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Volume 36 (1997)
- Issue 5 Pages 283-
- Issue 4 Pages 217-
- Issue 3 Pages 147-
- Issue 2 Pages 73-
- Issue 1 Pages 1-
Volume 36, Issue 2
Displaying 1-5 of 5 articles from this issue
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Osamu Fujiwara, Fujio Masuda, Tetsuya Sakai, Keisuke Fuse, Akira Saito1997 Volume 36 Issue 2 Pages 73-86
Published: May 31, 1997
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSThe Holocene marine terraces along the Boso and Miura peninsulas show repeated coseismic uplift at intervals in the thousands of years. However, there is little geologic evidence for inferring paleoseismic events around the Sagami Trough.
Three tsunami deposits generated by earthquakes were first found in Holocene bay-floor muds of Tateyama, in the southern part of the Boso peninsula.
The tsunami deposits consist of sand and sandy gravel beds in sequences that are increasingly finer upward, with abundant shell and wood fragments, unconformably covering the bay-floor muds. Molluscan shells in the beds indicate such mixed environments as muddy bay-floor, neritic sandy-floor, and coastal rocky bottom. Paleocurrents, in both landward and seaward directions, can be obtained from the sedimantary structures in some beds, although most bedsshow strong bioturbation.
These tsunami beds were deposited respectively 6, 300-6, 000yrs BP, 4, 800-4, 700yrs BP and 4, 500-4, 400yrs BP, as dated by the radiocarbon method in this study. The age of the lower bed agrees with the ages of the highest Holocene marine terraces along the Boso (Numa I Terrace) and Miura (Nobi I Terrace) peninsulas, and the middle and upper beds coincide with the emergence of the middle Holocene marine terraces along the Miura (Nobi II Terrace) and Boso (Numa II Terrace) peninsulas, respectively.
Other sand and gravel sheets are recognized in the Holocene. The ages of these beds, dated by this study, show they were formed every 100-200 years between 7, 400yrs BP and 3, 600yrs BP. These beds show sedimentary structures and fossil contents similar to the preceding tsunami deposits. Some beds may be tsunami deposits coincident with the emergent wave-cut benches on the Numa Terraces (Kayanne and Yoshikawa, 1986).View full abstractDownload PDF (4455K) -
Ryuta Hataya, Kazuhiro Tanaka, Yuji Saito, Tetsuo Hashimoto, Satoshi S ...1997 Volume 36 Issue 2 Pages 87-96
Published: May 31, 1997
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSWe investigate the applicability of sediment dating by ESR and TL, which are based on the assumption of the complete optically bleaching of some radiation defects in quartz grains in the processes of erosion, transportation, and/or sedimentation, and show that the TL method is probably appropriate for dating of the marine terrace sediments. We have carried out optical bleaching experiments using several light sources, and dated a marine terrace sediment whose age had already been estimated from geological information by the ESR and TL methods. The Ti signal is more light-sensitive than E'1 and Al signals, and is bleached completely by illumination. However, the age obtained using the Ti signal is much older than that estimated by geological data. On the other hand, the TL intensity decreases with illumination, approaching a constant value. This result shows that the TL consists of a light-sensitive component and a residual one, and that we can obtain the intensity of a light-sensitive component easily by an optical bleaching experiment using ultraviolet light. The TL age obtained using the light-sensitive component is in good agreement with geological data.View full abstractDownload PDF (1313K) -
Takashi Uchiyama, Masayuki Hyodo, Shusaku Yoshikawa1997 Volume 36 Issue 2 Pages 97-111
Published: May 31, 1997
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSArtificial ponds have been constructed over the past 2, 000 years in the Kinki district of Japan. Accumulated sediments covering the bottoms of two reservoirs, the Sayamaike and Hataoike ponds situated in Osaka Prefecture, were well exposed during the repairing of retention dams. Paleomagnetic studies were carried out with oriented samples directly collected from these bottom sediments. Consequently, high-resolution records of the geomagnetic secular variation were obtained. Based on the correlation with the secular variation reported from archaeomagnetism and sedimentary magnetism, it was discovered that the Sayamaike sediments, which are 630cm thick, record the changes of paleomagnetic directions for the past 500 years and the Hataoike sediments, 320cm thick, recorded those for the past 400 years. The behavior of fine fluctuations is found in both declination and inclination from 1500 to 1800 A. D. The Sayamaike deposits intercalate twelve horizons, designated as S-1-S-12 in ascending order, that indicate sign of liquefaction by earthquake and/or flooding. The results of paleomagnetostratigraphic dating suggest that horizon S-9 at 380cm and S-12 at 220cm date from around 1500-1530 A. D. and 1600-1630 A. D., respectively. The sediments range from the bottom to the S-9 horizon in the Sayamaike pond, around 250cm in thickness, and have magnetization of an almost uniform direction (D=8.2°, I=40.9°, α95=1.2°, k=263.4). The definite direction may suggest that magnetic grains in the sediments were once unlocked and were realigned with the geomagnetic field at the time of the earthquake that caused the liquefaction of the S-9 horizon.View full abstractDownload PDF (1592K) -
Michinobu Kuwae, Shusaku Yoshikawa, Yoshio Inouchi1997 Volume 36 Issue 2 Pages 113-122
Published: May 31, 1997
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSIn 1986, Takashima-oki core boring was carried out in the central part of Lake Biwa. The sediment samples are composed mainly of silty clay sediments extending over the past ca. 400, 000 years. The sediments are intercalated with many volcanic ash layers, and thus are rich in time control. We analyzed for diatom with high-resolution on the sediments, and have obtained the following results,
1. The variations of the total number of diatom valves in 1g dry sample, which is interpreted as the fluctuations of diatom productivity, have occurred widely off the main basin of Lake Biwa.
2. The profile of the total number of diatom valves during the past 400, 000 years shows good correlation with the marine oxygen isotope profile. The fluctuation of the total number of diatom valves in Lake Biwa reflects global climate changes sensitively. That is to say, the total number of diatom valves increases under warm and wet paleoclimatic conditions, and decreases under cold and dry conditions.
Stages 1 through 9 of the marine oxygen isotope stratigraphy can be recognized. Moreover, for the short-time fluctuations of the total number of diatom valves, correlation can be made with the detailed marine oxygen isotope stratigraphy. The Biwako diatom horizons (BDH) with rare diatom valves are well correlated with the coldclimate events of the marine oxygen isotope stratigraphy. The BDHs at 8.5m, 19.0m, 28.0m, 35.3m, 43.3m, 53.6m, 63.5m, 80.2m, 86.9 and 93.5m in depth correspond to stages 2.2, 4, 5.2 (5b), 5.4 (5d), 6.2, 6.4, 6.6, 7.4, 8.2 and 8.4, respectively.View full abstractDownload PDF (1197K) -
Tomio Otsuka, Nobuyuki Takahama, Hiroomi Nakasato, Satoshi Nomura1997 Volume 36 Issue 2 Pages 123-136
Published: May 31, 1997
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSThe authors have identified four liquefaction phenomena in tephra layers in the Karasu river valley, Gunma Prefecture, in central Japan. The last liquefaction must have occurred between 1108 and 1783, because the material spouted by the liquefaction penetrates the tephra layer of 1108 and is covered by the tephra layer of 1783. The other three liquefactions are judged to have occurred after 10, 000yrs BP, at about 17, 000yrs BP and between 21, 000 and 17, 000yrs BP, respectively, based on their stratigraphic horizons in the tephra layers.
The Gunma district has been considerd to be at low risk for earthquakes, because it has only a few active faults and has had only one historical earthquake, in 818. However, the last liquefaction reveals that a previously unknown historical earthquake occurred after 818, suggesting a higher frequency of earthquakes in this area than was previously thought.
In ancient liquefactions, “draw-in” phenomena can be distinguished from “intrusion-spout” ones . Although no previous papers have reported about the draw-in process, it is important because it follows the intrusion-spout process during the final stage of the liquefaction. Thus, comprehensive observation of all phenomena including the draw-in process is necessary for complete understanding of liquefaction.
Tephra layers contain a great quantity of geological information in addition to the ancient liquefactions reported here. We should not only use tephra layers as chronological sources but also study the information recorded in them.View full abstractDownload PDF (4126K)
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