On Microtremors

AKAMATU Kei

doi:10.4294/zisin1948.9.1_21

In the observations of microtremors and explosion seismics ground noises cause difficulties in picking out the initial movements in seismograms. In order to examine the above problem and determine the characteristic frequency of microtremors of a given locality, it is necessary to investigate these microtremors. This paper discusses microtremors with frequencies between 20c/s and 1c/s which were observed at Hongo in Tokyo and Mt. Tukuba. Equipments used were horizontal seismometers of moving coil type with a frequency of 1c/s, vertical seismometers with frequencies of 3c/s and 10c/s, and two amplifiers one with a band of amplification between 1-30c/s and the other with a band between 30-100c/s. Tripartite observations were conducted and parallel observations were made with different frequency seismometers for comparison. The data were analyzed by the methods of autocorrelation and crosscorrelation. The results of the observations and analyses are: 1. At Hongo for tremors of 7c/s to 10c/s the daily variation of amplitudes from day to night showed a ratio of 17:1. At Tukuba for the frequency of 20c/s the ratio of amplitudes in the day time to those in the night was 2:1. 2. At Hongo upon observation of tremors with seismometers with frequency responses of 1c/s, and 3c/s and 10c/s after calculation of the power spectra, tremors with frequencies of 2-5c/s, 7-9c/s and 10c/s were found; at Tukuba using seismomoter with 3c/s response tremors with 20c/s frequency were found. 3. By using the data from tripartite observation time difference of wave arrivals between two points was obtained by the method of crosscorrelation to determine the direction of wave propagation and the phase velocity. 4. Space correlation between two points decreased more rapidly for shorter period waves than for longer ones.

On Microtremors

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