An Effective Transducer for Observation of Strong Seismic Motion

Abstract

   A strong-motion seismograph with a wide frequency band and large dynamic range has been required these days, for example, in the current international programs to deploy strong-motion earthquake instrument arrays. The pendulum motion of the so-called servo-type accelerometer is examined to show that this type of accelerometer is of use as a transducer for the strong-motion seismograph. Comparative observation was performed between an ordinary high sensitive long-period transducer and low-cost commercial product JA-4 (a kind of servo-type accelerometer which covers 0-500 Hz and until±2000 Gal). When the surface wave from the Iran earthquake on September 16, 1978 (M_s: 7.4) was observed at Tokyo, quite similar seismograms were obtained because the recording system had been designed so that both frequency responses should become approximately equal. On the basis of the assumption that slight differences between the seismograms are due to a lack of resolution of JA-4, its minimum detectable signal is found to be less than 0.15 mGal^o-p in the low frequency band. Therefore, it can be said that this accelerometer is effective as a strong-motion seismograph.