Microtremors have been utilized in the earthquake engineering field as an effective tool to investigate underground structures from the ground surface measurements indirectly. In this paper, studies on measurements of microtremors are reviewed focusing on the engineering utilizations. Reviewing is carried out classifying the microtremors into two kinds of short and long period. Initially, short period microtremors are discussed. At the beginning of this century, the first measurement of short period microtremors was carried out, finding that microtremors reflect well ground vibrational characteristics during an earthquake. After that many studies continued to investigate the relation with the dynmamic characteristics of shallow underground structures. Among these studies Kanai and his colleagues' works are distinguished and along their results a standardized method for measurements and analyzing of microtremors is described here. Typical examples for practical applications are referred to including the Kanai's discrimination method for types of ground. Secondly, studies of long period microtremors are discussed which have been developed during the past 20 years in order to investigate deep underground structures up to several kilometers. These studies have been initiated and accelerated by the circumstances of the recent expansion of building height and width. Examples of the field measurements and their interpretation in earthquake engineering use are also introduced. In conclusion an applicability of microtremors to earthquake engineering field is summarized and further problems to be solved are pointed out.
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.
The average amplification characteristics of a sedimentary layer with a thickness of several kilometers in the Tokyo lowlands are estimated by using the SMAC's records obtained during the moderate earthquakes which occurred beneath the Kanto district. The average amplification characteristics are given by the average of ratios of the zero-damped velocity response spectra of surface strong-motion to those of bedrock motion.The zero-damped velocity response spectra of bedrock motion are given through the prediction models which were constructed by using data obtained by accelerometers installed at the bottoms of deep boreholes in the pre-Tertiary basement. The amplification factors obtaines at about 1Hz, are found to depend on soft alluvial layer ; high amplification factors as high as 20 are observed. The estimated response characteristics of the representative sedimentary layer in the Tokyo lowlands are explained by taking account of a frequency-dependent Q, i, e., Q = 50f (f =0.5-2 Hz) and Q=100 (f = 2-10Hz).
Dense instrument array observation systems by Public Works Research Institute are described. This kind of observation systems is recently introduced at several sites in earthquake-prone areas in the world, and is expected to give useful information to clarify characteristics of strong ground motion in detail. Preliminary results of the analysis on the recorded data are also introduced in this paper, these are principal axis of the ground motion, phase velocity, amplification and the ground strain.
The National Research Center for Disaster Prevention has deployed some strong-motion observation arrays in the Kanto district, in order to investigate the characteristics of seismic motions modulated by a sediment. Especially, we investigated the earthquake responses of the sediment with a thickness of several kilometers by means of down-hole arrays whose maximum depths are 600 meters or more below the top of the pre-Tertary basement. As observational results, we made sure two fundamental earthquake responses of the sediment-basement system. One is due to the multiple reflections of S-waves at the sediment-basement system. The sediment is treated as a horizontally layered structure. The other is due to a series of total reflections of S-waves both at the surface and at the upper boundary of dipping basement. The train of totally reflected SH pulses generates a pseudo-Love wave. More recently, we tried to separate the source spectrum, the path effect and the site amplification by using both the deep-borehole data and the surface array data. This paper shows the relation between the site amplifications and the geological conditions, estimated by using the data obtained during the E off Chiba Pref. Earthquake of Dec. 17, 1987 (M=6.7).
This paper studies how occupants behave and why they suffer casualty in an earthquake, based upon questionnaire, interview and inspection field survey data for recent large earthquakes in Japan. Major results obtained are as follows : 1) The most significant factor which governs their behavior is seismic intensity. 2) Personal characteristics and surrounding circumstances can explain multiplicity and behavioral patterns at and after an earthquake. 3) Losses of life as well as injuries are caused and amplified at home and nearby outdoors by the degradation of behavioral performance under seismic shakings, weak structural and non-structural elements, and environments with limited indoor and outdoor spaces.
The data of about 30, 000 borehole records centering around the Metropolitan area has been stored in the database. The data were produced at the fundamental survey for construction of various structures. Regional distribution of the ground properties such as thickness of soft ground, vibrational characteristics of ground, liquefaction potential of sand layer, etc. are grasped from the database. In addition, using the characteristics of earthquake motions in the basement and in the sedimentary layer estimated by the model constructed from the observed data at the network of strongmotions including three deep boreholes, regional distribution of the size of surface ground motions were estimated.