When an earthquake occurs, seismic waves spread out in all directions from the focus, and produce relative motions between the particles which make up the Earth. The effect may be investigated by recording the particle motion at a point, by recording the relative motion or strain between a pair of points, or by recording a component of stress, typically by means of a pressure-sensitive hydrophore. Any instrument used for the detection of earth motion may be called a seismometer, but the great majority of such instruments in seismological observatories are of the so-called 'pendulum' type. The seismometer must always be provided with a recording system which puts the instantaneous disturbance into permanent form.
The terminology now divides between instruments called 'seismoscopes' which produce only an indication that earth motion has occurred, and those in which the output of the seismometer is recorded as a continuous function of time. In this latter class the combination of seismometer, indicating system and recorder is called a seismograph. The record produced is called a seismogram.
Pendulum seismometers consist essentially of an inertial mass suspended on elastic members within a rigid framework. When the frame is disturbed by the passage of an earthquake wave, the inertia of the mass reacts against the forces transmitted through the suspension, and a relative motion occurs between the mass and the frame. These relative motions are detected and magnified by mechanical, mechanical-optical, electromagnetic or electronic methods.
Of the mechanical and mechanical-optical systems, the Wiechert, Mainka, Milne-Shaw, SMR-II and the WoodAnderson seismometers remain in fairly common use, the last-mentioned having provided the standard for the definition of the Richter scale of local earthquake magnitude: It is, however, hard to develop such systems to yield high magnification, and modern applications of the principle are restricted mainly to strong-motion studies.
'Strong-motion seismographs', which are used for recording earth movements of destructive intensity, are usually of the mechanical type. These instruments are designed to record highfrequency motion with very low magnification, and the recording system is designed to remain inoperative until triggered by an earthquake. 'Seismoscopes' are simple pendulums free to deflect in all horizontal directions, and to record on a fixed circular plate. Both of these classes of instrument are in the province of the earthquake engineer rather than the observatory seismologist, and will not be discussed further in this manual.
Electromagnetic transducers have the property that their output is proportional to the relative velocity of the elements rather than to relative displacement, which means that the effect of long-term drifts in the suspension or tilts of the base is attenuated in relation to effects of shorter period. When velocity transducers are used with the most sensitive short-period galvanometers, magnifications in excess of 500 000 are obtainable, and with long-period or heavily damped systems, useful response can be sustained over periods of hundreds or even thousands of seconds. In view of the great range of properties, which can be imparted to the overall system by the judicious choice of periods and damping constants, it should not be imaged that seismograms produced with aid of velocity transducers bear any particularly close relationship to particle velocity in the ground. Indeed, many of the finest registrations of ground displacement are made by using velocity transducers as the coupling device between a pendulum and an overdamped galvanometer, as in the Soviet 'SK' and the French 'APX' seismographs.
The Committee for the Standardisation of Seismographs and Seismic Records recognised five main classes of galvanometric seismographs, and drew up a typical response curve for each class. The magnification with which instruments of any class can be operated depends largely on local noise conditions. There is a range of period and damping constant within each class, but the specifications are close enough to ensure that records drawn from any class will be similar in general appearance, and that quantitative comparisons of corresponding phases of the earth motion will be possible.
In the classification which follows, Tg and Ts are the periods of the galvanometer and seismometer respectively, alpha and beta are the damping constants, and alpha2 is the 'coupling coefficient' which will be further defined below.
Short-period instruments, having maximum sensitivity in the period range 0.1-1.0 second. Four subclasses are in fairly general use, and are specified as follows:
|Tg||0.2-0.3 s||0.75 s||0.5 s||0.4 s|
|Ts||1.0 s||1.0 s||1.5 s||1.6 s|
Of the four classes, A1 and A2 typify configurations based on Willmore and Benioff seismometers, A2 being the standard short-period combination of the United States World Wide Standardized Seismograph Network (WWSSN). A3 is the French APX system, and A4 is the Soviet SKM-3 in the configuration which yields maximum bandwidth. All of these systems are capable of yielding magnifications of several hundred thousand, and the actual magnification which can be employed is determined by local noise level.
In the majority of existing stations, the choice of instruments, and of their adjustment is dictated by the general practice of one of the major networks, but in stations which have no network affiliation the choice may be determined by the character of the noise at the site. The narrowest band-width is achieved by system A2, particularly if variable reluctance seismometers are used, and this is most appropriate on sites affected by high-frequency traffic noise, and by natural microseisms of 2-5 seconds period. When the high-frequency noise is very low, the superior high-frequency response of system A1 will be advantageous. Systems A3 and A4 are widely used on quiet mid-continental sites, where the superior low-frequency response can be used without introducing excessive microseismic disturbance into the record. See also Aranovich and others (1968) and sections STA 2.2 and 2.3.
Long-period seismographs, which yield high magnification for periods considerably in excess of 100 seconds.
The sub-classes B1 and B2 give the instrumental constants for the WWSSN and for the French network. B3 is the galvanometric section of the High Gain Long Period (HGLP) system (Pomeroy and others, 1969), which also includes filtering and displacement output.
|Tg||100 s||90 s||100 s|
|Ts||15 s||15 s||34 s|
Seismographs having approximately constant magnification over the range of period from 1 to 10 seconds as typified by the Soviet SK system:
* Standard in new equipment
Long-period Benioff and other combinations, characterized by approximately uniform response to earth velocity over a wide range of period.
In existing 3-component sets the vertical component often has a shorter period than the indicated value, and the horizontals have longer periods. A method of matching to the preferred value is given in Section 5.2.
Magnification curves for all the above instruments are given in Fig. 1.1. In classes B-E inclusive, usable magnification is usually limited by the level of 5-second microseisms and is typically in the range of 1000-3000 at 5 or 10 seconds period.
A number of purely mechanical systems are being maintained in observatory service (as distinct from the widespread modern application in strong-motion work) but their limited magnification restricts their utility to the recording of large or nearby earthquakes. The determination of their constants (see Sections 3.1 and 4.1) is inherently simpler than is the case of electromagnetic seismographs, and their response characteristics can be obtained by inserting the appropriate scaling factors in the curves of Fig. 3.1a, Fig. 3.1b, and Fig. 3.1c.
The above set of standard classes must now be extended to include the instrumentation of the Seismic Research Observatories (SROs). In these, the detecting system consists of a 3-component wide-band accelerometer, from which the main output is taken in digital form (Unitech, 1974). These, therefore, are not describable in terms of the parameters of classical electromagnetic systems.
The increasing use of electronic seismograph systems, and of magnetic tape recording, has not only extended the range of recording characteristics, but has introduced the possibility of making changes in the transfer characteristic between the original record and the final display.
Suggestions have been made for the introduction of coding systems which could provide the necessary additional flexibility needed to describe in an abbreviated form, the characteristic relevant to any particular record, but there is as yet no international agreement on this matter. Essential information for types of instruments not included in this section must therefore be provided, at least for the time being, in complete graphical or tabular form.
Date created: 1/7/97 Last modified: 9/9/97 Copyright © 1997, Global Seismological Services Maintained by: Eric Bergman email@example.com