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Volume 45, Issue 1
Displaying 1-13 of 13 articles from this issue
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Shigeki TASAKA, Yoshimi SASAKI1992 Volume 45 Issue 1 Pages 1-9
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSWe developed a new radon observation system for 222Rn concentration determinations. A PIN photodiode (PD), an alpha particle detector, installed inside a container attracts radon families when charged electrostatically. Alpha particles emitted from 218Po(RaA), 214Po(RaC'), 210Po(RaF) and 212Po(ThC') are completely isolated from each other. This system can be applied to a long time period observation for wide variation of radon concentrations from 10Bq/m3 in indoor air to 10, 000Bq/m3 in underground water. The observed results of underground water radon concentration at a depth of 1, 000m in the Kamioka Mine showed that this technique is valid for earthquake prediction. A significant decrease in the radon concentration was observed about two and a half days before a magnitude 3.9 earthquake occurred on Atotsugawa fault about 49km from the observation station.View full abstractDownload PDF (883K) -
Data Base on Ground Survey and Its UtilizationSadaiku HATTORI1992 Volume 45 Issue 1 Pages 11-20
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe geotechnical data base in Tsukuba area was prepared and microzoning of ground amplification was made. Geological and geotechnical surveys had been carried out at boreholes of about 2600 sites. For more than 1500 boreholes, the following values were digitized; (1) depth of geological discontinuty, (2) N-value, (3) depth where N-value was measured, (4) class of soil, (5) age of soil, (6) coordinate of site and (7) level of site. In order to calculate the response spectrum of ground, the underground structure of S wave velocity (Ns) and density (ρ) are necessary for each site. It is possible, in general, to introduce empirical formulas proper to the area concerned, which give S wase velocity and density of the ground from geological data, through correlativity between results of seismic exploration and geological data. Unfortunately, there were not so many data of seismic exploration as to be able to create such formulas in Tsukuba area. In the present report, therefore, the formulas of Iida et al. were used. Response spectrum for each site was calculated by Haskell's method, when soil was treated as visco-elastic media of Voigt type. Analyses were made for almost all of segments in the area of about 21km in the latitude (35°58′45″ N-36° 10′00″N) and about 15km in the longitude (140°1′45″E-140°11′45″E), when the size of one segment was 380m in longitude and 460m in latitude. Regional distributions of three predominant periods in the period range from 0.1sec to 1.0sec and the corresponding peak amplitudes were obtained. The underground structure of P wave velocity in the depth of less than 400m was determined by seismic exploration which was carried out by the Geographical Survey Institute to investigate the ground condition in and around the borehole for earthquake observation. On the basis of S wave velocity, which was estimated from the above P wave data, response spectrum for the deep underground structure was obtained and predominant periods of 1.3sec and 2.6sec were recognized.View full abstractDownload PDF (4294K) -
Yoshihisa IIO1992 Volume 45 Issue 1 Pages 21-32
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe origin of high-frequency waves superposed on low-frequency waves was investigated using observed seismograms at hard rock sites. Two sets of data were used in this paper. One is the seismograms of main shocks and foreshock or aftershock sequences (M2.0-5.2) which occurred in the Kinki district, Japan. The other is the seismograms of the aftershocks of the Central Japan Sea earthquake (M5.3-7.1). For the earthquakes in the Kinki district, P-wave velocitywaveforms of a couple of events which occurred almost at the same location, were compared in the time domain. It was found that the high-frequency waves superposed on low-frequency waves generated by main shocks are very similar to the waveforms of their foreshocks or aftershocks. For the aftershocks of the Central Japan Sea earthquakes, the high frequency waves of every event had almost the same shape, even if its hypocenter is different from each other. Furthermore, their three component waveforms indicated that the high frequency waves did not arrived from the direction of their sources. It was concluded that almost all the high frequency waves analyzed in this paper reflect the site effect rather than the source process.View full abstractDownload PDF (1436K) -
Hiroyoshi SUZUKI1992 Volume 45 Issue 1 Pages 33-43
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe crustal stress in the Kanto plain and its vicinity is analyzed by using directional distribution of P- and T-axis of focal mechanisms of earthquakes within the earth's crust. The area is divided into six tectonic stress provinces which have different directions of P- and T-axis. In most cases, geological tectonic lines or active faults exist in the tectonic stress boundary zone which divides the tectonic stress provinces. This fact indicates that the stress distribution has close conection with the geological structure. The six tectonic provinces in due to the stress change at the weak zone of the crust. The stress change has its origin in the heterogeneity in the crustal strength. Average direction of P-axis in northern part of the Kanto area differs from that in the southern part, i. e., direction of the P-axis lie within NW-SE and N-S in the southern part and E-W in the northern part, which corresponds to the movements of Philippine Sea plate and Pacific plate, respectively. On the other hand, very complicated directional distribution of P-axis was seen in the northern part of Tokyo bay and the southwestern part of the Kanto plain. This stress state is interpreted in terms of increase of dip angle of subducting Philippine Sea plate just below the area.View full abstractDownload PDF (1332K) -
Kazuhiko ISHIHARA, Akio YOSHIDA1992 Volume 45 Issue 1 Pages 45-51
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThere are several gap zones in the intermediate-depth seismicity in the Philippine Sea slab beneath Kyushu. The focal depth of the deepest earthquake increases step-like toward the south, and dip direction and inclination of the slab change from segment to segment divided by the gaps. Further, seismicity around the depth of 80km, where the slab bends steeply downwards, varies with the segments. That is, the seismicity is very low for the northern segment, whereas fairly many earthquakes occur in the middle and the southern segments, although no earthquake with M≥4 was observed in the depth range deeper than 80km in the southern segment during the period 1926 through 1990. Sectionalization is seen in the seismic activities in the Hyuganada basin. Aftershock areas of large earthquakes in the Hyuganada basin are delineated by the boundaries of the segments. Moreover, a tendency of successive occurrence of large earthquakes, first in the northern section, then in the southern section, is clearly recognized.View full abstractDownload PDF (776K) -
Etsuzo SHIMA, Tokiharu OHTA1992 Volume 45 Issue 1 Pages 53-55
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
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Teruo YAMASHITA, Yoshimitsu OKADA1992 Volume 45 Issue 1 Pages 59-60
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
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Kiyoo MOGI1992 Volume 45 Issue 1 Pages 61-69
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe process of the occurrence of an earthquake is discussed from the mechanical point of view, and it is emphasized that the structural heterogeneity in and around the earthquake source region is one of the most important factors which cause the occurrence of precursory phenomena. Earthquake precursors are classified into the following two types. The one is the precursors which are caused by the increase in stress. In general, long-term precursors are those of this type. The gradual increase of earthquake swarm activity before the 1983 Japan Sea earthquake, the gradual increase in seismic activity in the surrounding region of the 1989 Loma Prieta earthquake before the main shock and the appearence of the seismic gaps of the second kind are discussed as examples of this type. The other is the precursors which are caused by a slowly progressive rupture process just before the sudden main fracture. Short-term precursors are mainly those of tis type. Foreshocks prior to the 1934 and 1966 Parkfield earthquakes are discussed as the typical examples of this type.View full abstractDownload PDF (988K) -
Akio YOSHIDA, Itsuo FURUYA1992 Volume 45 Issue 1 Pages 71-82
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSPattern of occurrence of precursory phenomena differs significantly in various respects for each earthquake. It is not the same even for earthquakes occurring in adjacent areas. Amplitude of precursory change is not necessarily large at nearer sites, and that it is very often larger than that of coseismic change. These features suggest that precursors occur locally at weak points or stress concentrating sites. It seems that desrimination of earthquake precursors from noises which are not directly related to the occurrence of earthquake is intrinsically difficult. Whether some observed change is a precursor or not may be expressed in a probabilistic way. When various kinds of anomalous change are observed at the same time, the possibility that they are true precursors will be increased. It is important to elucidate the occurrence mechanism of precursors in relation with structure and its change of seismogenic field.View full abstractDownload PDF (1238K) -
Tameshige TSUKUDA1992 Volume 45 Issue 1 Pages 83-96
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSeismic waves contain much information concerning to the physical properties of the medium in and around the source region of an impending large earthquake, where microcracks are believed to be extensively generated in the preparation stage of the event. Theoretical backgrounds and principles of seismic-wave methods for detecting temporal changes of the medium and their applications are reviewed with special attention to inherent problems and directions for future studies. The practical methods are clasified into the following categories: (1) Seismic wave velocity change; (2) Decay rate change of seismic coda waves; (3) Splitting of S waves; (4) Others including seismic spectra change, rectilinearity change of P-waves and so on.View full abstractDownload PDF (1898K) -
Tsuneo YAMAUCHI1992 Volume 45 Issue 1 Pages 97-105
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSCrustal movement observations for earthquake prediction research in and near fractured zones have attracted the attention of many researchers in recent years. Data obtained from fractured zones sometimes shows abnormal variations in relation to earthquake occurrence. Tectonic stress over a wide area tends to be concentrated in and near the fractured zone and anomalies were observed at sites situated far from the focal region. Tectonic stress over a wide area may provide more useful data than information from tectonic stress in a limited focal region. Data obtained at the same sites showed that anomalies preceding events sometimes resemble each other. It seemed that tectonic stress may show similar patterns in directly influencing such activities. One typical example of this was seen in data obtained at the Mikawa Crustal Movement Observatory (Aichi, Japan). Anomalies seemed to be related to the occurrence of nearby earthquakes within a specified region around the observatory.View full abstractDownload PDF (786K) -
Hideo YUKUTAKE1992 Volume 45 Issue 1 Pages 107-117
Published: March 24, 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSFracture nucleation in which instability occurs may be produced before earthquakes on the recent papers for fracture mechanics. The formation process of the fracture nucleation have been studied by the various models on physical and chemical bases, that is, slip weakening on a pre-existing fault, stress corrosion cracking and localization of microcracks. This paper suggests that a fracture nucleated region will be generated because of localization of microcracks as the differential stress in the crust is increased, in which ultimately dynamic rupture can be led due to sudden coalescence of cracks. We examined the precursor changes such as elastic wave velocities, attenuation, wave form, stress and strain in a seismic source region before earthquakes, on the basis of the fracture nucleation process which we have been investigated using an elastic wave tomograpy technique during triaxial compression tests. We also estimated the strength about 12km deep in the crust in order to evaluate a potential whether the fracture nucleation can be produced actually or not, considering the existence of water and pre-existing faults in the crust.View full abstractDownload PDF (2993K) -
1992 Volume 45 Issue 1 Pages 119
Published: 1992
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (35K)
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