Symposium on Microseisms Held at Arden House, Harriman, N.Y. 4-6 September 1952, Sponsored by the Office of Naval Research, and the Geophysical Research Directorate of the U.S. Air Force. (1953) / Chapter Skim
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Microseismic Period Spectra and Related Problems in the Scandinavian Area
Microseismic Period Spectra and Related Problems in the Scandinavian Area
Pages 56-73
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From page 56... ...
By periods we usually mean the periods corresponding to the maximum amplitudes. By constructing frequency curves of all periods existing at a certain time (period spectra)
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From page 57... ...
A too long false period may arise if a small quiet interval separates the two wave trains. Likewise too short false periods may occur where one wave train is replaced by another wave train without separation.
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From page 58... ...
As the main source of the microseisms in Scandinavia lies along the Norwegian coast, we understand that the shape of a frequency curve depends on the position of the station in relation to this coast. If the position is such that the coast length takes up a large distance interval, from the station, as the case is for Bergen, a wider and less regular spectrum is obtained.
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From page 60... ...
But the nature of the phenomenon is not clear: if it is mainly an actual increase of period with distance, or if it is only a more rapid extinction of the shorter periods. No decision seems to be possible only from a knowledge of the increase of T
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From page 62... ...
The total increase from B to H is in the mean about 1.5 sec. The results concerning the upper and lower limits of the period spectra strongly support the conclusion that the change of the spectrum with distance is due to a more rapid extinction of the shorter waves rather than to an actual increase of periods.
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From page 63... ...
5. A comparison of the upper and lower limits of the period spectra at the different stations clearly indicates that there is a greater extinction of the shorter waves, whereas there is no indication of an actual period increase.
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From page 64... ...
St. Louis University o In the brief space of a few pages Doctor Bath has assembled a surprisingly large volume of first hand observational data on the periods of microseisms recorded in the Scandinavian area.
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From page 65... ...
However, the Florissant periods are approximately the same as those listed by Thompson for the Palmer Land station in Antarctica and yet the probable source fronts were often very near to that station and sometimes over relatively shallow waters. Many seismologists will fail to see the cogency of the argument for a linear source at a given distance drawn from the variation of the plane of vibration of microseismic waves at single stations.
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From page 66... ...
ROMNEY Geotechnical Corporation at Troy Microseismic motions over a wide range of periods have been detected and reported in the literature; however, quantitative information on the ground amplitudes associated with the various periods is surprisingly difficult to find. Further, measurements describing the spectrum existing at a given locality and time are generally found to include only a narrow band of periods, usually of about one octave band width selected by the filter characteristics of conventional observatory seismographs.
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From page 67... ...
Both stations were in operation during the period from August 20 through August 23, at which time an intense hurricane was moving parallel to the Atlantic coast line between Cape Hatteras and a point south of Greenland. This storm produced a rapid rise in microseismic amplitudes at both stations, reaching a maximum on the early morning of the 21st, and decreasing to essentially the normal level by the 23rd.
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From page 68... ...
The instrumental setup consisted of a conventional vertical component electromagnetic seismometer whose output was amplified, played through a Krohn-Hite UltraLow Frequency Band-Pass Filter (model 330 Figure 2- Microseismic Spectrum for Pinewoods, 23 August 1952. Normal day.
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From page 69... ...
it was found necessary to further reduce the gain for longer periods; from this we infer that the ground amplitudes increase faster than the cube of the period. Calculated true ground amplitudes are shown on Figure 7, which shows an amplitude increase about proportional to the fourth power of the period in the range 0.5 <.
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From page 70... ...
Fourier analysis or autocorrelation methods may then be used to obtain true ground amplitudes over a wide frequency range. It may be observed that the short period Benioff seismograph when critically damped, has nearly the correct characteristics for the spectra discussed here, since the Benioff response decreases nearly as the cube of the period for periods longer than one half second.
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From page 71... ...
Father Macelwane had not examined short microseisms partly because they did not reach Florissant and partly because he had examined the most prominent groups. He was dealing with microseisms in the period range of GVfc to 814 seconds.
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From page 72... ...
Figures to the right of each line give pass band of filter, but not of combined seismometer-filter system. Large disturbance on third line due to near approach at automobile.
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From page 73... ...
Only relative ground motion is shown.
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