On the Complication of Earthquake Ground Motions

Abstract

With the aim of fundamental understanding about subsurface ground structure and earthquake ground motions, the results of seismic prospecting using explosion and the effects of deep underground structure on earthquake ground motions are discussed in the Kanto district. The Kanto district, which includes Tokyo and Yokohama, is well-known as the most densely populated and developed area in Japan. Therefore, a methodology to estimate the characteristics of strong ground motions during an expected earthquake is strongly required for engineering purposes.
In the research field of earthquake engineering, local effects on earthquake ground motions due to surface geology have mainly been discussed within Quaternary soft soil layers. It might be enough as far as problems of strong motions are discussed in short period less than one second. Actually, fundamental periods of major buildings and other structures were not so long, and resonance with predominant period by shallow Quaternary layers was much stressed. But today, many kinds of huge structures with longer natural period have been already planned and built. For example, fundamental periods of high-rise buildings distribute from 4 to 6 seconds, and those of bridges with long span or liquid in large oil tanks will reach more than ten seconds. The predominant period in such range is no longer expected nor evaluated by shallow Quaternary layers. And that is why recent earthquake engineering needs informations about the deeper ground structure.
On the other hand, research fields of geophysics and seismology have been treating the crustal structure of the Earth. The uppermost layer of the Earth's crust is considered corresponsing with pre-Tertiary basement rock. Therefore, there should be an interdisciplinary interest between Quaternary soil and pre-Tertiary rock. And it has been just a big problem in this decade.
Through many explosions which have been carried out in this attended area, the analyses by refraction method are being performed. As results at this moment, pre-Tertiary basement rock can be found in the whole part of this area, where P-wave velocity is about 5.5 km/s. Tertiary deposit is composed of two kinds of sedimentary layers having 1.8 km/s and 2.8 km/s as P-wave velocity, And it covers the basement with 2 to 3 km in thickness. An intermediate layer with P-wave velocity of 4.8 km/s can be seen only in the southwestern half of this area. Moreover, the existence of a vertical discontinuity is pointed out at the interface between the intermediate layer and the basement. It should be noticed that the discontinuity is just located on the south-eastern extension from the Tachikawa active fault. So it may be concluded that deep underground structure in this area is quite complicated and it has possibility to give much influence on earthquake ground motions.
Therefore in parallel to mentioned above, characteristics of earthquake ground motions are studied by means of simultaneous observation of seismograms with network stations in the same area. In this study, explosions are also regarded as a kind of earthquake with exact source. And the main discussion is concentrated upon the propagation mechanism of seismic waves in the Tertiary deposit to find out reasons why earthquake ground motions are so complicated. Most of all, significant later phases in the seismograms are discussed using deep underground structure obtained from the seismic prospecting.
By synthesizing these results, it will be concluded that the major part of complication of earthquake ground motions in this area is due to the existence of thick Tertiary deposit. And the effects of deep underground structure on earthquake ground motions are inevitable. Therefore, the authors would like to stress the importance to discuss a problem about seismic wave propagation in wide and deep underground structure.