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Strong Motion Seismology 113 STRONG MOTION SEISMOLOGY D. E. Hudson* ABSTRACT Strong motion accelerographs and data processing systems for the measurement of strong ground motion of damaging earthquakes are described with some comments on future developments. Some information is given on the earthquakes and instrumentation sites for which important records have been obtained. Studies of the areal distribution of ground motions during earthquakes are discussed in terms of transmission path effects. Some speculations are advanced concerning the establishment of limiting values of earthquake ground motions. INTRODUCTION have been based on such data, and (4) To suggest some lines of development for In the pre-instrumental era of earthquake future investigations. engineering, much was learned about the effects of earthquakes upon structures by STRONG MOTION INSTRUMENTATION a careful study of the damage caused by large earthquakes, and in this way many Because instrumentation to measure of the basic principles of earthquake strong ground motion is at present in a engineering became known in a qualitative rapidly developing stage with many current way. A more exact study of such problems proposals for new types of instruments and became possible only when the actual motions new approaches to network design, it will of the ground during strong earthquakes be of importance to give some background were directly measured in the early 1930's. detail. To measure the destructive ground Since that time and particularly since the motions associated with damaging earthquakes, late 1960' s, a rapid growth in the deployment the engineer requires an insensitive of special instrumentation to record strong instrument which will remain on-scale during ground motions throughout the seismic regions the largest ground motions likely to be of the world has resulted in the accumulation encountered and which will faithfully of a large data bank of useful records. record over the whole frequency range of These records have provided the fundamental structural importance. This has been information for a new science of strong motion accomplished by the development of the seismology and for the development of strong motion accelerograph, which records increasingly accurate quantitative methods accelerations up to one g or over with a in earthquake engineering. Until very resolution of the order of 0.001 g and a recently in the history of seismology the frequency range of 0.06 Hz to 25 Hz. There attention of geophysicists has been directed are now some 5,000 instruments of this kind almost entirely to far-field phenomena distributed unevenly throughout the seismic involving measurements of very small ground regions of the world - approximately 1,500 motions at large distances from earthquake in the United States, 1, 000 in Japan, and sources. Development of the required several hundred each in New Zealand, instrumentation and the analysis of data Yugoslavia, Iran, Mexico, etc. New Zealand for damaging earthquake ground motions have has contributed in a major way to the been carried out by earthquake engineers. development of such instrumentation through Recently seismologists have become more the MO 2 accelerograph, which in the 19608s interested in such subjects as earthquake was the first relatively low-cost instrument source mechanisms which also require which could be readily installed in the measurements of strong ground motions near large numbers necessary for adequate coverage. the source, so a fruitful collaboration between seismologists and earthquake engineers Rough estimates of the expenses of in the problems of strong motion seismology maintaining networks indicate that the is beginning to develop. direct cost of each useful strong motion accelerogram is of the order of $10,000 , The purposes of the present survey are which suggests that a considerable data (1) To briefly describe the instruments processing effort is justified in the interest required for such investigations and the of acquiring the maximum amount of information networks and data processing facilities that from each record. The data processing system are presently available, (2) To summarize should be thought of as an integral part of the characteristics of the recorded earth• the whole accelerograph network and is in quakes and of the instrumentation sites and fact the element for which the most significant to evaluate the adequacy of the data base, improvements in information collection are (3) To outline some significant studies which likely to be realised. The most important advance in data processing made in recent * California Institute of Technology, Pasadena, years is the routine introduction of CA 91125; Visiting Erskine Fellow, Depart• digital filtering. This filtering technique ment of Civil Engineering f University of based on running weighted averages is Canterbury, Christchurch. particularly well suited to digital BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, VOL.10, NO. 3, SEPTEMBER 1977 114 computations and has made it possible to carry Recent advances in digital instrument• out accelerogram integrations and transducer ation and in integrated circuit technology corrections over a considerably wider would seem to make it feasible to produce frequency range than had been possible by a practicable digital field accelerograph, other techniques. Not only can new records although the cost of current prototypes be routinely processed to a higher order having essentially the same overall of accuracy, but it is feasible to reprocess specifications of accuracy, dynamic range some of the old photographic paper records and frequency response is about twice that dating back to the 1930's and to recover of the standard analog photographic acceler• considerably more information from them ograph . An additional disadvantage of than had originally been thought possible. currently available digital accelerographs is the somewhat larger standby power The existing field accelerographs requirements which reduces the non-external produce records in the form of an analog power operating life by a significant amount. photographic trace on a 3 5mm or 70mm film. An advantage of the digital system is the For detailed analysis of the information, possibility of providing a short memory to the analog record is digitized using a recover the earliest portion of the triggering semi-automatic machine which combines hand- ground motion. In view of the relatively eye setting of a cross hair on the analog quick action of modern vertical triggers, trace with automatic readout and recording the memory is probably justified only if of time and acceleration coordinates on the additional complexity and power require• punched cards or magnetic tape. Currently ment does not compromise the field reliability under development is a completely automatic of the accelerograph. Digital accelerographs type scanning digitizer which will require would also be somewhat more dependent on operator intervention only for those portions laboratory-based playback equipment, since of the record which involve ambiguities or it will usually be essential for preliminary defects. A critical advantage of the current inspection and analysis to have an analog optical-photographic analog recorder plus record. It is also likely that more automatic digitization is that it permits elaborate field test equipment will be the widely dispersed field elements to be required with a correspondingly higher of a very simple form while the more complex level of training of maintenance personnel. digitization apparatus can remain in the The long range potential of such digital laboratory. Thus the special advantages of systems, however, is so attractive that an both analog and digital systems are combined extended period of careful field testing in an optimum way for this particular and evaluation is certainly justifiable. application. The first practical applications of completely digital systems will probably be in large A considerable effort is now being special instrument arrays rather than in made to develop a digital field accelerograph the widely dispersed individual stations which would directly produce the basic record which have characterized strong motion on computer-compatible digital magnetic tape. studies in the past. This would of course eliminate the need for the relatively laborious semi-automatic The overall capabilities of any measure• digitization of the analog photographic ment system can be measured by the relation• traces, at a cost of a more complex ship between noise characteristics which installation in the.field. The desirability limit the ability of the system to produce of such digital field systems depends- upon low level analyzable records and the size the extent to which rapid digitization of of the signal to be measured. As an example records is believed to be important. In my of the current state of the art, the perform• opinion digitization requirements do not ance of the instrumentation and data process• represent a decisive disadvantage for the ing system used in the present United States film recording system for the following strong motion accelerograph network is reasons. First, a large amount of information summarized in Fig. 1. Since frequency of immediate practical importance is quickly analysis of records is a frequent end available on the analog record without product, the instrument signal level can digitization or further processing. be conveniently represented by the Fourier Practical decisions
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