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Volume 33, Issue 3
Displaying 1-11 of 11 articles from this issue
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Tetsuo TAKANAMI, Hideki SHIMAMURA, Yoshinobu MOTOYA1980 Volume 33 Issue 3 Pages 269-287
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSAn earthquake swarm began south off Hakodate city on October 23, 1978. At the beginning, a seismological survey was temporarily set up in order to investigate a microearthquake activity in the neighbourhood of Minami-Kayabe which is about 30km northeast of the source area of the swarm.
Seismicity around there were monitored by a seismological network of tripartite composed of a routine station ESH (Esan) of the Hokkaido University and two temporary unattended stations in which the portable long-period cassette tape recorders were equipped. These temporary observation systems were the OBS (Ocean bottom seismographs) which were modified for the land use.
It is not a common experience that a temporary network caught the detail of the initiation of a swarm.
In the first stage, the source area of the swarm is concentrated in a region of about 4×8km2 of area with a north-south trend. Several weeks later another concentrated area is added at several kilometers southeast of the former region. Therefore, we can refer to the former in early about 40 days at the beginning stage of the whole activity off Hakodate which has been active for more than a year.
Item of the present paper is concentrated to the result of our seismological observation at the early stage of a relatively short period from October 23 through the end of November 1978.
In the first stage of about 40 days, the activity can be further divided into three periods. In the first period, which is five days since the appearance, very small shocks gradually increased in number.
When the number of earthquakes increased abruptly the second period began. It is midday of October 27, 1978. This was the most active period in the first 40 days.
The third period began on October 29, when the activity began decreasing.
The m-values of Ishimoto-Iida's equation were, m=2.8 for the first period, m=1.6 for the smaller events and m=2.0 for the larger events in the second period, respectively, by the monitored records of ESH. The value for the third period could not been obtained easily for the difficulty in applying the Ishimoto-Iida's relationship.
For the large earthquakes (250μ kines-2500μ kines in the maximum velocity amplitude), the m-value was obtained by the records at the nearest station OBS-1 for the period from Oct. 25 through Nov. 8, 1978. Its value was m=2.8.
No earthquake with magnitude more than 3.5 occurred during the first 40 days though the total number of the recorded earthquakes is about 3700 or more.View full abstractDownload PDF (3963K) -
Kan KATOH, Shozo NAGATA, Kichisuke ITO1980 Volume 33 Issue 3 Pages 289-301
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSWeekly observation to monitor radon content changes in the soil gas using a track etch method has been carried on, on the lines across the several faults such as Fujigawa, Himenoyu and Inatori-Omineyama faults since May 1978, as a part of the earthquake prediction research program. This conventional method measures relative radon concentration in the soil gas by counting the number of tracks per cm2·day recorded on a small piece of cellulose nitrate film which is sensitive to α-ray radiation. Examination of the weekly track data on the lower reaches of Fuji river revealed that:
1. Temporal change in the number of tracks is related well to that in the radon concentration in the water of the randon observation well nearby.
2. It is suggested that a truck etch method can be a simple alternative method for the observation of radon concentration in the water by deep well, in some cases.
Studies of the weekly track data at Himenoyu and Inatori-Omineyama faults in the Izu peninsla verified that:
1. Temporal changes in the number of tracks at Himenoyu and Inatori-Omineyama faults are related to that of the daily number of earthquakes, that is, the increase in the number of tracks follows after the increase in the number of earthquakes.
2. However, the pattern of the increase and decrease in the number of tracks is different between the two faults. The difference seems to be caused by a difference in the maturity between the two faults which are ascending paths of radon.
3. For the purpose of earthquake prediction, the observationsite should be selected on an active fault of the highest maturity.View full abstractDownload PDF (1176K) -
Nobuo HURUKAWA, Kazuro HIRAHARA1980 Volume 33 Issue 3 Pages 303-316
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA small array with a span of about 1km has been set up in the central part of the Kii peninsula to investigate the subducting Philippine Sea plate. Remarkable later P phases have been recorded from an earthquake occuring at a depth of 51km in the southern part of the Kii peninsula. These phases are interpreted as the waves reflected at deep discontinuities, on the basis of their apparent velocities and directions of wave approach. These analyses, combined with the results of the data recorded at other stations, reveal the structure of the Philippine Sea plate subducting into the continental Asian plate.
The interface between the Asian and the Philippine Sea plate is a thin low velocity zone, dipping northwestwards with an angle of about 40°. Earthquakes in the uppermost mantle there take place not within the subducting oceanic plate but within the continental plate, in contrast to the case in the Tohoku region, the northeastern part of Japan.View full abstractDownload PDF (1547K) -
Hiroaki TSUKAHARA, Ryuji IKEDA, Hiroshi SATAKE, Hiroshi TAKAHASHI1980 Volume 33 Issue 3 Pages 317-327
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn-situ stress measurements were made in a 450m deep well in tuff by the hydrofracturing technique. Six hydrofractures were created at depths between 263 and 436m. The results show that the maximum horizontal compressive stress, σHmax, increases with depth from 180 to 210 bars and that the minimum stress, σHmin, increases from 110 to 130 bars over the same depth range. The maximum and minimum stresses at a depth of 400m were determined to be 205 and 130 bars, respectively. A borehole televiewer was used to detect new fractures created by hydrofracturing. Two of six televiewer runs show clear new fractures and indicate the direction of maximum horizontal compressive stress to be N25°±10°E. This direction agrees well with those estimated from active faults, alignment of frank craters associated with Quaternary volcanoes, Quaternary dikes, and shallow earthquake mechanisms in the vicinity.View full abstractDownload PDF (2369K) -
The Structure derived from the Spectral Ratio of P WavesRyuji KUBOTA1980 Volume 33 Issue 3 Pages 329-342
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSUsing the Phinney's method which is based on the Thomson-Haskell matrix of the spectral responce of a layered crust, short-period P waves from distant earthquakes were analyzed from seismograms recorded at Mizumiiro, Toyone and Mikawa stations in the Tokai region. By using the ratio of the vertical component to the horizontal one, the author obtained the structure of the crust and uppermost mantle beneath this region. The Mohorovicic discontinuities were obtained as depths of about 33km in the western Shizuoka and about 30km near Mikawa. In the Tokai region, an “intermediate layer” just beneath the Mohorovicic discontinuity was detected, for which the compressional wave velocity has been suggested as 7.5-7.8km/s by explosion seismic studies. Thickness of this layer was obtained to be 15, 15 and 10km beneath Mizumiiro, Toyone and Mikawa, respectively. The present model including an intermediate layer does not contradict with the result from the surface wave observation.View full abstractDownload PDF (1154K) -
Haruo SATO, Hiroshi TAKAHASHI, Eiji YAMAMOTO, Nobuhei FUKUO, Masayoshi ...1980 Volume 33 Issue 3 Pages 343-368
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSContinuous monitoring of the crustal tilt is one of the most effective approaches for the purpose of earthquake prediction. Drilling a borehole is not so expensive as construction of an underground vault, which is necessary for installation of water-tube tiltmeters and horizontal pendulum tiltmeters, and a borehole station is possible to be made even in a plain area. The National Research Center for Disaster Prevention has not only developed the boreholetype tiltmeter itself but has improved borehole drilling techniques and installation methods for the tiltmeter. The force-balanced pendulum tiltmeter developed for down-hole observation is installed at the bottom of the borehole of which the depth is about 100m. The measuring range is ±2×10-4 radian, the resolution is 6×10-9 radian, and the long term drift is smaller than a few micro-radian per year after half a year since the installation. Clear records of the earth tides demonstrate the high quality of the instrument. Tilt changes of the order of micro-radian caused by precipitation are remarkable in the data of some stations. But, the crustal tilt observation by using borehole-type tiltmeters is expected to be very effective for detecting anomalous tilt changes of the order of 5×10-8 radian per day with a time constant of several hours to several tens of days.View full abstractDownload PDF (2641K) -
Kenji TANAKA1980 Volume 33 Issue 3 Pages 369-377
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn this paper, study is made on the spatio-temporal variation of the seismic gaps associated with 15 major earthquakes (M≥7.0, depth≤80km) in and near Japan. As a result, we classify the occurrence of major earthquakes into the following three types according to the process of the formation of quiescence in both time and space.
(I) Trench type; Smaller gaps, whose radii are about tens of kilometers, appear around the epicenters of major earthquakes before the occurrences of main shocks and the final radii expand to some hundreds of kilometers around the epicenters. A expanding of quiescence seems to appear after the occurrence of the main shock in both time and space around the epicenter of the main shock.
(II) Hyuganada type; Earthquakes occur at random in both time and space before and after the main shock. Seismic gaps cannot be found.
(III) Inland and Japan Sea type; Seismicity around the large event is always quite so low that quiescence looks like to exist always before and after the main shock.
For the case of Trench type, we found that the seismic gaps rapidly expand to the larger area than the epicentral area just before the main shock. No systematic relationship is found between the precursory time of the rapid expansion of the seismic gaps and the magnitude of the main shock, however, the following relationship is found between the maximum radius of the quiescence and the magnitude of the main shock.
logR=0.33M-0.77 (M≥7.0;R[km]).View full abstractDownload PDF (881K) -
Akira IKAMI, Katsutada KAMINUMA, Yoichiro ICHINOSE1980 Volume 33 Issue 3 Pages 379-389
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe summer-field party of the 20th Japanese Antarctic Research Expedition (JARE) carried out observations of explosion seismology to investigate the crustal structure of Soya Coast in Antarctica in January 1979. This was the first for JARE to carry out the explosion seismology for study on the crustal structure in Antarctica.
The objectives of the measurements by JARE-20 are to establish the techniques of explosion seismology in Antarctica, such as the laying of lines and instrumentation, development of ice drills, dynamite for the use at low temperatures, etc. JARE-21 will continue the observation along the 250km measurement line during the winter of 1980.
An upper crustal model is presented in this report. An overlaying thin layer of 4-5km/s P wave volocity is firstly estimated in the structure model of East Antarctica.View full abstractDownload PDF (852K) -
Kiyoshi SEYA1980 Volume 33 Issue 3 Pages 391-392
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (299K) -
Kazuaki MASAKI, Kumizi IIDA1980 Volume 33 Issue 3 Pages 392-394
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (360K) -
Hiroshi SATO1980 Volume 33 Issue 3 Pages 395-397
Published: September 25, 1980
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (428K)
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