A strong-motion accelerograph array with telephone line interconnections

Abstract

A basic problem in the use of self-triggered strong-motion accelerographs has been the reduction of starting time in the interests of recording as much of the early portion of the record as possible. It has been suggested, for example, that a short memory should be built into the instrument, perhaps by means of a tape loop, so that the very beginning of the ground motion could be recovered. Such expedients have the difficulty of requiring a more complicated, and hence more expensive and perhaps less reliable device. The system described in Part I below solves this problem by interconnecting for common start several accelerographs distributed over a five -mile region. The interconnection was accomplished using commercial telephone lines. In this way, whichever instrument first receives a ground motion sufficient to start, will also start some of the other instruments a second or so before the strong ground motion arrives. It is believed that in this way the advantage of early starting can be achieved for some instruments without a significant increase in instrument complexity. Once the interconnection for common start had been made, it was decided to additionally provide a common timing signal to the several recorders so that propagation of seismic disturbances might be studied. Part II of this report describes circuitry which provides common timing and also disconnects recorders from the interconnecting circuit after 8-1/2 minutes of continuous operation. The latter provision prevents exhaustion of film and batteries in the event of a short circuit of the telephone wires. The circuitry for common timing and short circuit protection has not yet been installed on the five-mile circuit, but has been built and tested only in the laboratory, with resistances used to represent the telephone wires.