All issues
Volume 29, Issue 3
Displaying 1-11 of 11 articles from this issue
- |<
- <
- 1
- >
- >|
-
Ikuei MURAMATU1976 Volume 29 Issue 3 Pages 223-232
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn this Paper, some seismograms of the strong earthquake motion obtained in Japan and in California were examined and we found that the particle velocity of the strong earthquake motion was almost constant for a certain range of vibration periods from 1 second to several seconds. We estimated further the maximum value of the particle velocity of earthquake motion as 30kines or 100kines. It is also noticed that the duration time of the strong motion caused by the great earthquakes in and near Japan is remarkably longer than that in California.View full abstractDownload PDF (1041K) -
Tadashi MAKI1976 Volume 29 Issue 3 Pages 233-245
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSP-wave velocity structure in the upper mantle beneath the Japanese Islands was studied by improved Gutenberg's method. Improvement of Gutenberg's method was made by fitting a series of segments to a travel-time curve and obtaining the slope dT/dΔ for each segment by the least-squares method. Maximum value was obtained from the curve of dT/dΔ versus epicentral distance for each earthquake, and corresponding velocities at depths in the upper mantle were obtained.
The velocity distribution, obtained from 227 nearby deep earthquakes beneath the Japanese Islands, represents the velocity situation of the descending slab and the upper mantle above it. Average velocities are extremely lower by 0.4km/sec than those under the oceans. Velocity difference between the slab and the upper mantle above it reaches to 0.8km/sec, even to the end depth of the slab. Features of the low-velocity layer and “20°-discontinuity” were clearly obtained. The low-velocity layer beneath the Japanese Islands is characterized by the range of depth from 120 to 170km, velocity contrast of 0.5km/sec, and minimum velocity of 7.5km/sec. “20°-discontinuity” beneath the Japanese Islands is at the depth of 364km and has the velocity jump of 0.4km/sec. Shallowness of the “20°-discontinuity” is consistent with the shallowness of deep seismic activity beneath the Japanese and Kuril Islands arcs.
Extremely low P-wave velocity in the upper mantle beneath the Japanese Islands is consistent with the fact that the shear velocities obtained from the studies of the surface wave dispersion are lower by 0.4km/sec than those under the ocean (KANAMORI, 1970), and that the ratios Vp/Vs are almost constant and equal to 1.77 in the upper mantle (UTSU, 1969).View full abstractDownload PDF (1367K) -
Susumu IIZUKA1976 Volume 29 Issue 3 Pages 247-263
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSLong-term premonitory change in Vp/Vs prior to the 1968 Tokachi-oki earthquake (M=7.9) was detected. However, only the latter half of the presumed precursory time interval may be identified because of the lack of the data before 1950. This Vp/Vs anomaly was disturbed by the 1952 earthquake swarm and the 1960 Sanriku-oki earthquake (M=7.5) which occurred in almost the same region of the aftershock area of the 1968 Tokachi-oki earthquake. The decreases found amounted to 6-18% of Vp/Vs during the period, assuming a normal value of Vp/Vs as 1.75. Utilizing the temporal variations of seismic activity and released seismic energy through the anomalous region as supplementation, the precursory time interval of the 1968 Tokachi-oki earthquake is estimated about 30 years.
In a region such as off Sanriku, Northeast Japan, where great submarine earthquakes have occurred frequently, the Vp/Vs, ratio changes incessantly and successive cycle of Vp/Vs variations will start soon after the occurrence of an earthquake.
The anomalous region spreads out with time, consequently it brings considerable regional differences on Vp/Vs variations. “Volcanic Front” seems to act as a barrier to stop the expansion of anomalous region.
A new model will be necessary to explain the mechanism of Vp/Vs variations related with great submarine earthquakes in stead of the dilatancy diffusion model.View full abstractDownload PDF (1623K) -
Susumu IIZUKA1976 Volume 29 Issue 3 Pages 265-275
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSPremonitory decrease in Vp/Vs ratio with 12 years of anomaly time duration was detected prior to the 1973 Nemuró-òki earthquake (M=7.4). This long-range precursory variation seemed to be clearly disturbed with two earthquakes which occurred in the neighbouring region of its aftershock area, east off Hokkaido.
A recovery phase to the normal Vp/Vs was found during the years of 1951-59, but no remarkable earthquakes relating with this precursory anomaly occurred within the aftershock region of the 1973 Nemuró-òki earthquake. Although we could not know the starting time of the anomaly for lack of the data, the velocity variation might be caused by both the 1958 Iturup (M=8.0) and the 1963 Urup (M=8.1) earthquakes.
The anomalous area of Vp/Vs could not extend to the ABJ station because “Volcanic Font” probably acted as a barrier to stop expanding the anomalous region.
The results suggest that it might be possible to predict not only the most likely location but also the time of occurrence and magnitude for the 1973 Nemuró-òki earthquake, if we could carefully moniter the developments both in seismicity gap and Vp/Vs variations, in connection with the other precursory phenomena related with great earthquakes.View full abstractDownload PDF (1087K) -
Masayuki KIKUCHI1976 Volume 29 Issue 3 Pages 277-285
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA two-dimensional self-similar crack is considered. The displacement velocity and stress fields around it are expressed by simple analytic functions. Then, the effective stress is related to the displacement velocity at the center of crack surface, while the stress concentrated ahead of the crack tip is related to the displacement velocity behind the tip. These relations are also obtained in explicit forms.View full abstractDownload PDF (651K) -
Toshihiko YAMADA, Hideki SHIMAMURA, Toshi ASADA1976 Volume 29 Issue 3 Pages 287-297
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSResolution in time difference between signals in separate channels of a cassette recorder for an ocean bottom seismograph is investigated. The small-size recorder utilizing compact cassette tapes has been developed especially for ocean bottom seismographs since 1971. Compact cassette recorder is better than open reel tape-recorder regarding size, cost, proof against shock, electric power consumption, and so forth. However, for maintaining required quality of observation at sea bottom, cassette-type recorders demand careful adjustments. It is because thin cassette tapes are very sensitive to the state of tape path and tape transport mechanism. Unstability of tape transportation extremely increases amplitude modulation noises in recorded signals and phase distortion between channels.
In the present paper, the following elements which are important for stable tape transportation are studied; (1) relative position of a cassette half to a pinch-roller and heads, (2) alignment of tape guide, head, capstan and pinch-roller, (3) tension of driving, belt, (4) parallelism of pinch roller axis to capstan.
When all these key factors which control the quality of recording are satisfied, the error in time resulted from recording and reproducing process is reduced less than 1/125sec.View full abstractDownload PDF (12263K) -
Hiroshi SEKIYA1976 Volume 29 Issue 3 Pages 299-311
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe Izuhanto-oki earthquake of May 9, 1974 (M6.9) was preceded by an anomalous seismicity as shown Fig. 1. The duration from the anomalous Seismicity to the mainshock was about 10 years and 4 months. Similar phenomena were also found for 10 events which occurred at shallow depths in Japan.
Considering that these data show a linear relation between the magnitude of earthquakes M and duration log T (day), the author calculated the coefficient of logT=bM+a by using the method of least squares as follows: logT=bM+a=0.77M-1.65
The relation between the above formula and that of TSUBOKAWA (1969), RIKITAKE (1975) and SCHOLZ et al. (1973), which indicate with a certain accuracy the magnitude and time of an expected earthquake was given by Fig. 7.
The author believes that the anomalous seismicity preceding earthquakes is one of the potential parameters which could be employed for earthquake prediction. On the other hand, in some certain fracture zones were found a premonitory change before some events. These phenomena generally are called foreshocks. Anomalous seismicity preceding smaller events cannot clearly be found by data of the Japan Meteorological Agency as shown Fig. 8. It is, however, not certain yet whether such earthquakes were not really preceded by anomalous seismicity.
Our study requires further accumulation of examples from the viewpoint of ascertaining the existence of anomalous seismicity preceding earthquakes by observation network for small earthquakes.View full abstractDownload PDF (1025K) -
Tetsuya WAKABAYASHI, Tsuneji RIKITAKE1976 Volume 29 Issue 3 Pages 313-315
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (328K) -
Toshihiko YAMADA, Toshi ASADA1976 Volume 29 Issue 3 Pages 316-318
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (408K) -
Mitiyasu OHNAKA1976 Volume 29 Issue 3 Pages 318-320
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (395K) -
Shoji SAKATA1976 Volume 29 Issue 3 Pages 320
Published: September 15, 1976
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (126K)
- |<
- <
- 1
- >
- >|