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Volume 35, Issue 4
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The Rotation of the Northern Part of Kanto DistrictTakuzo HIRONO1982 Volume 35 Issue 4 Pages 489-507
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSCHIHARA (1976) presented a hypothesis suggesting that the northern part of Kanto District, Japan, rotated 20° counterclockwise during Mesozoic-Palaeogene time about a center at the coast of Akita Prefecture. The Tanakura Fracture Zone, the eastern boundary of this rotated area, coincides with the front of the countercurrent against the Pacific Plate (CF). The countercurrent subducts from the Toyama Deep Sea Channel in the Sea of Japan, and reaches the surface of the subducting Pacific Plate at CF. This coincidence indicates that the rotational force driving the north Kanto District is the pressure of CF.
A 20° reversal of CF shows that there is possibility of further reversal of rotation, and the total angle of reversal attains 62°, involving three episodes each about 20° rotation of arc. During the first episode the old Ryoke Belt (RB) extending in the NE direction from its present situation over the Itoigawa-Shizuoka Tectonic Line (ISTL) was cut off at ISTL, and the Japan Island Arc bended 10° about ISTL. The position of CF came to the NW side line of the separated part of RB. During the second episode the RB massif was removed from its original location by the distance equal to its width, and the Pacific Plate suffers much deformation. During the third episode the massif was carried to the spot where Abukuma massif occupies now.
The rotation of CF caused the Pacific Plate to split into two parts along the Sagami Trough and its continuation to the NW direction. The split line starting from north reached an old trench axis off the Pacific coast by the end of the second rotation. Then it advanced eastwards along the present Sagami Trough with the trench deformed continuously until it reaches the present Japan Trench. Thus, the strain accumulated in the Plate during the first and second episodes dissolved totally in the course of the third episods.View full abstractDownload PDF (1766K) -
1982 Volume 35 Issue 4 Pages 507
Published: 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (70K) -
Akio YOSHIDA1982 Volume 35 Issue 4 Pages 509-521
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe statistical fault model for seismic spectrum is extended to include infinite number of correlation lengths. This model, compared to the usual statistical model which has one set of temporal and spatial correlation lengths related to the fault dimension, gives much enhanced spectral amplitude for high frequency range. In constructing the model we assume self-similarity for the distribution of correlation lengths. As is expected, the spectral density in high frequency range becomes larger by introducing more coherencies in small scale range.View full abstractDownload PDF (740K) -
Kin'ya NISHIGAMI, Tameshige TSUKUDA1982 Volume 35 Issue 4 Pages 523-537
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSInvestigation of the occurrence characteristic of earthquake swarms is one of effective ways to elucidate the generating mechanism of earthquakes. We analyzed Fukusaki earth. quake sequence, which occurred at the east end of the Yamasaki fault and consists of the main shock (M 4.3), 19 foreshocks and 99 aftershocks. From precise determination of hypocenters and P waveform analysis it was made clear that foreshocks and aftershocks have a clustering structure. Earthquakes in each cluster, which has a size of tens to two hundred meters, have very similar waveforms.
The most important characteristic of the cluster is that the largest event occurred at the final stage of each cluster's activity. From this we assume that one cluster corresponds to one weak area which has patches of weaker fracture strength, and that these patches break preceding the complete fracture of the whole area. This earthquake sequence occurred in two crushed zones (vertical and oblique) which are related to fractured zones of the Yamasaki fault. The main shock is considered to be a fracture of an asperity which existed in the oblique crushed zone.View full abstractDownload PDF (1185K) -
Yoshiteru KONO, Takeshi HIBI, Masayuki KUBO, Motoaki SUNAMI, Osamu MIC ...1982 Volume 35 Issue 4 Pages 539-545
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSGravity surveys were carried out at 2570 points over the northern part of central Japan (35°40′-37°20′N, 136°-138°20′E) by means of the LaCoste & Romberg gravimeter (model G) No. 348. Based on 5800 data including 3230 data measured by several institutions in and around the surveyed area, a complete Bouguer anomaly map was drawn over the central and northern parts of central Japan (35°20′-37°30′N, 136°-139°E). All of the data were refered to the International Gravity Standardization Net 1971.
Terrain correction term was calculated for all data over 80km (40′NS×60′EW) around each measurement point using the 500 meter-mesh mean topographic data file. Density was assumed to be 2.67gr. cm-3.
The complete Bouguer anomaly map thus obtained involves a lot of geophysical and geological information. Descriptions and analysis of the results will be given in succeeding papers.View full abstractDownload PDF (921K) -
Nobuhiro FURUSE, Yoshiteru KONO1982 Volume 35 Issue 4 Pages 547-556
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDetailed crustal structures in the northwestern part of the central Japan were obtained by taking recent detailed gravity data, crustal structure derived from the Atsumi- Noto explosion and geological information into consideration.
As the results, (1) undulation of the Moho derived from distribution of 1°×1°mean Bouguer anomaly agreed with that derived from explosion seismology; (2) the short wavelength Bouguer anomaly of less than 60km could be explained by the configuration of the crustal structure shallower than 10km. Therefore, the large negative Bouguer anomaly in the central part of the Japanese islands is presumably explained not only by undulation of the Moho but also by that of the shallower layers within the crust.View full abstractDownload PDF (864K) -
Yoshiaki IDA1982 Volume 35 Issue 4 Pages 557-565
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA graphic system that executes automatic tracing of curves has been developed to digitize contour maps, or other linear graphic data, such as seismograms. The graphic data are transferred by a TV camera to a video-frame memory as the distribution of the gray levels of 0 to 255 over 512×480 grid points covering the whole picture. A microcomputer traces a curve, successively seeking for a grid point with high gray level, and represents the curve in terms of a sequence of the chain codes that specify the directions of next grid points. The method was sucessfully applied to some simple examples of topographic maps.View full abstractDownload PDF (736K) -
Hwa-Chu SHEU, Masahiro KOSUGA, Hiroshi SATO1982 Volume 35 Issue 4 Pages 567-574
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe Hsinchu-Taichung earthquake (M=7.1) of April 21, 1935 is one of the greatest earthquakes in Taiwan. We analyzed this earthquake using the data of leveling, triangulation, and P-wave first motion. Surface deformation is modeled by a double-fault system in a elastic half-space on the basis of surface traces of the fault. The Hsinchu-Taichung event is a complex one characterized by a strike-slip and a reverse fault. The initial rupture inferred from fault plane solution, is a near vertical right-lateral strike-slip event striking N67°E. A seismic moment of this event is 9×1025dyne·cm. The reverse fault triggered by the initial strike-slip event is the main fault of the Hsinchu-Taichung event with seismic moment of 2.1×1026dyne·cm. An average stress drop is estimated to be about 40 bars. Strike of the reverse fault, N23°E, is consistent with the direction of motion of Philippine Sea Plate. Although tectonics of western Taiwan is a complex one, it may be largely affected by the movement of Philippine Sea Plate.View full abstractDownload PDF (645K) -
Kazuhiko IMAGAWA, Takeshi MIKUMO1982 Volume 35 Issue 4 Pages 575-590
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSeismic waveforms observed in the near-field during major earthquakes depend primarily on the dynamical rupture process on the fault, as well as on heterogeneous earth structures around the source, propagating path and recording sites. In order to investigate the faulting process of an earthquake, the expected ground motions are synthesized from record of several minor events such as foreshocks and aftershocks that occurred on the main shock fault, regarding these records as empirical Green functions which include the earth response. The technique used here is to convolve each of the phase delayed records with a specific time function correcting for the difference in the source function between the main shock and the smaller events, and to make summation over the entire fault surface.
The method described above has been applied to the case of the 1969 central Gifu earthquake (M=6.6), by using four aftershocks with magnitudes 4.3-4.8 which are distributed along the main fault. The results indicate that the synthesized waveforms provide a satisfactory agreement to long-period components (T>5 sec) of the JMA strong motion records at the Gifu station (Δ=51km), but that shorter-period waves (T=1-2sec) cannot be accounted for by this synthesis.
To provide a possible explanation to these short-period seismic waves, a stochastic fault model is tentatively introduced here with non-uniform displacements, slip angles and rupture velocities distributed over the fault. It is found that the most effective fault parameter to generate the short-period waves is the variation of the rupture velocities, and that this effect is enhanced with preserving long-period components if the rupture propagates with different velocities in large-scale fault segments. The synthesized waveforms, which have been obtained from the four aftershock records incorporating this type of incoherent rupture propagation, could better explain the general features of the strong-motion records including both of long- and short-period waves.
Since the above model with the complicated rupture process is only tentative, it seems necessary as a next step to examine the effects of earth structure on the recorded waveforms more closely, by synthesizing theoretical waveforms from the present model.View full abstractDownload PDF (1433K) -
Takeshi NAKAJIMA1982 Volume 35 Issue 4 Pages 591-606
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSRecently, in situ stress measurements have been carried out in Japan by various methods. Measured stresses, however, are apt to be different from regional tectonic stress field, because of such disturbing factors as topography, geological structure, material properties of rocks, and so on. Since the measurements are made at merely shallow sites, topographic effects on tectonic stress field are expected to be larger than other factors. Especially in mountainous area, they may become considerable.
In order to estimate topographic effects, a three-dimensional finite-element analysis is applied to some simple models, which represent mountainous area, on the assumption of homogeneous elastic properties. Furthermore, to geographically realistic models around measuring sites, the same analysis is applied.
The results show that (1) at relatively shallow points, stresses affected by topography are generally different from regional stress acting around the model, (2) therefore, measured stress obtained at shallow sites should be put in a topographic correction, but that (3) such topographic effects become negligible beneath the depth comparable to an amplitude of topographic undulation.View full abstractDownload PDF (1503K) -
Depth Distribution of the BasementTakashi TADA1982 Volume 35 Issue 4 Pages 607-617
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn order to detect buried faults under the plain and to estimate the activity of active faults, the structure of basement under the Kanto Plain was studied by the synthetic analysis of explosion seismic data, gravity anomaly data and deep test well data.
The depth of basement undulate in the western part; the depth of basement exceeds 3500 meters at the deepest point and is shallower than 1000 meters at the shallowest point. In the central part, the basement is comparatively flat and the average depth is about 3000 meters. The depth of basement in the western part shallows monotonously northeastward. Some dome structures, graben structures and fault like structures were detected under the Kanto Plain.View full abstractDownload PDF (1035K) -
The Meaning and Comparative Study of MethodsSadaiku HATTORI1982 Volume 35 Issue 4 Pages 619-621
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
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Kinya OKADA, Tetsuo SANTÔ1982 Volume 35 Issue 4 Pages 621-623
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
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Hiroshi SATO1982 Volume 35 Issue 4 Pages 623-626
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (340K) -
Yoichiro FUJII1982 Volume 35 Issue 4 Pages 626-629
Published: December 25, 1982
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (406K)
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