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The construction of a feedback seismograph station and an analysis of the Long Shot data from the Canadian seismograph stations Jensen, Oliver George

Abstract

A practical and versatile feedback seismograph station has been constructed. Using feedback techniques developed by R.D. Meldrum, the relative low frequency response and the damping ratio of a Willmore Mk. I seismometer have been significantly increased for use in a broadband, low frequency bandpass seismograph station. Within limits imposed by a very high ambient ground noise level at the University of British Columbia site, circuit noise and instrument amplifier characteristics, it is possible to vary the damping ratio and resonant period through modification of the feedback loop transfer function. The seismograph has been continuously operating since November 1965 and has recorded over 40 local tremors and distant earthquakes from as far away as the mid Indian Ocean. It has shown that it is a useful demonstration and research instrument. A pilot analysis of the Long Shot nuclear explosion data received by the Canadian seismic stations indicates a consistent compressional first arrival as expected from an impulsive explosion source. Significant travel time discrepancies are observed in the commencement of the P arrival which arrived up to 6 seconds early at all stations with the largest residuals at the most distant sites. A comparison earthquake in the Rat Islands area indicates a similar bias trend. The P arrival amplitudes appear to be anomalously low in the central B.C. area and high in eastern Canada. The effect is also evident in the unified magnitude determinations which are based on these amplitudes. The causes of the variations of magnitudes and the anomalously low amplitudes have not been explained. The average magnitude and standard deviation for all Canadian stations is shown to be 6.01 ±0.40 which agrees well with the world-wide average determination of 5.99 ± 0.52. Spectral investigations demonstrate that there are both common and individual characteristics among the ground amplitude spectra of the different stations. These characteristics have not been correlated to the explosion source mechanism or to geological structure although some causative suggestions have been made.

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