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Volume 47, Issue 1
Displaying 1-15 of 15 articles from this issue
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Tokutaro HATORI1994 Volume 47 Issue 1 Pages 1-9
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA tsunami was generated off the coast of Oshima Peninsula, the Japan Sea, at 22: 17 (JST) on July 12, 1993. The tsunami killed 231 persons and strongly hit Okushiri Is. located in the source region. A short-period waves of about 7min were predominant around the source area. According to JMA, the epicenter of the main shock was located at 42°47′N, 139°12′E, with a depth of 34km and earthquake magnitude of M=7.8. Based on tide-gauge records, the source area, tsunami magnitude and propagation of the present tsunami are investigated.
The source area estimated by means of an inverse refraction diagram is 150×70km2, extending N-S direction on the bathymetric line of 3, 000m. The tsunami source covers the aftershock area. The initial motion of the tsunami began with a down-direction at the tidal stations of Esashi and Iwanai, and the other stations were in an upward direction. It suggests that the sea-bottom on the east side of the tsunami source subsided and that on the west side laying the aftershock area uplifted. Judging from the attenuation of tsunami height with distance, the tsunami magnitude is m=3 on the Imamura-Iida scale. This value is one-grade (tsunami height: 2 times) high comparing with that of tsunamis of the same earthquake magnitude on the Pacific side.
The size of source area and the tsunami magnitude were comparable to those of the 1983 Nihonkai-Chubu tsunami. The tsunami also hit the tip of Noto Peninsula, Oki Is. and the east coast of Korea (37°-38°N). The present earthquake was occurred in the gap between the 1940 Shakotan-Oki earthquake and the 1983 Nihonkai-Chubu one, and the southern part of the source area overlapped with the source of the 1741 Kanpo Oshima-Oki tsunami (m=3.5). In the future, there is a possibility of fault motion extending toward the south direction.View full abstractDownload PDF (1264K) -
Shozo KIMURA, Kennosuke OKANO1994 Volume 47 Issue 1 Pages 11-19
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe seismic activity in Shikoku is characterized by two different focal depth ranges. One of the ranges is in the upper crust and another one in the uppermost mantle to about 40km depth. This study examines the seismic activity in the latter region. We reveal that the thickness of the focal distribution of these earthquakes is at most 5km by improving of hypocenter determination. Such a thin focal distribution suggests that the generation of these mantle earthquakes is not due to descending of the oceanic plate, but is more easily comprehensible as a boundary phenomenon between the continental and the oceanic basements.
The other focal distribution which appears to descend under Kyushu nearly to the west is observed in the region west to Shikoku. This study shows a possibility that these two focal distributions are not directly related with each other and that the generation of these mantle earthquakes in Shikoku is restricted by the Median Tectonic Line.View full abstractDownload PDF (1176K) -
Ryoichi NAKAMURA, Kunihiko SHIMAZAKI, Toshihiko HASHIDA1994 Volume 47 Issue 1 Pages 21-32
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSAttenuation of seismic ground motions is affected by the 3-D attenuation structure. The distribution of seismic intensity, of the 1993 Kushiro-Oki earthquake can not be predicted by conventional attenuation relations. We attempt to explain this phenomenon on the basis of the 3-D attenuation structure of the Japanese islands. We first obtain the 3-D attenuation structure by tomographic inversion of a large quantity of previous seismic intensity data and then simulate the seismic intensity distribution of the 1993 Kushiro-Oki earthquake using the attenuation model. Results are summarized as follows.
(1) The obtained 3-D attenuation structure shows that the Pacific plate and the Philippine Sea plate tend to show High-Q. These results are consistent with the reported results of P wave velocity tomography.
(2) The source acceleration of inland earthquakes whose depths are 0-30km is smaller than that of subduction zone earthquakes at the same depth range.
(3) It is possible to reproduce the seismic intensity distribution of the 1993 Kushiro-Oki earthquake using the obtained 3-D attenuation structure and the relationship between source acceleration and JMA magnitude.View full abstractDownload PDF (1306K) -
Geological Structure down to Pre-Neogene BasementKazuo YAMAGUCHI, Tsutomu KIGUCHI, Toshiyuki YOKOTA, Takanobu YOKOKURA1994 Volume 47 Issue 1 Pages 33-41
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA seismic reflection survey was conducted in Shimodate city, several kilometers west of the Yamizo mountains in Kanto plain. Results of seismic interpretations are (1) RMS velocities are 1800m/s in shallow part and increase to more than 2400m/s below 1.0s in two-way time, (2) Top of the pre-Neogene basement shows steep dip of 30° which falls down westward from 650m into 1150m in depth, (3) Layers above the steeply dipping basement also show steep dip as the basement up to around 400m in depth, but layers whose depth is shallower than around 400m are horizontal and not affected by the basement slope, (4) Unconformities are found, one in depth between 400m and 270m, the other 600m and 380m. The geological structre around the survey area is inferred as follows, based mainly on this seismic data as well as on gravity, surface geology and well data. The basement outcrops in the Yamizo mountains, extends westward with very gentle dip of 1° near the surface ranging from several tens meter to one hundred and several tens in depth, falls down into 1200m in depth with dip of 30° near 140°E and the dip becomes gentle again in the west of 140°E. The basement in the east of 140°E relatively upheaved after the sediments below 400m deposited. This upheavement of the basement is observed as gravity anomaly which could be correspond to the Karasuyama-Sugaonuma fault.View full abstractDownload PDF (1411K) -
Part-2: A Simple Method to Calculate the Secondary Love Wave Generated at the Edge of the Sediment-Filled ValleyMichihiro OHORI, Arihide NOBATA, Kunio WAKAMATSU, Yuzuru YASUI1994 Volume 47 Issue 1 Pages 43-51
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSWe propose a very simple method to calculate the secondary Love wave generated at the edge of the two-dimensional (2-D) sediment-filled valley. From engineering point of view, we consider the seismic response due to horizontally incident plane SH wave. The rectangular sediment-filled valley is used in the analyses. In our approach, we neglect the displacement and stress distribution along the depth in the Love wave propagation and convert the 2-D sediment-filled valley to the one-dimensional (1-D) vertical stratified media. Considering the Love wave propagation in the layered media, Haskell matrix technique is used for such geological structure. The physical meaning and applicability of our approach is discussed in comparison with the formulation by Hisada. From some demonstrations, it has been shown that the validity of our method is confirmed.View full abstractDownload PDF (958K) -
Hiroshi YAKIWARA, Toshiki KAKUTA, Kazuhiko GOTO, Hiroshi SHIMIZU1994 Volume 47 Issue 1 Pages 53-61
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSOn January 30, 1992, a shallow earthquake of magnitude 4.9 followed by about 300 aftershocks occurred in a northern area of the Koshiki channel, north-western Kagoshima Prefecture. We located seismic events observed at two stations of NOEV (Nansei-toko Observatory for Earthquakes and Volcanoes) and four stations of SEVO (Shimabara Earthquake and Volcano Observatory), using Joint hypocenter determination (JHD). Hypocenters of the mainshock and its aftershocks were nearly vertically distributed at depths from 5km to 13km in a small area.
Initial motions at the seismic stations of NOEV, SEVO and FMO (Fukuoka Meteorological Observatory) suggest a focal mechanism of strike slip fault type with a T-axis of NNW-SSE direction: the mechanism is very similar to those reported for the earthquakes in and around the area. The nodal plane striking in NE-SW agrees with trends of the fault system in the channel and the other WNW-ESE plane is parallel to the earthquake alignment along Amakusanada-Izumi-Kakuto areas. Hypocenters of the event and aftershocks nearly vertically distributing are, however, not consistent with any of the planes.
In March of 1991, about 10 months before the M 4.9 event, an earthquake swarm (Mmax 2.9) occurred at depths around 5km almost within the same epicentral area. Namely, two different types of earthquake sequence occurred at different depths in the same area: the swarm occupied a shallower zone than the focal zone of the M 4.9 event. Although some volcanic process may be inferred from hypocenters vertically aligning, it is probably difficult to explain the fact that the earthquake swarm at shallow depths occurred about 10 months before the M 4.9 event at a deeper depth without accompanying any notable foreshocks.View full abstractDownload PDF (1348K) -
Hidenori MOGI, Tatsuo OHMACHI1994 Volume 47 Issue 1 Pages 63-72
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSRayleigh waves in an elastic medium lying over a rigid basement were analyzed in the frequency-wavenumber domain. First, the Rayleigh function and representation of displacements were derived. Next, the function was closely investigated with respect to causality and locations of its zeros in the complex wavenumber plane. This is because, in case of the elastic and rigid basement model, all the singularities of the representation of the displacements are due to the zeros of the Rayleigh function. The Rayleigh function for the rigid basement model is known to have the Rayleigh poles with negative group velocity. In this study, it was shown that these poles violate causality. Finally, surface displacements were calculated by means of both the mode superposition and the discrete wavenumber method for several epicentral distances, and compared with each other in the frequency and time domain. The comparison showed that the mode superposition method tends to give considerable errors due to neglected complex poles especially at a short epicentral distance and in a low frequency range.View full abstractDownload PDF (1185K) -
A Possibility of the 1586 Tensho EarthquakeShinji TODA, Daiei INOUE, Nobukazu TAKASE, Akihiko KUBOUCHI, Nobuyoshi ...1994 Volume 47 Issue 1 Pages 73-77
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
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Takeshi NISHIMURA, Yu'ichi MORITA, Hiroyuki HAMAGUCHI1994 Volume 47 Issue 1 Pages 79-83
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
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Motoaki TAKAHARA, Kazushige OBARA1994 Volume 47 Issue 1 Pages 85-88
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (374K) -
Tokuji UTSU1994 Volume 47 Issue 1 Pages 89-92
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (511K) -
Tokuji UTSU1994 Volume 47 Issue 1 Pages 93-95
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (325K) -
Hitoshi ODA, William M. VISSCHER1994 Volume 47 Issue 1 Pages 97-100
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
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Exploration of Subsurface Structures: Reflection Seismic Method and VSP (Vertical Seismic Profiling)Takeshi IKAWA1994 Volume 47 Issue 1 Pages 103-112
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSeismic reflection method is useful to explore deep subsurface structures in details, not only for prospecting of oil but also for engineering seismology. As a wide band of frequency from several tens of Hz to 0.1Hz is required for precise prediction and simulation of strong motions in latest studies, the subsurface structures are necessary to explore, up to the seismic basement whose velocity is around 3km/s for S wave or over 5km/s for P wave. Recently, some seismic reflection surveys of this kind have been done in the vicinity of large cities, such as Tokyo and Osaka. However, althogh the velocity structures are very important for the simulation of strong motions, the velocity analyses based on reflection data are usually used to optimize the stacking quality of CDP (Common Depth Point) data and the geological structures are mainly discussed in the processed seismic sections.
Therefore, in this paper, the author will review the CDP stacking method of reflection data, examining the principles of velocity analysis in simple and complex structures, and Vertical Seismic Profiling (VSP) as a good tool to calibrate the seismic reflection data.View full abstractDownload PDF (2198K) -
Yutaka OHTA1994 Volume 47 Issue 1 Pages 113-136
Published: June 14, 1994
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSEssentially composed of 3 major chapters, this paper attempts a review of recent studies on seismic strong motions as key information to explore better earthquake disaster prevention strategies. Followed to the introductory chapter, the existing earthquake disaster prevention strategies for local-to-regional administrative districts and earthquake-resistance regulations for man-made structures in Japan are overviewed to recognize how effectively the accumulated knowledge, in these decades, on seismic strong motions has been incorporated.
In chapter 3, a few major indexes characterizing seismic strong motions are described based on not only the instrumental but also noninstrumental data. Discussed are the maximum (acceleration, velocity and displacement) amplitudes, duration times and envelope curves etc. as characteristic indexes instrumentally-derived, and seismic intensities due to questionnaire and other noninstrumental surveys. Chapter 4 outlines the off- and on-line ways of assessment to indentify a damaging earthquake and to predict associated disasters, which are useful tools to make up prior and immediate protection strategies against an earthquake. The final chapter summarizes a short comment for further development of earthquake disaster prevention strategies in more cooporation of the information on seismic strong motions.View full abstractDownload PDF (4401K)
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