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Volume 40, Issue 2
Displaying 1-13 of 13 articles from this issue
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Execution and ResultsYutaka OHTA, Hiroshi KAOAMI, Hitomi OHASHI1987 Volume 40 Issue 2 Pages 145-157
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSBased on the investigation scheme described in the previous paper a principal survey for examining the household safety against an earthquake was set up in a size of 800 households from 16 block areas in Kawasaki City, Central Japan. The selection of surveyed block areas was made in consideration of their natural and social backgrounds. The questionnaires were mailed to 40-50 households per block area and the collection rate attained was above 60% in 2 months later.
After developing a causal chain model with 11 essential and sequential disaster aspects from direct to indirect, the diagnostic analysis per household was conducted first for the individual safety to each of earthquake disaster aspects and then for the total earthquake safety. The final diagnosis which classified the total earthquake safety of a household into 3-5 grades from “safe” to “risky” was made accordingly, and the results with some guidelines for improving the family level earthquake preparedness were sent back to the surveyed households.
A comparison made clear that the aerial changes of the earthquake safety averaged in block unit are in good correlation with those of land use and other natural and social characteristics and that this questionnaire survey is effective as for the first-stage screening method. Short comments for further developments was remarked. One for an extention to the full-scale survey in an administrative district and the other for an upgrading toward the second-stage screening survey.View full abstractDownload PDF (1477K) -
Koichiro NAGAMINE1987 Volume 40 Issue 2 Pages 159-168
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSeismicity is active around the Izu Peninsula. For the purpose of earthquake prediction, chemical compositions of bubble gas from a hot spring have been continually observed since April 1983 at Atagawa on the east shore of the Izu Peninsula. Variations of gas compositions at the spring were observed before the earthquake swarms which occurred off the east coast of the Izu Peninsula. He/Ar ratios of the spring gas remarkably decreased six months before the occurrence of earthquake swarms and H2/Ar ratio increased along with He/Ar variations. The variations of these ratios range in two orders of magnitude. Since deep seated gases with high ratio of He/Ar may be driven out of depth under a high pore pressure, the decrease of He/Ar ratio may be caused by the decrease of the pore pressure which is due to the microcrack opening by tectonic activities eventually resulted in earthquakes. Whereas the increase of H2/Ar ratio may be due to the production of H2 by the chemical reaction between groundwater and fractured rock in microcracks. The relationship between N2/Ar ratio and He/Ar ratio supports this interpretation. The continual observation of chemical compositions of spring gas which are subjected to tectonic activities can thus be regarded as a useful tool for earthquake prediction.View full abstractDownload PDF (1042K) -
Toshikazu TANADA, Shigeru WATANABE, Teruyuki KATO, Shin-ichi KOIZUMI, ...1987 Volume 40 Issue 2 Pages 169-181
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe microseismicity and focal mechanisms were examined in the vicinity of Fuji River around Suruga Bay, central Japan, during the period from June 1981 to August 1985. The total, number of located events amounted to more than 1800 and 84% of them were less than M=1.0, namely, ultra-microearthquakes.
The seismicity is unusually low in the innermost part of Suruga Bay and its neighboring area, and around Mt. Fuji. The seismicity pattern changes across the Itoigawa-Shizuoka tectonic line and Fuji River fault. Most earthquakes occurred within the depth range of 10km to 30km and their focal depths tend to be deeper from east to west in the region. The seismic pattern may be interpreted as a northern extension of the seismic activity within the subducted Philippine Sea plate from the Suruga trough.
The fault plane solutions were determined for 36 events with M=0.0 to 4.7. The results show 21 strike-slip and 9 reverse types. Pressure and tension axes indicate nearly N-S compression and E-W tension, respectively, except for the events occurred beneath OKY and FUJ. These results are consistent with the earthquake mechanism data already published around Suruga Bay and in the southwestern part of Yamanashi prefecture.
It may finally be concluded that these microearthquakes are generated by underthrusting of the Philippine Sea plate toward west.View full abstractDownload PDF (1482K) -
An Example from Jusan, Shiura-mura, AomoriKoji MINOURA, Shyu NAKAYA, Hiroshi SATO1987 Volume 40 Issue 2 Pages 183-196
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSOrganic-rich deposits were cored in the inter-dune ponds of Maekata and Myojinnuma on the Japan Sea coast in the extreme northeast of Honshu. The cores of 2m in the maximum length are composed of black organic-ooze intercalating sandlayers. Five sand-layers were found in the core of Maekata and the top one was formed by the tsunami of the Japan Sea Earthquake of May 26, 1983. The cored sediments of Myojinnuma yield four sand-layers and the lower two are composed of gravelly coarse sands. Sedimentological and geochemical studies about the sediments suggest that most of those sand-layers were brought by density currents of dune sand-seawater mixtures. It can be supposed that the currents were made during the passing of seawater through cracks formed in the dunes by seismic shocks. The rise of the sea surface caused by the tsunamis enabled seawater to flow into the inter-dune ponds. The radioisotopic (210Pb) dating of the layers shows that the traces of the tsunami correspond to the ancient tsunamis practically kept on the historical record.
The method and interpretations introduced in this study can be applied to older sediments. We suggest that the traces of the prehistoric earthquakes with tsunami can be found exactly in the sediments of coastal regions.View full abstractDownload PDF (1480K) -
Takashi TADA, Gaku KIMURA1987 Volume 40 Issue 2 Pages 197-204
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDetailed analysis of the geodetic survey data in Hokkaido, Japan, has revealed an interesting horizontal crustal deformation in the southwestern margin of the Kurile Arc. The horizontal crustal deformation shows that the Outer Kurile Arc (the Kurile Sliver) has drifted toward southwest and collided against middle Hokkaido at the Obihiro Tectonic Line. This crustal deformation is concordant with the tectonic feature which is expected from the oblique subduction of the Pacific Plate under the North American Plate (or the Okhotsk Plate) and the mechanical decoupling between the inner and outer Kurile Arcs, and the geological researches in Hokkaido.View full abstractDownload PDF (805K) -
The Boso-Oki Tsunami of September 19, 1984Tokutaro HATORI1987 Volume 40 Issue 2 Pages 205-211
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSBased on tide-gauge records, the Boso-Oki tsunami of Sept. 19, 1984 is investigated. Adding the present data, the magnitudes of the tsunamis generated near the trench triple junction off Boso Peninsula since 1927 are discussed in relation to earthquake magnitude (M, JMA scale and Ms). Judging from the diagram of attenuation of tsunami height with distance, the magnitude (Imamura-Iida scale) of the 1984 tsunami was determined to be m=0. This grade is very large for an earthquake with M=6.6 and Ms=6.7. The estimated source area agrees with the aftershock area, and the source length is 35km which is the normal size for the earthquake magnitude.
According to the statistical relation, the magnitude values of the Boso-Oki tsunamis are one to two grade larger than the average tsunami magnitude. Such tsunami energy is five to ten times larger. Its characteristic behavior seems to be caused by the low-frequency earthquake and the high-angle (dip angle 34°-74°) dip-slip fault. For future tsunamis generated off Boso Peninsula, there is a need of careful precautions.View full abstractDownload PDF (672K) -
Epicenter and MagnitudeTsuneji RIKITAKE1987 Volume 40 Issue 2 Pages 213-223
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSAnalysis of earthquake precursor data so far obtained in Japan makes it possible to determine the dependence of maximum detectable distance of a precursor (Dmax) on main shock magnitude (M). The M-Dmax relation thus obtained is different from discipline to discipline of precursor.
When a precursor-like phenomenon is observed at an observation point, we can draw a circle centering at that point with a radius of Dmax peculiar to that discipline provided a certain value of M is assumed for the main shock. If there are a number of observation points in an earthquake-threatened area and if anomalies are found for various precursor disciplines, we may have many circles with different radius. In that case it is likely that the epicenter of the soon-to-occur earthquake is located in the area, which is common for all the circles. When a magnitude, which is too small, is assumed, the circles do not overlap, so that the lower limit of magnitude can be derived. On the contrary, the overlapped area, which may be regarded as the epicentral one, becomes so wide that it cannot be accepted as the actual area over which the seismic energy is accumulated when an unreasonably large magnitude is assumed. In such a way we may have a rough idea about the main shock magnitude.
As examples, the method is applied to the precursors of the Izu-Oshima Kinkai (M=7.0, 1978) and Niigata (M=7.5, 1964) earthquakes achieving a remarkable success in predicting the epicentral area and main shock magnitude, which are more or less the same as the actual ones.View full abstractDownload PDF (842K) -
Izuo OZAWA1987 Volume 40 Issue 2 Pages 225-234
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSContinuous observations of the crustal tilt by use of a water-tube tiltmeter have been recorded in the old Osakayama tunnel of former Tokaido-line since 1965. The effects of monthly precipitation on the monthly changes of the tilt have been calculated. Compensation for these effects has been made in calculating the amount of tilt for each month.
In orderr to detect anomalous changes of the crustal tilt in these observations, first the average values of each monthly tilt were estimated for every overlapping three years periods thus: 1966-1968, 1967-1969, 1968-1970 etc.. Next, the monthly deviations of the observed tilts from the average tilt were calculated. Anomalous tilts larger than monthly mean square errors of the average tilts have been noted.
Large anomalous tilts have been detected for the periods of 1967-1969, 1977-1978 and since 1984. The seismicity around the observatory was very active in the period 1967-1969. A check of anomalies since 1984 performed by comparing the automatic recordings with the visual measurings of the water-tube tiltmeter itself and another water-tube tiltmeter which is installed at deeper place in the tunnel and parallel to the first one. Also, the ambient temperature and the ground water level in the tunnel were measured to estimate their effect on the anomalies.
The schematic models of the anomalous crustal tilt are studied to interpret the mechanisms of this recent anomalous change.View full abstractDownload PDF (970K) -
Akira SANGAWA1987 Volume 40 Issue 2 Pages 235-245
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn the morning of September 27, 1611, a strong earthquake hit the Aizu Basin and its surroundings. The damage and its location are thoroughly described in old documents. Landslides occurred in many places on the upthrown side of the Aizu active fault system, which borders the western fringe of the basin. The earthquake fault occurred along the active fault system and dammed up the Nippashi (Ohkawa) River, forming a small lake. The altitude of the lake is inferred to have been 174.5-175m above sea level and depth about 2-3m. The lake is assumed to have been 2-2.5km wide and 4km long, which coincides with the description of old documents. Some villages were submerged and moved outside of the lake. Some part of the ancient Echigo Highway was also submerged and were rebuilt about 4km southward. The earthquake is believed to be caused by the Aizu active fault system. The magnitude of the earthquake is calculated at about M7.3, based on the vertical displacement (2.5m) of the earthquake fault.View full abstractDownload PDF (3479K) -
Masanori HORIKE1987 Volume 40 Issue 2 Pages 247-259
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe AKI and LARNER method has been extended to an absorbing medium with plural curved interfaces to predict earthquake motions on a sedimentary basin. Several features of a seismic response to a plane SH wave incidence were investigated using this new method. In comparison with the response by the Haskell method, the spatial change in wave form and duration time of S wave portions is much larger due to the generation of surface waves by curved interfaces. The effects of such parameters as number of layers with curved interfaces, Q value, and incident angle were also examined and the following results were obtained:
(1) The addition of a low-velocity surface layer makes a response larger in amplitude and longer in duration.
(2) The decrease of Q value mainly results in the decay of surface waves.
(3) The response to a small-angle oblique incidence is little different from that to a vertical incidence, but a large-angle oblique incidence generates large surface waves over the boundary edge where the incident wave arrives earlier.View full abstractDownload PDF (1128K) -
Tetsuo TAKANAMI1987 Volume 40 Issue 2 Pages 261-267
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (2738K) -
Shozo MATSUMURA1987 Volume 40 Issue 2 Pages 267-269
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (365K) -
Hiroo WADA, Takeshi MIKUMO, Makoto KOIZUMI1987 Volume 40 Issue 2 Pages 270-274
Published: June 25, 1987
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (526K)
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