All issues
Volume 39, Issue 2
Displaying 1-14 of 14 articles from this issue
- |<
- <
- 1
- >
- >|
-
Kazushige OBARA, Akira HASEGAWA, Akio TAKAGI1986 Volume 39 Issue 2 Pages 201-215
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThree dimensional P and S waves velocity structure of the crust and the uppermost mantle beneath the northeastern Japan arc is investigated by an inversion of P and S waves arrival times observed at the seismic network of Tohoku University from local earthquakes.
The results show that, in the uppermost mantle, there exists a dipping high velocity zone of P and S waves which corresponds to the descending Pacific plate. Beneath active volcanoes low velocity zones of P and S waves are clearly seen, which are dipping westward and extend to a depth of about 100km.
Although the pattern of P wave velocity distribution is very similar to that of S wave velocity distribution, change in absolute value of S wave slowness perturbation is bigger than that of P wave slowness perturbation, indicating the spatial change in VP/VS value. Within the Pacific plate VP/VS values are generally low, whereas the low velocity zones existing beneath active volcanoes have high VP/VS values.View full abstractDownload PDF (1548K) -
Yasuhiro UMEDA, Akio KUROISO, Kiyoshi ITO, Yoshihisa IIO, Tatsuo SAEKI1986 Volume 39 Issue 2 Pages 217-228
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSMany boulders were thrown off out of their former sockets by the Western Nagano Prefecture Earthquake of September 14, 1984. Anomalous high accelerations of 5-30g, in the frequency range of 5-10Hz, are estimated from the displacement of these thrown out boulders.
Almost all of the thrown out boulders were found on the tops, ridges and suddles of the mountains covered with the volcanic ash. The amplification effects by topography and soil sediment of the surface are estimated from the observation of the aftershocks recorded on the mountain-top, -foot and rock. The spectral ratios of seismic waves with the frequencies of 5-10Hz, namely mountain-top/-foot and soil/rock, are 2-7 and 2-10, respectively.
The accelerations on the basement rock are obtained dividing the accelerations estimated on the mountain-tops. The high accelerations exceeding 1g distribute within a small area with a length of 3km and a width of 1km. This small area corresponds to the large dislocation portion of the assumed fault and the low active region of aftershocks.View full abstractDownload PDF (1329K) -
Shuzo ASANO, Katsumi MIURA, Yoshihiro INOUE, Reiko MIURA, Yukio ISHIKE ...1986 Volume 39 Issue 2 Pages 229-240
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSRecently the telemetering seismic network of Shiraki Micro-Earthquake Observatory, Earthquake Research Institute, University of Tokyo, has been constructed in Chugoku District and its vicinity, Western Japan by the fourth stage of National Earthquake Prediction Project. There are seven seismic observation sites in the network, three of which are linked by the telephone lines and four of which, newly installed, are linked by radio to Shiraki Micro-Earthquake Observatory. The latter four are located in the southern part of the network. Thus, the accuracy of hypocenter determination has been improved, especially in the southern part. According to hypocenter determination based on the revised Ichikawa-Mochizuki travel-time table, the following features of the seismic activity in the period from July, 1983 to October, 1984 were obtained:
1. In the land area of Chugoku District, earthquakes occurred in the upper part of the crust.
2. In the southern region of 34°N latitude line, most of earthquakes took place in the upper mantle. An east-west cross section of hypocenters clearly shows a simple Wadati-Benioff seismic zone with a thickness of about 20km, but a north-south cross section of hypocenters shows complicated seismic zones, possibly more than two. The dip of the seismic zone west of 132°E is a little larger than that east of 132°E in the east-west cross section.
Furthermore, detailed description is given to main events during the period from July, 1983 to October, 1984, such as earthquakes M6.8 and M5.2 on August 26, 1983 and June 25, 1984, respectively.View full abstractDownload PDF (1237K) -
Haruo SATO1986 Volume 39 Issue 2 Pages 241-249
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe decay rate of coda envelopes of small local earthquakes is characterized by QC-1. Analyzing records of 95 local earthquakes at 10 stations in the Kanto-Tokai district, we found regional differences in QC-1. Frequency dependence of QC-1 from 1 to 16Hz was measured from the logarithmic decay curves of RMS band-pass filtered seismograms. The fQC-1 value is low in the northern Kanto, high along the coast of the Pacific Ocean, and is 1.1×10-2s-1 on average, where f is frequency in Hz. The regional difference in a power low dependence of the form QC-1∝fn was studied. The n value is close to -0.6 in the southeastern Kanto, and close to -0.9 in the Tokai and the western Kanto, and -0.86 on average.View full abstractDownload PDF (699K) -
Harumi AOKI1986 Volume 39 Issue 2 Pages 251-256
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSFast computation of focal mechanism is achieved with the use of Fourier analysis for a P-wave polarity distribution on a focal sphere. If the focal sphere is rotated so as to give a vertical null axis, the polarity becomes a function of azimuth only. Let every positive and negative polarities be given a value of +1 and -1, respectively, then the polarity distribution is approximated by a periodic square function of period π, which provides a new technique for the focal mechanism solution. The direction of a nodal plane is fundamentally constrained by the phase angle of Fourier component at period π for the function. However, the null axis should systematically be sought on the focal sphere till the best fit to the polarity data is obtained.View full abstractDownload PDF (619K) -
Takashi TADA1986 Volume 39 Issue 2 Pages 257-265
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSCrustal strain field in the Northeastern Japan Arc has been revealed by repeated geodetic surveys. The crustal strain field is divided into four tectonic provinces, that is, the Kitakami Mountains Area (outer arc), the Uetsu Area (inner arc), the Honjo-Matsushima Tectonic Zone and the southern part of the Northeastern Japan Arc. These strain provinces relate closely to the geophysical and the geological phenomena in their provinces. The extentional strain field in the Kitakami Mountains Area shows that the compressional stress which is caused by the subduction of the Pacific Plate has not conveyed beyond the aseismic front. It is considered that the extentional strain field in the Kitakami Mountains Area is related to the continuous uplift phenomena of the Kitakami Mountains Area since the middle Miocene. The E-W compressional strain field in the Uetsu Area agrees with the folding zone which shows the E-W crustal shortening, and supports a new idea that the Eastern Japan Sea Margin is a plate boundary between the Eurasian and the North American Plates. The strains in the Honjo-Matsushima Tectonic Zone are small. It is considered from the geophysical and the geological data that the Honjo-Matsushima Tectonic Zone is a deep crustal fracture zone which divides the Northeastern Japan Arc into the northern and the southern blocks.View full abstractDownload PDF (820K) -
Multichannel Pulse-Height AnalysisIkuo KATSURA, Susumu NISHIMURA, Taro SADAHIRO, Kazuo MINO, Tohru MOGI, ...1986 Volume 39 Issue 2 Pages 267-275
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe γ-ray survey using a multichannel analyzer (hereafter called MCA) with NaI (Tl) scintillation counter was carried out across the Obaku active fault, which is extending to about N-S direction in southern part of Kyoto Prefecture. This study reveals the increase of γ-radiation due to 214Bi and 40K over the fault plane necessarily coincides to each other. The increase of γ-radiation due to 214Bi is clearly attributed to the radioactive decay of gaseous 222Rn emanating through the active fault plane, while the increase of γ-radiation due to 40K attributed to the concentration of potassium in the artificial works and biogenic products may be superimposed on the increase by the dissolution of potassium from rocks and adsorption to clay minerals along the active fault plane. It is considered that the accurate location of active fault can be recognized by the peak intensity of γ-ray radiating from 214Bi.
The γ-ray measurement by MCA shows that the Obaku fault is assumed as an active fault, which is composed of, at least, three parallel fault planes in the fracture zone of about 100m width extending to N10°W direction.View full abstractDownload PDF (1069K) -
Yoshinobu TSUJI1986 Volume 39 Issue 2 Pages 277-287
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe Tenmei Odawara Earthquake outbroke in early morning of 23th August, 1782, and damages of building and houses took place at the castle town of Odawara and its vicinity, 80km SW of Tokyo. USAMI et al. (1984) had newly found out several kind of old documents and had made clear that intensity V or more was felt within the circle with the radius of 45km around Odawara. No reliable record of tsunami had been found out, and it had been considered that no tsunami was accompanied. But in the recent few years two old documents with tsunami disaster records at the fishery harbor of Ajiro in Atami City, 100km SW of Tokyo, were newly found out, inundation height was surveyed as 4 meters, and tsunami magnitude was estimated as m=1. The magnitude of the earthquake was judged by comparison of magnitude of tsunami, and areas of intensities III-IV, V, and VI with the 1982 Urakawa-Oki Earthquake-Tsunami (M=7.1, tsunami magnitude m=0), and was estimated as M=7.2. The Location of the epicenter was suggested to be situated at (35.1°N, 139.2°E), about 10km in the offing of Atami City.View full abstractDownload PDF (1472K) -
Experimental Apparatus and Hypocenter DistributionOsamu IDEHARA, Takashi SATOH, Osamu NISHIZAWA, Kinichiro KUSUNOSE1986 Volume 39 Issue 2 Pages 289-300
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn order to obtain higher quality AE (acoustic emission) signals during triaxial compression experiment, we designed a new co-axial type outlet which is insulated from the pressure vessel. The ratio of signal to noise was improved by about 15 db using this outlet.
A cylindrical sample of Yugawara andesite was triaxially compressed under a confining pressure of 20MPa. AE waveforms and strains were simultaneously detected by 20PZT transducers and 6 strain gauges attached to the sample surface. The sample fractured in a brittle manner at a differential stress of about 390MPa with fracture planes nearly parallel to the loading axis. Hypocenters were determined for 163 AE events that occurred at the differential stress more than about 60% of the fracture strength. The hypocenters were randomly distributed below about 94% of the fracture strength. Just above this stress, AE events began to swarm around one of the fracture planes. Near the swarm region, anomalous decrease of local volumetric strains preceded to the onset of the swarm activity. Temporal variation of the anomalous strain behavior seems to be fairly consistent with the swarm activity.View full abstractDownload PDF (1372K) -
Kiyoshi ITO, Yasuhiro UMEDA, Akio KUROISO, Hiroshi MURAKAMI, Yoshihisa ...1986 Volume 39 Issue 2 Pages 301-311
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe source characteristics of the Japan Sea earthquake, May 26, 1983 (M=7.7) is inferred from the seismic observation system with wide-frequency and large-dynamic range at the Abuyama Seismological Observatory. The duration of oscillation of the long-period low-gain seismogram (T0=25s) is much longer than those of other earthquake with nearly the same magnitude and nearly the same epicentral distance, which implies that the earthquake is a multiple shock. The relationship between a multiple shock and duration of oscillation is more clearly indicated in the figure of double amplitude envelope to eliminate the difference in amplitude by magnitude and focal mechanism. This simple method is applicable to detect multiple shocks in seismograms at one station, especially in historical seismograms with a few instrumental records.
Seismograms of the main shock of the Japan Sea earthquake recorded by Wiechert seismographs and those of middle-period (T0=10s) low-gain velocity seismographs show a clear onset of the second event at about 22 seconds after the first arrival. Since no such second arrival is seen on the seismograms of the aftershocks at the same station, the phase is not a crustal phase but a P-wave arrival of the second event of the main shock.
The main shock recorded by the middle-period low-gain velocity seismograph contains more complicated high frequency waves than the largest aftershock. This indicates that the rupture process of the main shock is much complicated compared with that of the aftershock. Further, comparing the spectrum of the first event of the main shock with that of the second event, the average amplitude at a low frequency (5-10 s) of the first event is smaller than the second event, while that at high frequency (1-2s) is larger than the second event. This suggests that the main shock is composed of double events of different rupture type; the rupture of the first event is smaller and radiated much high frequency waves than the second event.View full abstractDownload PDF (1277K) -
Mitsuhiko SHIMADA, Akio CHO1986 Volume 39 Issue 2 Pages 313-317
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (665K) -
Naoshi HIRATA, Naoki SHINJO1986 Volume 39 Issue 2 Pages 317-321
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (602K) -
Part 6. A Numerical Model ExperimentMichio OTSUKA1986 Volume 39 Issue 2 Pages 321-323
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (390K) -
Ryohei IKEGAMI1986 Volume 39 Issue 2 Pages 323-324
Published: June 25, 1986
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (173K)
- |<
- <
- 1
- >
- >|