the recording station from remote sources.
Traditionaly the activities of an earthquake observatory involved alot of daily routine. The photographic paper film or magnetic tapes had to be changed everyday, so as the previous days earthquakes could be anaylsed. With the invention of computer technologies advanced seismological observatories record seismic signals to computers as a regular series of discrete numerical data. Much of the daily drugery formerly required by the seismologist in traditional observatories has gone because of digital recordings.
To see realtime seismic recordings use this link Live Internet Seismic Server
Seismologists require skill in pattern recognition to pick out the P and S waves that have travelled along different paths inside the earth. They look for the sudden increase in amplitude, often associated with a change in wave frequency, that signals the arrival of a particular earthquake wave. The analysts then read the time of arrival of the firt onset of this wave as closely as possible, to the nearest tenth of a second. Often the amplitude and period of the wave are also read.
Digitally recorded earthquakes give an arithmetic sample of the continuous signal. The seismologist scans the recorded ground motion on a graphics screen and makes selections as they would on drum or photographic recordings. With digital recordings of earthquakes the seismologist can flag the arrivals of P and S waves according to preprogrammed rules. In addition a check can be done for the accompanying time code and print or record the arrival time and amplitude of each selected onset.
After recording the arrival time of every main wave, the seismologist identifies each selected wave according to its type and path and assigns to it a standard symbol. The entire process is much like that carried out by a cryptologist attemting to break a secret code.
The final step is to record the arrival times, amplitudes, periods, and identify these onsets and transmit them to a catalogue or directly to regional or international seismological centres.