Analog analyses of seismograms recorded on magnetic tape

Abstract

Routine recording of long-period seismometer signals in analog form on magnetic tape makes it possible to perform a wide variety of analyses rapidly and efficiently. Examples of analog analyses of seismic waves from eight distant earthquakes, a large nuclear explosion, and the microseism storm associated with a hurricane illustrate some of the advantages of this type of recording. Inverse and selective filtering, with and without filter phase shift, are used to remove instrumental effects, improve signal-to-noise ratio, and emphasize desired phases. Fourier, energy, and power spectrums are obtained by several analog methods. Comparison of results of an analog Fourier analysis of the Palisades record of a large Soviet nuclear explosion near Novaya Zemlya in 1961 with digital analyses of two smaller 1958 explosions in the same area recorded at Palisades shows remarkable similarity in the spectrums. The only significant difference between the two sets of data is that the larger event is richer in long-period energy. Plots of frequency versus time and of cumulative signal, in different frequency bands, versus time present seismic data in a form more convenient for certain studies than the conventional record. Various combinations of the original three-component signals (N-S, E-W, and vertical) permit easy identification of seismic wave types by surface particle motion, improve the precision and reliability with which seismic arrivals can be identified, and aid in the detailed study of near-station effects on seismic signals. Results from the rotation of apparent instrumental coordinates to longitudinal horizontal, transverse horizontal, and vertical; multiplication of vertical and longitudinal components; and integration of this product serve as examples of the advantages of this type of data transformation. The analog magnetic tape seismograms can be digitized for any digital analyses desired, using a relatively simple, automatic, low-speed digitizer (two 3-decimal digit readings per second).