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Volume 38, Issue 1
Displaying 1-16 of 16 articles from this issue
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Takashi TADA1985 Volume 38 Issue 1 Pages 1-12
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSThe N-S extensional strain field in the Kyushu is understood as the result of the crustal deformation caused by the rifting and the spreading of the Okinawa Trough which crosses the Central Kyushu in the E-W direction. The spreading velocity and the spreading direction of the Okinawa Trough in the Kyushu have been estimated as 1.4cm/a and N-S direction, respectively, by the analysis of the distance survey. The horizontal and the vertical crustal movements, the normal active faults, the gravity anomaly, the volcanism, and the fault plane solutions of the earthquakes in the Shimabara Peninsula suggest that the Unzen Volcanic Graben is an active rift of the Okinawa Trough. The beginning of the spreading of the Okinawa Trough in the Kyushu has been estimated to be during the period from 0.8Ma to 0.5Ma B. P. by the spreading velocity and the width of the Unzen rift. The distributions of the active faults, the low gravity anomaly belts, the hypocenters of the shallow earthquakes and their fault plane solutions suggest that Okinawa Trough in the Kyushu consists of the three rifts whose strikes are E-W direction and the three transform faults like shear fault zones whose strikes are the NE-SW direction.View full abstractDownload PDF (1136K) -
Ryuji IKEDA1985 Volume 38 Issue 1 Pages 13-23
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn order to investigate the detailed underground structure and fault displacement, we carried out geophysical explorations such as observation of explosion seismic waves, electrical sounding and resistivity well log around the estimated basement fault called the Karasuyama-Sugaonuma fault. Synthetic analysis of these data revealed that the underground structures from the pre-Tertiary basement to shallower layers have the following characteristics. (1) scale basement fault that has a gap more than 1000-1500m, extends SSE-NNW from Sugaonuma A large to Sashima-cho, (2) the eastern side of this fault, the basement rises at a gentle slope toward Mt. Tsukuba, accompanying a small scale basement fault and (3) shallow layers above the basement are much disturbed around these faults.
By investigating resistivity well log data at 33 shallow wells, we found steep inclination of overriding Quaternary layers that is harmonious with the offset of the large scale basement fault. The slope decreases monotonously from deeper to shallower layers belonging to the Shimosa Group. Those facts strongly suggest that the fault activity has continued through the Quaternary period. Rate of the vertical displacement is roughly estimated to be 0.04mm/year.View full abstractDownload PDF (1327K) -
Takeshi MIKUMO, Makoto KOIZUMI, Hiroo WADA1985 Volume 38 Issue 1 Pages 25-40
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSeismicity and focal mechanism of earthquakes in the northern Hida region, central Japan, are closely investigated, and seismo-tectonic features of this region are discussed, mainly on the basis of five years' data obtained at seismic network stations of the Kamitakara Geophysical Observatory in cooperation with some other observatories.
More than 5, 800 earthquakes with magnitudes greater than 0.5 have been located. It was found that high seismicity is concentrated along the Atotsugawa fault and around the Ushikubi and Mozumi faults, all of which are typical, Quaternary active faults in this region, and just beneath the Hida mountain range, which is highest in the Japan Islands. Also noticeable is relatively high seismicity around conjugate sets of minor faults northeast of the Atera fault, and in the southwest part of the Toyama plain.
Most of larger earthquakes along and around the Atotsugawa fault indicate strike-slip type mechanisms consistent with right-lateral slip motion that had been continued since Quaternary ages. Earthquakes at shallow depths beneath the northern part of the Hida mountains also show strike-slip mechanisms with a nodal plane nearly parallel to the axis of the mountain range, and sometimes with a small component of normal faulting. In the southern part of the mountains southeast of Mt. Ontake, however, two different types of earthquakes, one with strike-slip, another with reverse faulting, are concentrated in a limited area, suggesting complex structures there. All earthquakes northeast of the Atera fault indicate strike-slip mechanisms.
The maximum pressure axes (P-axes) derived from these focal mechanism solutions are found to be oriented nearly in the ESE-WNW direction (N60°-80°W) over the northern Hida region, but with small, counter-clockwise local deviations in some areas. The direction is parallel to those inferred from the conjugate sets of Quaternary faults in this region and derived from geodetic triangulation surveys, and is also consistent with the direction of relative motion between the Pacific and Eurasian plates rather than between the Eurasian and North American plates. This suggests that the northern Hida region including the Hida mountains appears strongly subjected to the tectonic compressive stress due to the plate movements.View full abstractDownload PDF (1718K) -
Shuzo TAKEMOTO, Kunihiko WATANABE, Kensuke ONOUE, Kunihiro SHIGETOMI, ...1985 Volume 38 Issue 1 Pages 41-55
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSOn May 5, 1984, a moderate earthquake with a magnitude of 4.6 occurred at the border of Kyoto and Osaka prefectures in Japan. Accompanying the earthquake, strain steps of the order of 10-7-10-9 were observed with extensometers at seven crustal movement observatories in the Kinki district. Observed strain steps were compared with theoretical values expected from the dislocation fault model. Some of them are found to be one or two order larger than the theoretical ones. This discrepancy can be attributed to mechanical instability of instruments or local inhomogeneities around the observation sites. Observed data are fairly well consistent with the expected values, except the extremely large ones.View full abstractDownload PDF (1542K) -
The Yamasaki Fault RegionTohru MOGI, Susumu NISHIMURA, Kazuo MINO, Taroh SADAHIRO1985 Volume 38 Issue 1 Pages 57-66
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSGeological and geophysical surveys were carried out in the eastward extended area of the Yamasaki fault. In this area, any faults have not been recognized, but many microearthquakes occur.
From the geological survey, three remarkable outcrops and some topographical evidences which show the existence of the fault were found in this area. The hidden fault was traced by γ-ray and ELF·MT surveys. γ-ray intensity was measured at 12sites. The places where high γ-ray intensities are obtained, are approximately on a straight line in the direction of N70°-80°W.
The ELF·MT surveys have been conducted in the Yashiro and Sanda regions. The spatial distribution of the obtained apparent resistivities reveals the existence of low resistivity belts along the fault which was inferred from the geological surveys. These belts have a width of more than 1.5km in both regions. The results of these surveys show that this new fault named the Yashiro fault, running from the Fukusaki to the Sanda regions in the Hyogo prefecture, is the eastward extension of the Yasutomi fault that is a part of the Yamasaki fault system. On the other hand, the results of the surveys also suggest that the Yashiro fault extends to the Jumantsuji fault which is one of the members of the Arima-Takatsuki tectonic line.
The large fault which consists of a series of three faults, Yasutomi, Yashiro, and Jumantsuji faults, bounds this region into two blocks. They show different patterns in each region in the topography, the distribution of active faults and the occurrence of microearthquakes.View full abstractDownload PDF (1379K) -
Takashi MIYATAKE1985 Volume 38 Issue 1 Pages 67-79
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSIn many earthquakes, aftershock activity tends to cluster near the endzone of the fault. This clustering is expected from elastic theory, i. e., two dimensional crack causes high stress region near two endzones of the fault. But, in many shallow strike-slip earthquakes, we can find only one of them. In this paper, we will solve why the clustering is activated near either of the endzones. We study aftershock distributions and source processes of inland long, shallow faults in which we can easily distinguish the clustering near the endzones. It is shown that in bilateral case we can see two clusters at two endpoints of rupture propagation, but in unilateral case one cluster at an endpoint of rupture. The dynamic crack model shows that accelaration, particle velocity and dynamic stress change near the stopping point of rupture are larger than those near the starting point. This may cause much activation of aftershocks near the stopping point of rupture than near the starting point, i. e., hypocenter of main shock.View full abstractDownload PDF (1273K) -
Ken'ichiro YAMASHINA, Kazuaki NAKAMURA, Takaaki FUKUDOME, Tamao SATO, ...1985 Volume 38 Issue 1 Pages 81-91
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSBased on a few tens of photographs, height change of Kyuroku-shima Island, which locates very close to the focal region of the Japan Sea earthquake (M=7.7) on May 26th, 1983, is analyzed. 1) The island and adjacent small islets are considered to have subsided by about 30-40cm associated with the 1983 earthquake. The average offset obtained by 13 reliable photographs is 32±9cm. 2) Precursory deformation and secular change of the height were not found during 1964 through 1982. No detectable deformation was caused by largest aftershock (M=7.1) on June 21, 1983, and other aftershock activity. 3) Coseismic tilt and local collapse were not observed in the island. 4) In some cases, a height can be inferred even from snapshots with the error less than several percents. Crustal uplift and subsidence more than 10-20cm may be possible to be detected photographically at the seashore and lakeside.View full abstractDownload PDF (4228K) -
Kennosuke OKANO, Shozo KIMURA, Tadashi KONOMI, Masao NAKAMURA1985 Volume 38 Issue 1 Pages 93-101
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSFrom examining the spatial and depth distributions of earthquakes in Shikoku and its surrounding regions, some interesting inferences are drawn, as described in the following. The extremely low activity of crustal earthquakes shallower than about 10km depth in the northern Kii channel between Shikoku and the Kii peninsula is supposed to be attributed to hydropressure in the uppermost part of the crust, resulting from permeation into rocks with water. This feature is observed from place to place in Japan. The Median Tectonic Line is a boundary between the two regions of different distributions of crustal earthquakes.
Intermediate-depth earthquakes beneath Kyushu and mantle earthquakes beneath Shikoku are generally considered to be intraplate earthquakes in the Philippine Sea Plate generated by its subduction under the Eurasian Plate. The subducting direction of the plate is nearly E-W for Kyushu and NW-SE for Shikoku. It is natural to expect, therefore, that fissures produce between the two plates and thus earthquakes scarcely occur in the fissure region. However, such phenomena are not noticed from the distribution of microearthquakes. Crustal and mantle earthquakes beneath Shikoku occur in zones dipping down and up from north to south, respectively, and then the two seismic zones appear to make contact each other at the south of about 20km from the Muroto promontory.View full abstractDownload PDF (1096K) -
Hiroshi OGASAWARA, Takeshi NAKAJIMA1985 Volume 38 Issue 1 Pages 103-113
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSAn “8-element-gauge” method is one of the stress relief techniques for the in situ stress measurements. Using this method, the measurements have been carried out at twelve sites by the Research Group for Crustal Stress in Western Japan. During the in situ processes and the data analyses for these measurements, many problems have been encountered. These problems were investigated in detail and important ones were settled. Hence the “8-element-gauge” method was significantly improved. Based on the results of the investigations, the stress values were revised into more reliable ones. In addition, characteristics of this method were examined in detail. The examinations show that the stress values are unavoidably affected by systematic errors associated with distinctive eigen vectors of the Jacobian matrix of the observation equation, unless certain conditions, such as properties of rocks, are satisfied. They also show that the stress values for several sites are suspected to include considerable amount of these errors.View full abstractDownload PDF (1164K) -
Tadayasu SAIJO, Akira HASEGAWA, Norihito UMINO, Akio TAKAGI1985 Volume 38 Issue 1 Pages 115-124
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA large number of local microearthquakes have been observed at a high-sensitivity seismograph station near the Kamafusa Dam since the filling of reservoir started in 1970. Microearthquake activity near the dam site abruptly increased two months after the start of filling and this reservoir-induced seismicity lasted about 18 days. About three years later seismicity near the dam site increased again. The high seismicity period lasted six years including seven major earthquake swarms. These earthquake swarms occurred at a regular interval of 14.5 months except one which accompanied the 1978 Miyagiken-oki Earthquake with magnitude 7.4, the distance between the epicenter of this event and the dam site being about 130km. Focal areas of these earthquake swarms, without overlapping each other, form a belt-shaped epicenter distribution trending to the NW-SE direction, which is almost the same as the strike of the fault plane of the largest eathquake (M=4.6) that occurred near the dam site since the filling of reservoir.View full abstractDownload PDF (1006K) -
Akio YOSHIDA1985 Volume 38 Issue 1 Pages 125-136
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSeismic activities in and around the Izu Peninsula during the period from 1926 through 1982 are investigated. In this sea region earthquakes are apt to occur in the shallow areas, and some earthquake swarms seem to be related to volcanism. Several “seismic gaps” are recognized in this region, and the possibility of earthquake occurrence in the “gap” observed at the Oiso hills, the north-west coast of Sagami Bay, is examined with reference to the location of active faults, crustal movements during the Quaternary, and historical data of earthquakes as well. The most conspicuous features in space-time distributions of earthquakes in and around the Izu Peninsula are “non-superpositionality” and “migrationality”. Succesive active regions do not overlap each other and have a tendency to move to adjacent areas. The meaning which these characteristics suggest on seismogenesis is preliminarily discussed.View full abstractDownload PDF (1359K) -
Keiichi NISHIMURA, Wataru MORII1985 Volume 38 Issue 1 Pages 137-140
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
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Fumiaki TAKEUCHI, Fumio AMAIKE1985 Volume 38 Issue 1 Pages 141-143
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (386K) -
Kyozi TAZIME1985 Volume 38 Issue 1 Pages 143-144
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (213K) -
Junji KOYAMA1985 Volume 38 Issue 1 Pages 144-146
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (376K) -
Kei TAKANO1985 Volume 38 Issue 1 Pages 147-150
Published: March 25, 1985
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSDownload PDF (312K)
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