All issues
Volume 46 (2007)
- Issue 6 Pages 445-
- Issue 5 Pages 399-
- Issue 4 Pages 305-
- Issue 3 Pages 167-
- Issue 2 Pages 83-
- Issue 1 Pages 1-
Volume 46, Issue 6
Displaying 1-9 of 9 articles from this issue
- |<
- <
- 1
- >
- >|
Special Issue on the Symposium “Tsumami Deposits and Seismo-turbidites : Understanding of Sediment Records for Earthquake Hazard Mitigation”
-
Osamu Fujiwara, Kazuhisa Goto, Kazuomi Hirakawa, Ken Ikehara, Fumihiko ...2007 Volume 46 Issue 6 Pages 445-450
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSThe Japanese Islands are at a high risk of earthquake and tsunami disasters, because of concentrated population and industries in the coastal areas facing troughs and trenches along active plate convergent boundaries. We are able to learn much about past earthquakes and tsunamis from the geological records, including tsunami deposits and turbidites, and to make use of such information for disaster prevention. Evaluation of the physical properties of past earthquakes and tsunamis from geological records, such as their size and recurrence intervals, would be useful for evaluating the risk of future earthquake and tsunami disasters.
Collaboration of scientists and engineers is needed to better understand past events and predict future events. This new attempt will solve the existing problems in the various study fields, e.g. geology and tsunami engineering. Better understanding of the risk of natural disasters in our milieu through these collaborative efforts will prevent future disasters.View full abstractDownload PDF (613K) -
Osamu Fujiwara2007 Volume 46 Issue 6 Pages 451-462
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSStudies of tsunami deposits have greatly progressed, both in the earth sciences and in disaster prevention technology, during the last 20 years. Understanding of tsunami depositional processes has been advanced by both field surveys (including sedimentological and paleontological research) and numerical experiments using computers and flumes.
The long-term recurrence history of tsunamis reconstructed from the geological record contributes to estimating future events and mitigating earthquake and tsunami disasters. Tsunami histories, about 7,000 to 10,000 years long for eastern Hokkaido and the Sagami Trough, and over 3,000 years long for the Nankai Trough, have been reconstructed from the geological records. Linkage of tsunami deposits, as indicators of tsunami height and inundation limits, and numerical tsunami simulations lead to progress in the use of fault models and estimation of tsunami inundation areas.
However, identification of tsunami deposits from other events, such as storms, is problematic, and accurate records of tsunami deposits are still few. Quantitative evaluation of tsunami wave heights, flow speeds, and inundation limits from their deposits will be the major targets for promoting the public use of paleo-tsunami data. Wide collaboration across various fields, such as geology, seismology, tsunami engineering, economics, and social science, will contribute to progress.View full abstractDownload PDF (1433K) -
Kazuhisa Goto, Fumihiko Imamura2007 Volume 46 Issue 6 Pages 463-475
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSSediment transport by tsunamis causes serious human injury and damage to buildings, coastal topography, and marine ecosystems. To understand processes of sediment transport by tsunamis, experimental and numerical studies as well as field investigations are important. In this study, we summarize experimental and numerical models for transport of sandy sediments or a large block, and then discuss their limitations and possible improvements. Although some problems and difficulties remain, these models generally are able to reproduce experimental results, and they can be adapted to the modeling of sediment transport by real-scale tsunami events. When these models have been improved to handle mixed grain sizes and multiple blocks, it will be possible to estimate hydraulic properties (hydraulic force, current velocity, and inundation area) of historical and prehistoric tsunamis more precisely by constraining the models with information on existing tsunami deposits and boulders. Moreover, sediment transport models are useful for estimating the spatial distribution of sediments eroded and redeposited by tsunamis ; thus, such models will be helpful in the selection of drilling sites for core samples in investigations of historical and prehistoric tsunami deposits.View full abstractDownload PDF (745K) -
Ken Ikehara, Kazuko Usami2007 Volume 46 Issue 6 Pages 477-490
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSDeep-sea turbidites have potential for paleoseismicity analyses, especially in connection with large earthquakes that occurred in marine areas. Seismo-turibidites related to the 1993 Hokkaido-Nansei-oki earthquake were found in three cores collected from Shiribeshi Trough and Okushiri Basin, in the northern Japan Sea, located near the epicenter of the earthquake. Turbidite in a core from Shiribeshi Trough has a clear upward fining graded structure and a thick turbidite mud, and contains benthic foraminifera living in a wide water depth range from shelf to basin floor. On the other hand, turbidite in a core from Okushiri Basin has no turbidite mud, and contains benthic foraminifera of shelf and upper slope. Because slope sediments in this area have higher mud content than shelf sediments, the difference in turbidite origins might result in the differences in thickness of turbidite mud among the cores.View full abstractDownload PDF (1905K) -
Fumihiko Imamura, Kazuhisa Goto2007 Volume 46 Issue 6 Pages 491-498
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSIt is rather difficult to reconstruct and understand the details of past tsunami events and behaviors because of limited information. Not only historical documents but also traces of tsunamis accumulated in the sediment can contribute to solving this difficulty. Although the documentation contains useful and valuable information to understand historical tsunami events, these descriptions of tsunami behavior are limited and there are many uncertainties involved. Thus, scientific evaluations of these data using the results from sedimentological study and numerical simulation with hydraulic models are required. In particular, since tsunami wave current and hydraulic force could be recorded in tsunami deposits, this information can be used for developing and calibrating the models. Study of sandy tsunami deposits is important to prepare for future tsunami events, because it is possible to understand the hydraulic properties of past tsunami events using sandy tsunami deposits. Not only the sandy particles but also the boulders transported by tsunamis are useful to understand past tsunami events. In this study, we introduce the numerical analyses of boulder transport by the 1771 Meiwa tsunami at Ishigaki Island, Japan.View full abstractDownload PDF (1008K) -
Shunichi Koshimura2007 Volume 46 Issue 6 Pages 499-508
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSUsing a GIS database and numerical modeling technique, the author developed a method of obtaining a quick estimate of the impact of a great tsunami disaster. The estimates can pinpoint the area where more precise damage detection technologies such as high-resolution satellite imagery or other remote sensing data should be deployed, to maximize a limited amount of time and resources. During the 2004 Indian Ocean tsunami disaster, we spent a great deal of time comprehending the overall damage within the entire Indian Ocean. The present research strongly suggests that research communities from various fields should collaborate to enhance the capability of quick and effective post-tsunami disaster response to the next possible tsunami disaster.View full abstractDownload PDF (1200K) -
Nobuo Shuto2007 Volume 46 Issue 6 Pages 509-516
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSExamples of topographic changes caused by tsunamis, as well as data on height and current velocity of the source tsunamis, were collected from documents. Tsunamis cause both erosion and deposition undersea and on land environments. Repeated tide-level change also accelerates to extend the openings cut by tsunamis on sand spits, tombolos, and sand bars. The extent of damage to coastal embankments caused by tsunamis is quantitatively explained with reference to the hydrodynamics of the tsunamis, including the wave height.
Depth of erosion and thickness of deposits resulting from tsunamis ranges widely from cm-order to meter-order according to the physical condition of the sites. However, numerical simulations have not fully succeeded in reproducing the scale of erosion and deposition. Improved hydraulic formulas are needed to accurately simulate the tsunami erosion and sedimentation in various environments. As a first step, a formula for the relationship between the sediment transport distance and physical properties of tsunamis, such as wave heights, is discussed.View full abstractDownload PDF (1031K) -
Yumi Sasaki, Toshiaki Irizuki, Kohei Abe, Jun-ichi Uchida, Osamu Fujiw ...2007 Volume 46 Issue 6 Pages 517-532
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSDetailed changes of fossil ostracode assemblages in Holocene deposits along the Tomoe River, Tateyama, Boso Peninsula, central Japan, were analyzed. At least seven layers of Holocene tsunami deposits are exposed at the study site. We focused on fossil ostracode assemblages from the lower two tsunami deposits (T2 and T2.1 ; ca. 8,000-8,100calBP and 7,900-8,000calBP, respectively) and normal deposits above and below them (lower mud, middle mud, and upper mud in ascending order). As a result, 124 ostracode species were identified from 134 samples. Normal deposits contain many well-preserved enclosed muddy bay species. On the basis of these ostracode data, paleo-Tomoe Bay at the study site was a calm muddy bay at water depths of about 10-15m at the time of the T2 and T2.1 tsunami events. Some peaks of mud content were recognized especially in the T2 tsunami deposits. Tsunami deposits with high mud content include many thin-shelled species of enclosed muddy bays ; those with low mud content yield hard-shelled species of open sandy shelves and outer bays. The T2 tsunami deposits include many open-sea species and contain rarely broken valves of middle sublittoral to upper bathyal species, while the T2.1 tsunami deposits contain many muddy middle bay species. One of the reasons for the difference in assemblage compositions between two tsunami deposits was that the density of muddy middle bay species just before the time of the T2.1 tsunami event was relatively higher at the study site. After the T2 and T2.1 tsunamis, values of species numbers and species diversity became gradually lower as we move upward in the sequence due to the deepening and enlargement of the bay and biogenic sediment mixing process.View full abstractDownload PDF (1904K) -
Jun-ichi Uchida, Kohei Abe, Shiro Hasegawa, Osamu Fujiwara2007 Volume 46 Issue 6 Pages 533-540
Published: December 01, 2007
Released on J-STAGE: March 26, 2009
JOURNAL FREE ACCESSForaminiferal tests are often yielded in tsunami deposits and provide important information on the source of sediment supply. Water depth of sediment source areas and transport distance from the source areas estimated from the foraminiferal tests may be useful criteria to identify tsunami deposits.
Tractive force by tsunami waves is in inverse proportion to the water depth at depositional sites. Horizontal distance of sediment transport by tsunami waves is proportional to the period and amplitude of the waves. Thus, the use of threshold amplitude and period of tsunami waves to explain sediment transport depends mainly on the grain size of sediment and the water depth of depositional sites.
Some washover and near-shore deposits yield foraminiferal tests transported long distances from the deep marine bottom. These deposits are likely to result from tsunamis with large wave periods and amplitudes. Foraminiferal tests reported from some tsunami deposits indicate that they originated from 100m or deeper sea floors and were transported long distances -up to several kilometers- to the coasts. However, the theoretical amplitude of tsunami waves that can explain the transport of the foraminiferal tests is unusually large. Change in the action of the tsunami water mass affected by local topography, such as submarine canyons, is convincing as a cause of the contradictions in sediment transportation estimated from foraminiferal tests and from theories of tsunami propagation.View full abstractDownload PDF (825K)
- |<
- <
- 1
- >
- >|