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Volume 45, Issue 4
Displaying 1-9 of 9 articles from this issue
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Michihiro OHORI, Hiroshi TAKENAKA, Kazuki KOKETSU, Tadao MINAMI1993 Volume 45 Issue 4 Pages 357-363
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSAki-Larner method (ALM) is a very powerful technique to calculate the seismic responses of layered media having irregular interfaces. However, it is necessary to solve complex simultaneous equations with very large full matrices so that it takes very long time and consume much memory. Recently, Ohminato et al. (1990) proposed a Galerkin method and suggested that wavenumber coupling between far separated elements in the submatrices of the full matrices are very week if the lateral irregurality is gentle. Their idea seems to be applicable to the analyses of ALM, too. Therefore, in this paper, we estimated possible reduction of the memory consumption in the ALM by neglecting most of non-diagonal elements of the submatrices. Through the 2-dimensional (2-D) analyses, we calculated responses due to vertically incident plane SH-wave and studied the effect of the neglect of non-diagonal submatrix elements on the accuracy of results. Six types of alluvial valleys with different shape were used and the error caused by the neglect was evaluated in comparison with results obtained by the conventional ALM. As a result, we can treat submatrices as band matrices with 1/4 or 1/6 width if the lateral irregularity is gentle, and this leads to reduction of the memory consumption in the Aki-Larner method for the simulation in high frequencies or 3-D problems.View full abstractDownload PDF (721K) -
Tokutaro HATORI1993 Volume 45 Issue 4 Pages 365-371
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSOn the tsunamigenic earthquakes generated off the Kurile Is. to Tohoku regions during the last 98 years, 1894-1991, the characteristic of seismic intensity, tsunami frequency and tsunami height at 10 tidal stations along the Pacific coast is investigated. At the many stations, the tsunamis were most observed when the seismic intensity reaching I=4 (JMA scale). However the number of tsunamigenic earthquakes is 1/10 times or less for the frequency of earthquakes (I≥4).
According to the height-distance diagram for the Kurile-Hokkaido tsunamis, the observed tsunami heights at Hachinohe exceed more than 1.8 times higher than the heights inferred from the average tsunami magnitude. Tsunami heights along the Sanriku coast are relatively large, but those at Kushiro, Muroran and Hakodate show a tendency to be small. For the Sanriku tsunamis, the tsunami heights along the Sanriku coast nealy correspond to the tsunami magnitudes, but those in the Hokkaido region are a litlle small excep Urakawa. The deviations from the average tsunami magnitude caused by the submarine topography indicate the measure of tsunami hazards.View full abstractDownload PDF (535K) -
A Case Study for the Izu RegionKenji MAEDA1993 Volume 45 Issue 4 Pages 373-383
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSWe propose an empirical alarm criterion for the occurrence of mainshocks (M≥5 or M≥6) in the Izu region on the basis of the clustering activities of earthquakes which appear immediately before the mainshochs. The criterion consists of three parts: 1) elimination of aftershocks, 2) selection of candidates for foreshocks on the basis of the grade of clustering which is measured by the number of earthquakes (Nf) with magnitude larger than or equal to Mf0 in unit time (two days) and in unit space (D°(latitude)×D°(longitude)), 3) setting up the alarm period with Ta days and the alarm segments with the unit spaces where candidates for foreshocks are observed. In this criterion Nf, Mf0, D and Ta are treated as parameters. After the investigation of the effects of the parameters on the alarm results, the value of each parameter was selected as Nf≥10, Mf0=3.0, D=0.2°and Ta=4.0 days. This set of values gives us the following practically useful alarm results: the alarm rates are 68% and 71% for mainshocks with magnitude M≥5 and M≥6, respectively; the occurrence rates of mainshocks in the alarmed time-space per Ta (=4 days) and unit space (0.2°×0.2°) are 0.24 and 0.08, respectively; the probability gain, the ratio of the occurrence rate of mainshocks in alarmed time-space to that in all time-space concerned, for either magnitude range is as high as about 850. As for Ta, it is found that four days are enough to give the alarm. This means that the proposed foreshocks are not true foreshocks but merely seismic swarms if no mainshock follows during the four days after the first alarm is raised by the proposed foreshocks.
All these results are obtained from the data of earthquakes almost homogeneously observed in the Izu region by J. M. A. through 1977 to 1991, with the range of depth ≤50km and magnitude ≥3.0. The numbers of mainshocks contained in this data with magnitude M≥5 and M≥6 are 22 and 7, respectively.View full abstractDownload PDF (1281K) -
Phase Velocity Anomaly of Love Waves across a Mountain Root StructureMitsuru YOSHIDA1993 Volume 45 Issue 4 Pages 385-394
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSPropagation of Love waves across a symmetrical mountain root structure with a high dip angle is investigated by use of the finite difference method. The structure is modelled by a set of steps of the Moho discontinuity, and has crustal thicknesses of 35 and 87.5km at the thinnest and thickest portions over a distance of 250km, respectively. The dip angle of the Moho discontinuity is about 20°. The important and interesting results obtained are as follows:
(1) The phase velocities for the downdip propagation are higher than those for the updip propagation in the period range 18-37s. They are inverse in the period range 38-46s.
(2) The phase velocities for the downdip propagation oscillate several times in the period range 18-46s, while the phase velocities for the updip propagation increase monotonously with increasing period in the period range 22-46s.
(3) At the periods of 18, 26 and 38s, the phase velocities for the downdip propagation accord with those for the updip propagation.
(4) The average phase velocity dispersion curve across the mountain root structure shows the middle one between the dispersion curves for the downdip and updip propagations.
(5) The average dispersion curve across the mountain root structure does not show the middle one between two typical dispersion curves for the thinnest (S-structure) and thickest (D-structure) crustal structures.
(6) The average dispersion curve is close to that for the D-structure at short periods and approaches that for the S-structure at long periods.View full abstractDownload PDF (1093K) -
Kazuki MIYAOKA, Akio YOSHIDA1993 Volume 45 Issue 4 Pages 395-405
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSWe investigated seismic activities before large interplate earthquakes which occurred in the subduction zone along the Japan trench since 1970s and found that following features are commonly observed: (1) conspicuous quiescence appears in the focal region of the main shock several years before its occurrence, (2) in the deeper zone than the focal region seismic quiescence also appears at the same time or somewhat earlier, but the seismicity recovers preceding occurrence of the main shock, (3) migration of seismic activities from deeper parts to shallower regions—from the land side to the trench side—is observed, (4) remarkable foreshock activity appears one to several days before the occurrence of the main shock. Besides these common features before large interplate earthquakes, we found that seismic activity in the sea region off-Fukushima prefecture decreased obviously about two years before the 1978 Miyagi-ken-oki earthquake and the seismicity recovered when an M6.7 earthquake occurred off-Miyagi prefecture on February 20, 1978. Further, it is found that a clear seismic quiescence appeared around the focus of the 72km-depth earthquake with a magnitude of 6.6 occurring in the Sanriku seashore in 1987. Recovery and migrational tendency of seismic activities in the deeper part of the interplate coupling zone before large earthquakes are considered to reflect a ductile feature in that region.View full abstractDownload PDF (1129K) -
Norio MATSUMOTO, Makoto TAKAHASHI1993 Volume 45 Issue 4 Pages 407-415
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSMultiple regression and autoregression analyses are applied to the ground water level observed at Hamaoka observation well, Shizuoka Prefecture, central Japan for the purpose of detecting anomalous changes of the water level due to earthquakes. These analyses can eliminate barometric response, tidal response and response due to precipitation from the observed water level. During the period from April 1983 to March 1992, a change of corrected water level is detected just after Western Nagano Prefecture Earthquake that occurred on September 14th, 1984. This anomalous change is the only coseismic change detected at Hamaoka well. Whereas, thirteen coseismic fluctuations of the corrected water level are detected during the same observation period at Haibara well about 20km north of Hamaoka well. Because a aquifer observed at Hamaoka well is less confined than that at Haibara well, the corrected water level at Hamaoka well is less sensitive to earthquakes than that at Haibara. The corrected water level at Hamaoka well is compared to volumetric strainmeter of Japan Meteorological Agency at Hamaoka, 150m northwest of the Hamaoka well. The strainmeter is changed only after the Western Nagano Prefecture Earthquake during April 1983 to October 1991. The change of corrected water level at Hamaoka well is thought to express strain nearby the well.View full abstractDownload PDF (1010K) -
Takeshi MATSUMOTO, Masaaki KIMURA1993 Volume 45 Issue 4 Pages 417-426
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSHistorical record shows that a large-scale tsunami named “the Great Meiwa Tsunami” or “the Yaeyama Earthquake Tsunami” occurred on 24th April 1771 in the sea region of Miyako and Yaeyama Islands, westernmost part of Ryukyu District caused by a large earthquake and the tsunami inundated up to the inside of the islands. Swath bathymetric survey in this area was conducted by the use of SEABEAM and HS-10 systems on board the Research Vessels KAIYO and YOKOSUKA, in November 1990, April 1991, January and April 1992. Precise topographic contour map was completed in the area of 22°40′N-24°10′N, 122°50′E-126°20′E, including the westernmost part of the Ryukyu Trench and tsunami source area (around 24.0°N, 124.3°E). The survey area is classified into five different topographic domains which are arranged zonally. Those are:
Zone 1: north of 24°00′N, which is characterised by the distribution of well developed submarine canyons along the southern coasts of Yaeyama Islands.
Zone 2: 23°40′N-24°00′N, which is characterised by deep sea fans and its large-scale subsidence edged by steep scarp near 23°55′N-24°00′N, 124°10′E-124°20′E. The observed relative height of the depression is about 200-300m.
Zone 3: south of 23°40′N on the landward slope of the trench, which is characterised by quite complicated topography such as several escarpments and ridges and troughs of which trend is not clearly recognized.
Zone 4: trench axis area with depth about 6500-6600m, which is characterised by broad plain. Maximum width of the axial plain is about 40km.
Zone 5: seaward slope of the trench, which is characterised by horsts and grabens which are samely distributed in the sea area of the south of Okinawa Island. The strike direction of the horsts and grabens is NW-SE off Miyako Islands, and WNW-ESE off Yaeyama Islands.
The 1771 Meiwa Tsunami is considered to be originated by the large-scale subsidence of the deep sea fans which is composed of soft sediment derived from the submarine canyons just north of the fans, and the wave might be propagated along the canyons from the position of the tsunami source.Tention of N-S direction is suggested by the topographic features in the survey area, and the tention stress in this area is considered to be the origin of the large-scale subsidence which induced the tsunami.View full abstractDownload PDF (1278K) -
Seismic Hazard Map of Colombia, Ecuador and VenezuelaSadaiku HATTORI1993 Volume 45 Issue 4 Pages 427-436
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSeismic risk analyses, which mean here to get regional distributions of maximum earthquake motion and convert them to seismic hazard maps, were made for Colombia, Ecuador and Venezuela, by using the seismic data, attenuation models and the method of extreme value fitting. Attenuation models for peak earthquake motion were put into three categories of maximum acceleration, maximum particle velocity and maximum displacement, and the least upper bound and the greatest lower bound of three categories were examined. In the present paper, Oliveira·McGuire model and Kanai model were used for maximum acceleration and maximum particle velocity, respectively. Colombia and Ecuador were attacked by 79 and 62 earthquakes with damage in the period 1800-1989, severally. The dead in the same period were 111, 855 by 46 events in Colombia and 105, 793 by 27 events in Ecuador. Colombia and Ecuador, therefore, are very intent on making seismic risk analyses and reducing earthquake disasters. Venezuela is located on the northern part of South American plate which make contact with Caribbean plate and seismic activity at the northern part of its territory is very high. During the period 1800-1989, Venezuela was hit by 39 earthquakes with damage and the dead in the period was 93, 834 by 24 events. The above facts have urged Venezuela to making the building code and mitigating earthquake disasters. The regional distributions of maximum earthquake motion and the seismic hazard map, whichwere investigated here for Colombia, Ecuador and Venezuela, will furnish basic data for these countries to make countermeasures against earthquake disasters.View full abstractDownload PDF (1412K) -
Yoshinobu MOTOYA1993 Volume 45 Issue 4 Pages 437-440
Published: March 24, 1993
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (487K)
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