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Volume 50, Issue 1
Displaying 1-15 of 15 articles from this issue
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Masashi MATSUOKA, Saburoh MIDORIKAWA, Yasuo UCHIYAMA1997 Volume 50 Issue 1 Pages 1-10
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA dense strong-motion array in the central part of Odawara city, Japan has been installed for understanding the detailed spatial distribution of ground motion characteristics. To evaluate the local site effect on the observed earthquake motions, the spectral ratios at soil sites with respect to two reference sites are calculated for several events. When the reference site KNB located about 3km from the soil sites is used, the spectral ratios are scattered in different events. For the reference site SRY which is very close to the soil sites, however, the spectral ratios are stable. One possible interpretation for the scatter in the ratios with respect to KNB may be large attenuation of seismic waves in the propagation path between KNB and the other sites. The spectral ratios with respect to the reference site SRY are compared to the horizontal/vertical ratios of microtremor in order to clarify the applicability of microtremor to site effect evaluation. Both ratios show spectral peaks at the same periods. The correlation is also found in levels of both ratios at the peaks.View full abstractDownload PDF (1298K) -
Part 2 Reproduction of the Displacement Records at the Takada Observatory based on the Fault ModelTakashi NOZAWA, Masayuki TAKEMURA1997 Volume 50 Issue 1 Pages 11-20
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSRecords observed at the Takada observatory by an Imamura's type strong motion seismograph are useful in Japan to investigate a source process of the 1923 Kanto earthquake (M=7.9). Ground motion in the vertical component is completely recorded to the end. Those in horizontal components are also recorded to the end, though the amplitudes of them are saturated in the midst of the records. The instrumental response of the seismograph has been already clarified by TAKEMURA and NOZAWA (1996), and many records from the recent earthquakes occurring near the focal region of the Kanto earthquake are also obtained at the same site with modern instruments. First, a crustal structure model from the source to the station is established so as to explain the dispersion characteristics of Love-waves observed for the recent earthquakes, and their records are simulated by the normal mode theory using the obtained structure to confirm its validity. Secondly, the records from the Kanto earthquake are reproduced using the same structure model. NOZAWA et al. (1995) proposed the two possible source models for the Kanto earthquake through the simulation of the records at the Gifu observatory. Two big subevents with a time interval of about 12s are assumed on the fault plane in both the models, while the focal mechanism and the focal depth of the second subevent are different between these models. It is found that these models similarly explain the records at the Takada observatory from the Kanto earthquake. The maximum displacements of the reproduced horizontal ground motions are estimated about 20cm in NS component. and about 10cm in EW component. These results are consistent with the fact that the amplitudes of the records in NS component are strongly saturated compared with those in EW component.View full abstractDownload PDF (1454K) -
Manabu HASHIMOTO1997 Volume 50 Issue 1 Pages 21-27
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe previous calculations of static stress changes for the geodetic fault model of the 1995 Kobe earthquake in HASHIMOTO (1995) contains an error in input of parameters. We correct the figures of the distribution of calculated stress changes. The conclusions derived in HASHIMOTO (1995) are not significantly affected, but there are two major differences. (1) The largest aftershock (M 5.4) might have occurred in the zone of CFF increase for the nearly E-W trending right lateral mechanism, while it was hard to find correlation between this aftershock and CFF changes because this event is located in the region of little CFF change in the previous study. (2) The increase of CFF for the westward dipping thrusts in the Osaka plain in the previous study turns to a slight decrease by 0.04MPa.View full abstractDownload PDF (1393K) -
The Distribution of Tsunami Earthquakes around the Japan IslandsHideo WATANABE1997 Volume 50 Issue 1 Pages 29-36
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA method for judging tsunami earthquake is proposed. The method uses a simple formula for calculating the judging factor α from JMA seismic intensity and epicentral distance. α corresponds to the difference between the tsunami magnitude by WATANABE (1995) Mto and JMA earthquake magnitude MJ. [WATANABE (1994)]
We apply this method to some recent and some historical tsunamigenic earthquakes around the Japan Islands. The results are as follows.
1) Among 19 tsunami earthquakes, eight earthquakes have α values of 0.6 or more and one has α value of 0.5.
2) The criterion α≥0.6 is suitable for identifying tsunami earthquake, because number of tsunami earthquakes of α≥0.6 is almost the same as the number of earthquakes of Mto-MJ≥0.6.
3) Historical earthquakes of 1605, 1611 and 1677 were tsunami earthquakes. However, the earthquake of 1793 had a normal tsunami.
4) Tsunami earthquakes around the Japan Islands are distributed along the Japan trench, off the southeast coast of Hokkaido and along the Nankai trough. It must be noticed that two tsunami earthquakes of 1611 and 1896 occurred off the Sanriku coast, and the two tsunami earthquakes occurred simultaneously in 1605 off the Kii peninsula and the Tokai coast.View full abstractDownload PDF (985K) -
Tomomi OKADA, Toru MATSUZAWA, Satoshi MATSUMOTO, Toshio KONO, Koichi N ...1997 Volume 50 Issue 1 Pages 37-48
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe Nikko area, the northwestern part of Tochigi prefecture, is one of the most seismically active regions in the northeastern Japan. In recent years, seismic activity has become higher and isolated volcanic tremors have begun to occur beneath the Nikko-Shirane volcano.
To investigate the cause of these changes, we determined moment tensors of microearthquakes in and around the Nikko area by using broad-band seismic waveforms observed near the hypocenters. In order to stabilize the solutions, we utilized long-period band-passed waveforms to suppress the effect of short wave-length crustal inhomogeneity and to emphasize the near-field term.
The smallest event whose moment tensor could be determined is as small as Mw 1.3. We also tried to estimate the moment tensors of isolated volcanic tremors just beneath Nikko-Shirane volcano. The result from the analysis of the beginning parts of the tremors shows little amount of non-doublecouple components, which implies that these tremors contain ordinary double-couple sources at least in their beginnings. The stress held estimated from the moment tensors is considerably different from the tectonic stress field in this region. The extremely low-velocity high-attenuation body beneath Nikko-Shirane volcano is presumably the cause of the stress disturbance. The pattern of the disturbance indicates that the pressure inside the low-velocity body have a little increased recently.View full abstractDownload PDF (1323K) -
Tokutaro HATORI1997 Volume 50 Issue 1 Pages 49-56
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSTwo tsunamis were generated off Kikai Island on October 18 and 19, 1995, after 84 year's silence since the 1911 Amami-Oshima earthquake (M=8.0). Inundation heights at Kikai Island were ranging 1-3 meters above M. S. L. According to JMA, the epicenters of the main shock and the largest aftershock were located at 28°02′N, 130°23′E and 28°07′N, 130°17′E with earthquake magnitudes, M=6.7 and M=6.6, respectively. The source area of the tsunami of 18th which nearly corresponds to the aftershock area is 60×35km, extending along the Ryukyu Trench. Tsunami magnitudes on the Imamura-Iida scale are estimated to be m=1 and m=0, respectively, by judging from the diagram of the attenuation of wave-height with distance. The magnitudes of those tsunamis exceed by two grades (tsunami height: about 5 times) larger than those generated by the earthquakes with the same magnitude and similar size in other regions. It suggests high-angle faulting mechanism, causing a larger deformation of sea bottom. The velocity of the maximum wave propagated along the Ryukyu Islands to central Japan is 376km/h which is a little slower than those of the tsunamis along the Mariana Trench.View full abstractDownload PDF (1017K) -
Takashi SATOH, Kinichiro KUSUNOSE, Akio CHO, Tamotsu KIYAMA, Fumitaka ...1997 Volume 50 Issue 1 Pages 57-65
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSCrustal stress measurements by the hydraulic fracturing method were carried out using a 1000m- and a 100m-deep boreholes drilled at Inagawa town, Hyogo prefecture, where earthquake swarm activities with very shallow hypocenters have intermittently occurred since July 1994. On borehole televiewer record, borehole breakouts were clearly observed at a number of depth intervals of the 1000m-deep borehole. The borehole breakout data as well as the hydraulic fracturing data was used to estimate orientation of the maximum horizontal compressive stress.
Magnitudes of the horizontal stress down to 700m in depth are comparable to the standard stress gradients with depth in western Japan derived by TANAKA (1986). On the other hand, the maximum and minimum horizontal compressive stresses at about 950m in depth are about 70MPa and 40MPa, respectively, which are about twice as large as the standard stress gradients with depth in western Japan. The maximum horizontal compressive stress below 600m in depth is generally oriented E-W to NW-SE.View full abstractDownload PDF (1043K) -
Boming ZHAO, Masanori HORIKE, Yoshihiro TAKEUCHI, Hiroshi KAWASE1997 Volume 50 Issue 1 Pages 67-87
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSWe compare site-specific response characteristics inferred from microtremors with those from seismic shear waves in terms of (1) predominant frequencies (PP), (2) horizontal component spectral ratios relative to a reference station (HH), (3) horizontal-to-vertical spectral ratios (HV). We furthermore compare the ratios of horizontal-to-vertical ratios of microtremors between stations (RHV) with the spectral ratios of seismic motions to examine the validity of the method proposed by Nakamura. The predominant frequency of seismic motions is compared with two kinds of predominant frequencies which are estimated from the power spectra and the horizontal-to-vertical spectral ratios of microtremors. To examine microtremor source effects we utilized data from two different-size arrays. The diameters of these arrays are 0.5km and 6km, respectively. We obtained the following conclusions from the small-array data: (1) The predominant frequencies estimated from seismic motions are coincident with the two kinds of predominant frequencies only if their spectral peaks are conspicuous. In comparison with the power spectra the spectral ratios (HV) is superior as a method for inferring the predominant frequency in that they give fewer spurious predominant frequencies. (2) The spectral ratios HH of microtremors are in agreement with those of seismic motion below 5Hz. (3) The spectral ratios RHV of microtremors do not agree with the spectral ratios HH of seismic motions. (4) The spectral ratios HV of microtremors are in agreement with those of seismic motions. For the large-array data, we obtained the same conclusions, except for conclusion (2). This indicates that spectral ratios HH can be used within an area in which microtremor source characteristics are spatially uniform.View full abstractDownload PDF (2916K) -
Akiko TANAKA, Tsukasa NAKANO1997 Volume 50 Issue 1 Pages 89-99
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA map of coseismic displacement field can be drawn by interferometric processing of synthetic aperture radar (SAR) images taken before and after an earthquake. This differential interferometric SAR (INSAR) technique allows us to detect subtle changes in the surface of the Earth without any ground-based measurements in place before an earthquake. The resulting interferogram is a contour map of the component of the displacement in the slant range direction, that is from the ground to the antenna. Geometric distortions such as foreshortening, layover and shadow occur in SAR image, because SAR measures the distance between terrain features and the antenna. These limitations aside, it is required for repeat-pass interferometry that the SAR data must be acquired such that the speckle in the image pairs is correlated. In order to understand the observed interferogram, it is necessary to know the data acquisition geometry. We simulate the differential interferogram of the spaceborne SAR images using the location of antenna and theoretical ground deformation. We determine the orbit of the SAR satellite by piecewise polynomial interpolation and calculate the theoretical changes in range due to the earthquake using the elastic dislocation model. The slantrange-ward displacements predicted by these simulations help us to analyze and interpret observed interferograms. We apply this simulation to the 1995 Hyogoken-Nanbu earthquake to detect the effect of geometric distortion of the observed SAR images. The simulation shows the theoretical total displacement due to the earthquake is considerably larger than the slant-range-ward displacement. Geometric distortion due to topography is not so large in the surveyed area, because the range of the altitude is less than 800m. Although differential INSAR derives deformation map over periods of days to years with very high accuracy at any time, its potential applications, characteristics and technical limitations need to be more explored.View full abstractDownload PDF (5354K) -
Hidenori ENDO, Kazuaki WATANABE, Masahiko MAKINO, Yasuaki MURATA, Tsut ...1997 Volume 50 Issue 1 Pages 101-114
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe crustal movement was investigated by the leveling along four routes with short interval of about 25m in the Kobe area, during the period of one year from March, 1995 to February, 1996, after the 1995 Hyogoken-Nanbu Earthquake. The results are summarized as follows: 1) Vertical tectonic movements occurred in a broad area in the plain, and the displacement reached more than 20mm in maximum. 2) During the period from March to July, 1995, the distribution of vertical movements was continuous along each leveling route, and the uplifts like buckling were recognized in the plain. Oppositely, during the period from October, 1995 to February, 1996, stepping displacements were recognized around active faults, and the lowering of the coastal area occurred. 3) Those vertical movements could be attributed to the aftereffect of the 1995 Hyogoken-Nanbu Earthquake. It is possible that horizontal stress in the shallow subsurface was increased during the earlier period of the measurements, and then the fault displacements reached close to the ground surface. 4) The delay of the displacements after the earthquake may be attributed to the existence of thick soft sedimentary rocks, and the branched fault system in the Quarternary structural basin.View full abstractDownload PDF (2294K) -
Yutaka MAMADA1997 Volume 50 Issue 1 Pages 115-118
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (523K) -
Masanao SHINOHARA, Naoshi HIRATA, Shigeo MATSUDA1997 Volume 50 Issue 1 Pages 119-124
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (2413K) -
Hiroo NEMOTO, Hiroaki NEGISHI, Kojiro IRIKURA1997 Volume 50 Issue 1 Pages 125-129
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (770K) -
Koji MASUDA, Orson L. ANDERSON, Dave ERSKINE1997 Volume 50 Issue 1 Pages 131-133
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (316K) -
Jun KAWAHARA1997 Volume 50 Issue 1 Pages 135-156
Published: May 28, 1997
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThis paper reviews numerical studies on acoustic as well as elastic wave propagation in random media, to model the earth's crust. For a long time, stochastic properties of wave propagation in random media, such as scattering attenuation, coda decay, velocity dispersion, and fluctuations of waveforms and traveltimes, have been studied analytically and approximately. Recent development of computer capability enables us to investigate them by solving the wave equation directly and numerically. Numerical wave simulations have also become an important strategy in seismic exploration. We concentrate here on types of inhomogeneities described suitably by autocorrelation functions of the elastic fluctuations of media, and summarize how to model them. For simulating waves scattered by such “random fluctuations”, the finite difference scheme is popularly used and the pseudospectral method is also in use. Isotropic and continuous fluctuations are usually assumed in the modeling, though anisotropic and/or discrete ones, that are often recognized in geological observations, have also been considered just recently.View full abstractDownload PDF (9620K)
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