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Volume 25, Issue 1
Displaying 1-12 of 12 articles from this issue
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Mitsuo NOGOSHI1972 Volume 25 Issue 1 Pages 1-15
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe attenuation characteristics as the fundamental nature of microtremors are discussed at the present paper. The experimental field is Hakodate City in Hokkaido.
Recently, the attenuations of Rayleigh waves and Love waves were investigated. However, the attenuations of microtremors have not yet been investigated. As the first step, the present paper described the discussion on the attenuation characteristics of microtremors by means of energy of wave motion.
Energy ratio contours agree very well with the structure of the streets in Hakodate City. Namely, the higher level places of artificial disturbance ; factories and streets having larger traffic noises, show larger values of the energy ratios.
This fact agrees well with the propagation directions which were obtained by tripartite observations of microtremors.
Therefore, it is suggested that the source of microtremors may have a special frequency characteristics as a source function. Next, the attenuation of microtremors was obtained by calculating the distance-energy relation along an observation line. Fig. 15 and Fig. 16 show these results. Therefore, it is clearly found that both the vertical and the horizontal components of microtromors are attenuated as the invese first power of distance.
These attenuation characteristics of microtremors seem to indicate a nature of surface waves.
Attenuation coefficients were obtained as reasonable values.View full abstractDownload PDF (1614K) -
Kunihiko SHIMAZAKI1972 Volume 25 Issue 1 Pages 16-23
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSA statistical method of detecting unusual seismic activities has been developed. Stationary and random earthquake occurrence is assumed throughout the present study. The probability for earthquakes of which the number is larger (smaller) than m to occur during a time-interval S when n earthquakes occurred during a preceding interval T is calculated. It turns out that such a probability is expressed by a negative binomial distribution. When this probability is smaller than a given significance level, the occurrence of more (less) than m earthquakes during S can be recognized to be unusual.
The number of observed micro-earthquakes at the Shiraki Microearthquake Observatory, having the same S-P interval, is analysed. An unusual seismic activity is detected 20 days before the occurrence of the Mivoshi Earthauake Swarm of 1970.View full abstractDownload PDF (890K) -
Kunihiko SHIMAZAKI1972 Volume 25 Issue 1 Pages 24-32
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSOccurrence of destructive earthquakes in Tokyo area during historical time has been investigated by several authors. Possibility of 69-year as well as 36-year periodicity of earthquake occurrence was proposed on the basis of periodgram analyses. A basis of these hypotheses is Schuster's criterion which is applicable only to any one harmonic term obtained from a random sampling. Although the above periodicities are obtained by applying the criterion to the largest of the harmonic terms, the writer believes that, in order to test the largest, we must adopt Fisher's test.
A periodgram of a discrete time series of the earthquake frequency is computed for the Tokyo area. It is shown that the periodicity corresponding to its greatest peak is not statistically significant by Fisher's test at significance level 0.1.View full abstractDownload PDF (1008K) -
Hisashi MIYOSHI, Kiyoshi MAKINO1972 Volume 25 Issue 1 Pages 33-43
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe lowlands of southwestern islands of Okinawa are dotted with many blocks of coral which were quarried and conveyed from the reef on the floor of the sea, by the giant tsunami of April 24, 1771. We investigated the distribution of blocks in Ishigaki Island. We chose the third biggest block in this island, the approximate weight of which is 750 tons. It lies some 2.5 kilometers away from the nearest coast line, and its location reaches some 30 meters above sea level. We pierced a tunnel through the base of this huge block, which is the hardest to be imagined to have been conveyed by the tsunami, and demonstrated that this block takes no root. In the opinion of some persons, these blocks are the negro heads of the erosion type. But our tunnel crushed down this opposition. One is apt to feel that this capacity of the tsunami is rather fantastic in view of wave dynamics. But, for example, the giant wave observed in Lituya Bay (Alaska) in 1958, the energy of which was ascertained accurately, could have conveyed the supposed coral pillar whose cross section is some 27.3mm2, to the same position of the above-mentioned block, losing only 0.73% of its wave energy.
Then we considered the action of the tsunami of 1771 upon the tied-island called Funakuyâ in Ishigaki Island. We confirmed that it is traditionally said that the northern part of this island has never been isolated from the main part of the island.View full abstractDownload PDF (1730K) -
Kumizi IIDA, Kazuo SAKABE1972 Volume 25 Issue 1 Pages 44-55
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe western extension of the Fuk6zu fault associated with the Mikawa earthquake in 1945 was investigated by the present geological survey. It was ascertained that the extension of the fault has begun near the pass between the villages, Kiriyama and Miyabasama and has been followed as far as Shikoyamachi, Nishio city, a total distance of about 9km. As the result of this survey a total length of the earthquake fault formed by the Mikawa earthquake in 1945 became about 28km as shown in Fig. 3. The fault is of a reverse type, and it has manifested itself as a northeastward upthrusting of the land on the southwest side of the fault amounting up to about 1.8m.
The fault also may presumably continue to the northwest and further to the Nobi earthquake fault which was formed in 1891, based on the seismological standpoint. It is concluded that a number of destructive earthquakes in the past might occur in connection with a chain of faults such as the Fukui, the Nobi, and the Mikawa earthquake fault.View full abstractDownload PDF (4251K) -
Mitsuru YOSHIDA, Katsuhiko ISHIDA1972 Volume 25 Issue 1 Pages 56-66
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSThe spectral analysis of non-stationary processes based on the concept of “evolutionary spectra” was mathematically developed by M.B. PRIESTLEY. In his study the local energy distribution of waves is analized. Then the modulating function must be determined from an original wave.
The present paper shows one method of making the modulating function and presents the reasult of statistical test of semi-stationary and non-stationary processes of two earthquake accelorograms. In this analysis the acceleaograms are treated as piecewise-seperable, and the frequency independent modulating function is estimated by the envelope of accelerograms.View full abstractDownload PDF (966K) -
Seismicity of Microearthquakes in Mikawa District.Tooru OOIDA, Isao YAMADA1972 Volume 25 Issue 1 Pages 67-74
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSIn the first paper under the same title, the seismic activity of microearthquakes in the vicinity of the Neo Valley fault was studied and the seismicity of this region was elucidated. However, in the region to the sourth of Inuyama seismological observatory few information had been obtained. In order to study the seismicity of this region, the observation of microearthquakes in Mikawa district was carried out in 1969 for 75 days from June 25 to August 31. Four temporary observation stations which were installed in this region, and all the stations of Inuyama seismological observatory were used in this study, During this observation, about 600 earthquakes were recorded and about 120 hypocenters could be determined. The magnitudes of these earthquakes were less than 2.
One of the results deduced from this observation was that microearthquakes occurred frequently at depths between 30km and 35km, and shallow earthquakes were very few in this region.View full abstractDownload PDF (807K) -
Akira IKAMI, Kumizi IIDA1972 Volume 25 Issue 1 Pages 75-82
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSIn this paper, seismicity of Japan was discussed on the basis of seismic energy released by earthquakes.
It was found that seismicity of the north-eastern Japan was higher than that of the south-western Japan, and the high seismic activity in the former was deeper than in the latter.
In Kyushu district, the high seismic activity was deeper in the central part than that in the northern and southern parts, where it changed alternately each other, In Nankaido disrict, the high seismic activity in the eastern part was deeper than that in the western one. In Chubu district, the high seismic activity was shallower than other districts, and migrated from north to south. In Kanto district, the high seismic activity was 30-40km deep. In south Tohoku district, the high seismic activity in the northern part was deeper than that in the southern part, and seismicity of the eastern part was higher than that in the western part. Off Tokachi district, the high seismic activity was deeper than 30km. In east of Hokkaido district, the high seismic activity was shallower in the eastern part than in the western part and it migrated from west to east.View full abstractDownload PDF (1205K) -
Yoshinobu MOTOYA1972 Volume 25 Issue 1 Pages 83-91
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSUsing the data obtained at the Urakawa Seismological Observatory (KMU) and the Urakawa Weather Station (URA) of JMA, the seismicity near Urakawa, Hokkaido, was investigated during the period from July 1967 to December 1970. Earthquakes treated in this study are those having P-S interval at KMU less than 8.0 sec and magnitude 2.0 and larger.
The 1968 Tokachi-Oki earthquake of magnitude 7.9 occurred off the south coast of Hokkaido on May 16, 1968. The region investigated here is about 150km from the epicenter and does not overlap the aftershock area of this earthquake. However, the seismicity near Urakawa might be affected by its occurrence; the number of earthquakes remarkably increased and several earthquake sequences were observed since May, 1968. A change in magnitudefrequency relations before and after this great earthquake was also found, especially in the magnitude range M>3.5.View full abstractDownload PDF (935K) -
Shigeki HORIUCHI, Kinya EMURA, Tomowo HIRASAWA1972 Volume 25 Issue 1 Pages 92-104
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSReliability of the pressure and tension axes in double couple mechanisms of deep earhquakes determined from P-wave initial motions is estimated for each subdivided region of the typical seismic zones in and near Japan. Our method of estimating the reliability is explained as follows: Based on the known results that mechanisms of earthquakes occurring in a small region are so similar to one another that we may define a representative mechanism in the region, we first imagine a hypothetical earthquake of the mechanism located at a point in each of appropriately divided regions. We next assign data of P-wave initial motion directions expected theoretically for the representative mechanism to 112 standard stations of the J.M.A. Network. The data are now regarded as observational data at these 112 stations. We can calculate in reverse the focal mechanism of hypothetical earthquake by the use of these fictitious data, where the representative mechanism from which the data are originally computed, of course, satisfies the data completely. However, if the pressure and tension axes are allowed to move to any direction on a focal sphere as far as the number of inconsistent stations is less than a prescribed small number including even zero, we can calculate permissible variations in directions of the pressure and tension axes, leading to an estimation of the reliability.
The main results are summarized as follows: A wide variation in orientation is permissible for both the pressure and tension axes in the seismic zones beneath the Japan Sea near U.S.S.R. and beneath the Pacific Ocean south off the coast of the Honshu (the main island of Japan), where distributions in azimuth of stations are not sufficiently good. In these regions both the axes are able to move on the focal sphere up to an angular distance of 50 to 70 degrees (90 degrees in some cases) in a particular direction, even though the number of inconsistent stations is taken to be only one. Determinations of both the pressure and tension axes are quite stable in the seismic zones beneath the Japan Sea coast near the central part of Honshu, where the azimuthal distribution of stations is satisfactory.View full abstractDownload PDF (1510K) -
Ryôhei IKEGAMI1972 Volume 25 Issue 1 Pages 105-107
Published: July 30, 1972
Released on J-STAGE: March 09, 2010
JOURNAL FREE ACCESSDownload PDF (388K) -
1972 Volume 25 Issue 1 Pages 113
Published: 1972
Released on J-STAGE: February 18, 2011
JOURNAL FREE ACCESSDownload PDF (205K)
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