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Volume 30, Issue 2
Displaying 1-11 of 11 articles from this issue
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Takuji KOBORI, Yuzo SHINOZAKI1977 Volume 30 Issue 2 Pages 127-142
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThis paper deals with the scattering and diffraction of SH waves from surface irregularities of a soil medium. The boundary value problems are precisely formulated in terms of coupling integral equations, which are reduced to the finite difference equations to be easily evaluated.
Two types of irregularities are considered: Alluvial valley with an arbitrary cross section and a step-like irregularity in the surface of the soil medium. It is shown that the effects of irregularities on the amplitude characteristics of surface displacement are highly dependent upon not only incident angle and wavelength of SH waves but also the impedance ratios between the alluvial valley and the half-space and the configuration on the surface irregularities. The results are also compared with those derived from the flat-layer theory.View full abstractDownload PDF (1166K) -
Shoji SAKATA1977 Volume 30 Issue 2 Pages 143-149
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe change of Vp/Vs value before an earthquake corresponds to the change of the elastic constants of a part of the crust. The change of elastic constants distribution in the crust under initial stress field will induce strain changes. If those strain changes can be detected by ordinal geodetic surveys, they will serve as good earthquake precursors as well as Vp/Vs changes.
In order to estimate the values of expected strain changes, a simple plane strain model was adopted for calculation. The results are as follows.
(1) It is very difficult to detect any substantial horizontal strain changes using an ordinal one-wave optical distance meter.
(2) It is well possible to detect the tilting of the free surface as vertical movement by ordinal levelling.View full abstractDownload PDF (485K) -
Tameshige TSUKUDA, Kajuro NAKAMURA, Yoshimichi KISHIMOTO1977 Volume 30 Issue 2 Pages 151-162
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSOn September 21, 1973 an earthquake of magnitude 5.1 occurred in the vicinity of the Yamasaki fault, western Kinki district. In this region, it was the largest event we had experienced since the occurrence of the earthquake swarm of 1961.
Using data obtained at three temporary observation stations and routine stations of the Tottori Microearthquake Observatory, we determined hypocenters of the main shock-aftershock sequence. Large three shocks (M>3) including the main shock have their foci concentrated in a small volume of 1-2km in diameter at a depth of 11km, where many aftershocks are also located. Another concentrated region of aftershocks is located 2km south of the first one. Consequently the size of the aftershock area is about 2km×3km, which is roughly consistent with the Utsu-Seki's formula log A=M-4.
Focal mechanism of the main shock is of a thrust type, and its maximum pressure axis lies nearly horizontal and a N64°E direction. Such type of mechanism has not been found difinitely so far for the Yamasaki fault area where events of strike-slip are large in number. Major aftershocks are estimated to have rather different types of mechanism as compared to the main shock. This implies some complexity of focal process or faulting for this earthquake.
Aftershock activity had decayed in accordance with the formula N(t)=k·t-p (p=1.5) during the period of ten days after the main shock, and had continued for more than two months. Gutenberg-Richter's b value for the aftershocks is estimated to be 0.66-0.73.View full abstractDownload PDF (1084K) -
Yoshiteru KONO, Masao AMANO1977 Volume 30 Issue 2 Pages 163-178
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe effects of sedimentation on the thickening process of the lithospheric plate and surface heat flow are examined numerically. For this purpose, we revised the program for the thickening plate model which was used by KONO and YOSHII (1975) to take sedimentation into account.
The results are as follows:
(1) The effect of sedimentation is negligible when the sedimentation rate is smaller than 10-4cm/yr, which is a typical rate for the ocean basins.
(2) There is, therefore, no need to revise the results of KONO and YOSHII (1975) and YOSHII et al. (1976), which were calculated neglecting sedimentation.
(3) However, when the sedimentation rate is as high as 5×10-3cm/yr, which is some times realized in the marginal seas and ocean basins near continents, the surface heat flow is decreased considerably.
(4) The sedimentation prevents the thickening of the plate, but this effect is very small compared with the effect on the surface heat flow.
We applied the present model to describe the origin and development of the Japan Sea, a typical example of a maginal sea, to its present state. Two cases were considered for a date of opening of the Japan Basin; the first assume a date of opening of 25 M. Y. B. P. (Miocene) and the second a date of 50 M. Y. B. P. (Palaeogene). The calculations present favorable results for the hypothesis that the Japan Sea opened at about 25 M. Y. B. P. If, however, we take the uncertainty of the parameters used in the present article into consideration, it is difficult to conclude that the opening time of the Japan Sea is in Miocene time only from the present calculations. Therefore, we concluded that whether the opening of the Japan Sea is in Miocene or Palaeogene times, the history of the Japan Sea, based on the thickening plate model, is consistent with the present geophysical and geological observations.View full abstractDownload PDF (1628K) -
Tokuji UTSU1977 Volume 30 Issue 2 Pages 179-185
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSTwo kinds of probabilities p1 and p2 are considered in connection with the earthquake prediction, p1 is the probability that a prediction will be successful, and p2 is the probability that an earthquake will be predicted. Both “prediction” and “earthquake” have been defined by some criteria X and Y, respectively. The status of the prediction based on each observational element can be indicated by a point on the p1-p2 plane. The effectiveness of the prediction is related to p1 and p2 by an equation in the form E=p2(a-b/p1)-c where a, b and c are constants. If two or more observational elements are considered, p1 and p2 for the combined elements are calculated from p1 and p2 for each element under some assumptions. Formulas for such calculation have been derived. In these formulas the probability p0 that a random prediction (under criterion Y) will be successful plays an important role. For example, if a prediction is done when precursorlike anomalous phenomena are observed for two independent elements A and B, the probability of successful prediction is given by p1(A ∩ B)=1/1+(1/Ap1-1)(1/Bp1-1)/(1/p0-1) where Ap1 or Bp1 is the p1-value for element A or B alone. The effectiveness of the multielement prediction is discussed by using these formulas.View full abstractDownload PDF (568K) -
Masami OKADA1977 Volume 30 Issue 2 Pages 187-199
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe clustering in time and periodicity of earthquake occurrence are investigated statistically by the use of historical data of strong earthquakes in Tokyo. In order to discuss the periodicity of main shocks, the author examines the homogeneity of historical materials and the distributions of the intervals between any two earthquakes and between two successive earthquakes. It is found that one third to one fourth of events can be regarded as fore- and aftershocks in a wide sense. By the use of Monte Calro method, it is tested statistically whether the occurrence of main shocks is represented by a stationary random process or a periodic process. As a result, the 69-year periodicity, which is the highest peak in the periodgram calculated from Kawasumi's table, is not statistically significant at a 95% confidence level, but becomes significant if we lower the confidence level down to 80%. From the table of Usami and Hisamoto, the 36-year periodicity, which is the most predominant in their table, is found to be insignificant even at the 80% confidence level.View full abstractDownload PDF (1195K) -
Kiyoshi ITO, Tadashi ANNAKA1977 Volume 30 Issue 2 Pages 201-212
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe deep and intermediate-depth earthquakes tend to cluster spatially in the central part of Japan, the corner between the Izu-Bonin and the North-Honshu arcs. Focal mechanism solutions have been determined for the earthquakes in five clusters which are beneath the Sea of Japan, the Wakasa Bay, the Osaka Bay, the Kumanonada and Takayama.
Most of the mechanism solutions can be divided into three types in relation to the main feature of the seismic zone and each cluster has its own predominant types of focal mechanism. The axes of maximum compression (P axes) of the earthquakes beneath the Sea of Japan, the Osaka Bay and the Kumanoda dip nearly westwards, though the dip direction of the seismic zone changes from northwest to southwest. There are two types of focal mechanism in these regions according to their directions of the axes of minimum compression (T axes). T axis of one type is perpendicular to the seismic plane while the null axis (B axis) is pararell to the strike of the seismic zone (R-type). The other type has the interchanged B and T axes respect to each other (S-type). R-type is said to be the predominant type for the ‘down-dip compression’ mechanism and the solutions beneath the Osaka Bay seem to have this type but half of the solutions beneath the Sea of Japan and the Kumanoda have S-type. These S-type solutions probably reflect the local change of dip or strike direction of the seismic zone. The solutions beneath the Wakasa Bay and Takayama have a different type focal mechanism of which null axis is pararell to the dip of the seismic zone while T axis is pararell to its strike (N type). These solutions strongly reflect the effects of the arc-junction.View full abstractDownload PDF (1106K) -
Tadashi ANNAKA1977 Volume 30 Issue 2 Pages 213-225
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSFocal mechanism solutions of 60 deep and intermediate earthquakes (depths greater than 200km) which occurred during the period from 1931 to 1974 in the Izu-Bonin and the Northeastern Japan arcs except in Central Japan, where ITO and ANNAKA (1977) have investigated, have been determined from the first motions of P-waves. Fourteen well determined mechanism solutions published by the previous investigators were also adopted.
Regional variations of the focal mechanism have been studied in detail, with reference to the spatial distribution of hypocenters. The Izu-Bonin arc is divided into eight regions, and the Northeastern Japan arc is divided into two regions, based on the discontinuity of the spatial distribution of hypocenters or the change of the configuration of the deep seismic zone. The predominant types of the focal mechanism in each region are determined.
The axes of maximum compression for the deep earthquakes in most of the regions are dipping nearly westward in the dip angle about 35°. The local dip direction of the deep seismic zone changes remarkably in those regions, but it hardly influences the direction of the axis of maximum compression. The directions of the axes of minimum compression in those regions change from region to region, and they seem to be related to the configuration of the deep seismic zone. The axes of minimum compression are dipping nearly eastward, or nearly horizontal in the north to the south, for the earthquakes in the regions where the local dip of the deep seismic zone is nearly westward. On the other hand, they are dipping nearly northeastward for most of the earthquakes in the regions where the local dip of the deep seismic zone is nearly southwestward, or the configuration of the deep seismic zone changes abruptly.
For the intermediate earthquakes in two arcs, for the deep earthquakes in the southern end region of the Izu-Bonin arc, where the deep seismic zone is near vertical, and for some of the deep earthquakes in the Northeastern Japan arc, the axes of maximum compression aren't dipping nearly westward in the dip angle about 35°, but are dipping nearly northwest-ward. And the axes of minimum compression are approximately perpendicular to the deep seismic zone.
The hypocenters seem to be distributed on one plane for three regions where the earthquakes occur in small and isolated area. And the plane corresponds to one of the two nodal planes of the focal mechanism for the earthquakes in the region. It may suggest that the earthquakes are caused by the relative motion between two sides of the plane.View full abstractDownload PDF (1167K) -
Kazuo SHIBUYA, Akiko HORIE1977 Volume 30 Issue 2 Pages 227-230
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (420K) -
Hideki SHIMAMURA1977 Volume 30 Issue 2 Pages 230-233
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (2144K) -
Masanori SAITO1977 Volume 30 Issue 2 Pages 233-235
Published: August 05, 1977
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (348K)
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