1999 Volume 51 Issue 5 Pages 12-18
What change does a seismic ground motion undergo under the influence of complex topological and geological conditions like mountainous areas? In order to make that clear through observations of actual seismic ground motions, we studied characteristics of seismic ground motions at the Shirasu (pyroclastic flow deposit) and granitic areas with aftershocks of the 1997 Kagoshima-ken Hokuseibu Earthquake. Furthermore, we tried to estimate characteristics of seismic ground motion from microtremor at each geological area.
At the top (STP) and foot (SFT) of the Shirasu terrace, about 33 m in difference of altitude and about 500 m away in a straight line, aftershocks were observed simultaneously. Consequently, while a seismic wave was passing through the Shirasu layer, remarkable changes appeared in the frequency characteristics of the horizontal seismic ground motion, i.e. amplification of vibration of 2.5 to 4.5 Hz and damping of vibration of 6 to 10 Hz. Moreover, as a result of the simultaneous observations of the aftershocks at STP and a granitic area (GTM), relatively long-period vibration of 2.5 to 6 Hz was predominating at STP the same as the above-mentioned, and on the other hand short-period vibration of 5.5 to 15 Hz was predominating at GTM.
Microtremors were observed at (1) the top (STP, SGM) of the Shirasu terrace of the Ito pyroclastic flow origin, (2) the granitic area (GTM, GKS) intruded in the Neocene, and (3) Shimanto group (MSB, MOS) of the Mesozoic. Predominant frequencies in spectral ratios were as follows : (1) 2 Hz, (2) 6 to 10 Hz, and (3) 15 to 20 Hz. The amplification characteristics of the surface ground were roughly presumed from the microtremors.
By means of the relation between the predominant frequency of the earthquakes and the nature frequency of the ground, we can probably explain features of the distribution of landslides, e.g. few landslides at the Shirasu area and many ones at the granitic area.