The 1983 Nihonkai-chubu earthquake occurred of Noshiro City, Akita prefecture, Japan on May 26,1983. Damage due to soil liquefaction was found at many sites in Noshiro City and areas heavily damaged by soil liquefaction were located between sand-dune and soft alluvium with a very shallow level of ground water. The results of investigations on the characteristics of microtremors around Noshiro City, which suffered great damage, were described in conjunction with S-wave velocities of ground surface, geological conditions(microtopography and sdbsurface geology) and the areas damaged by the 1983 Nihonkai-chub earthquake. The short-period microtremors were measured at sites in the above areas by 3-component seismometers with a natural period of 1sec. connected to amplifiers and data recorders in field, and were analysed by means of micro-computer in laboratory. The digitized time ⊿t of microtremors was taken equally at 0.02sec. considering the frequency range (1-25Hz) under investigation. Fourier spectra were calculated by means of FFT from 2048 digitized data of wave forms of microtremors. Overall frequencycharacteristics were flat between about 1.0 and 25Hz. Results of this investigation are as follows: 1) All spectra of the microtremors were classified into four types(A,B,C and D) for about 1.0 to 10 Hz. Type A has a single peak below 1,5 Hz in the spectra, type B has a single peak below 1.5 Hz and a peak from 2.0 to 3.0 Hz, type C has a peak from 2.0 to 3.0 Hz, and type D has plural peaks from 1.0 to 10Hz. Furthermore, a diferent classification type a and b, from the previouse one, type A,B,C,and D were examined from the point of view of soil liquefaction as a peculiar damage. The type a has peaks above 4 Hz in the spectra and the type b has no peak there. 2) Average amplitudes of microtremors are particularly small at terrace, and they are slightly larger at natural levee and point bar than at terrace. Type A of microtremor spectra is found mostly at terrace and type B tends to be found more at older alluvial soil. 3) Type a of microtremor spectra is measured mostly in areas damaged by soil liquefaction. The spectral peaks at frequencies higher than 4Hz are found to be related to large velocity contrast between layers. Therefore, microtremor measurements can be useful to estimate the damage due to soil liquefaction in earthquakes.
物理探査 41 (1), 47-63, 1988-01-01