The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 123, Issue 10
Displaying 1-10 of 10 articles from this issue
Special Issue Paleotsunamis of Japan
Preface
Review
  • a review of case studies from Sendai and Hirota Bays
    Daisuke Sugawara
    2017 Volume 123 Issue 10 Pages 781-804
    Published: October 15, 2017
    Released on J-STAGE: January 25, 2018
    JOURNAL FREE ACCESS

    This review summarizes geological research on the tsunami caused by the 2011 Tohoku Earthquake, with a focus on the sediment dynamics and budget, and relationship between hydraulics and deposit characteristics, in two areas with different settings. The tsunami sedimentation in Hirota Bay, Sanriku Coast, was characterized by offshore transport of beach sediments due to an intensive backwash. Onshore transport of marine sediments was inferred from paleontological data, as well as visual observations and numerical modeling of the tsunami. In contrast, geological evidence, video footage, and numerical modeling suggest that onshore transport of marine sediments was limited in Sendai Bay. These differences are best explained by the topographic and bathymetric settings of these sites, and the properties of the tsunami wave. The field of tsunami sedimentology has been significantly advanced through geological studies of the Tohoku tsunami deposits. Further challenges are identified regarding the application of the Tohoku tsunami deposit research to paleotsunami studies and hazard assessment.

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  • Focusing on the tsunami deposits of Northern Territory and fault models of the western Pacific coast of Hokkaido
    Yasuhiro Takashimizu
    2017 Volume 123 Issue 10 Pages 805-817
    Published: October 15, 2017
    Released on J-STAGE: January 25, 2018
    JOURNAL FREE ACCESS

    Studies of recent and paleo-tsunami deposits from coastal lowlands along the Pacific coast of Hokkaido, northern Japan, are described with a focus on deposits in the Northern Territory (Hoppo Ryodo). Seafloor fault models facing the southern Kuril Trench and northern Japan Trench on the Pacific side of Hokkaido were also considered. The number of tsunami deposits found on Shikotan Island, (located close to the Kuril Trench), off the Pacific coast of Hokkaido, during the past ~7000 yr is more than those found on Kunashiri Island. This indicates that the coastal lowlands of Shikotan Island can record both major and minor tsunami events. In contrast, Kunashiri Island is only capable of recording large-sized tsunami events on its coastal lowlands, because large tsunamis can reach the island. Reconstructions using several seafloor fault models on the Pacific side of Hokkaido can explain the distribution of tsunami deposits in coastal lowlands. The significance of fault models for “Tohoku-hokubu-oki (off the northern Tohoku)” is pointed out for the source area along the western Pacific coast of Hokkaido. Tsunami deposits in the Iburi area of Hokkaido prefecture and Higashidori area of Aomori prefecture indicate that only one tsunami deposit has been preserved in this area, in the past 2500 to 2800 years.

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  • Yuki Sawai
    2017 Volume 123 Issue 10 Pages 819-830
    Published: October 15, 2017
    Released on J-STAGE: January 25, 2018
    JOURNAL FREE ACCESS

    This paper reviews research on paleotsunami deposits in the Tohoku region, which began with investigations of lacustrine deposits along the coastal area of the Japan Sea in the 1980s. Subsequently, deposits of the 1611 Keicho, 1454 Kyotoku, and 869 Jogan tsunamis were identified. There is abundant historical and geological evidence of the 1611 tsunami, although its source is still unclear, with possibilities including offshore of Sanriku coast or the southern Kuril Trench. The other two historical tsunamis (1454 and 869) are modeled as having an offshore Sendai source, with a moment magnitude (Mw) of greater than 8.4–8.6. In addition, tsunami deposits older than the 869 Jogan event have been reported in the Tohoku region, but their sources remain unknown.

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  • Their achievements and upcoming challenges
    Osamu Fujiwara, Koichiro Tanigawa
    2017 Volume 123 Issue 10 Pages 831-842
    Published: October 15, 2017
    Released on J-STAGE: January 25, 2018
    JOURNAL FREE ACCESS

    Paleotsunami studies within the Nankai Trough region started in the late 1990s and have contributed to our understanding of the tsunami history of this region over the past 6000 yr. Notably, the discovery of unusually thick tsunami deposits on the western Nankai coast suggests that exceptionally large tsunamis have occurred in this area. This has led to a hypothesis that there is a “hyperearthquake cycle” of 300-500 yr, which is distinct from the ordinary Nankai Trough earthquake cycle of 100-150 yr. However, the number of reliably documented tsunami deposits is still too small to accurately reconstruct the size and source area of each paleotsunami. In this review, we propose two main fields as future research directions for tsunami deposit research in the Nankai Trough region. The first involves verifying the presence or absence of great earthquakes during the historical period for which there is no written record of Tokai or Nankai earthquakes. Although the last three Tokai and Nankai earthquakes (i.e., after the 18th century) occurred in pairs with a short time interval between them (<2 yr), it remains unknown if the older Tokai and Nankai earthquakes also occurred in pairs. This is fundamental for improving our understanding of the fault rupture pattern along the Nankai Trough subduction zone. The second field of research involves quantification of the size of past tsunamis. Quantification of paleotsunami inundation height and area is needed to verify the “hyperearthquake cycle”, and to delineate the maximum tsunami size that should be adopted in hazard mitigation measures.

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  • Kazuhisa Goto
    2017 Volume 123 Issue 10 Pages 843-855
    Published: October 15, 2017
    Released on J-STAGE: January 25, 2018
    JOURNAL FREE ACCESS

    Paleotsunami research along the Ryukyu Trench has been undertaken for ca. 50 years. In particular, studies of tsunami-transported boulders have shown that large tsunamis have repeatedly occurred in the Sakishima Islands along the southern Ryukyu Trench, whereas no large tsunamis have affected the Amami and Okinawa islands along the northern and central Ryukyu Trench in the past 2300 yr. These observations are supported by seismological research and imply that earthquakes and tsunamis occur unevenly along the Ryukyu Trench. In contrast to studies of tsunami-transported boulders, sandy and gravely deposits related to tsunamis have been less extensively studied. Future research on such sandy deposits is expected to reveal more detailed paleotsunami histories, which include small- to moderate-scale events.

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  • Gentaro Kawakami, Yoshihiro Kase, Atsushi Urabe, Yasuhiro Takashimizu, ...
    2017 Volume 123 Issue 10 Pages 857-877
    Published: October 15, 2017
    Released on J-STAGE: January 25, 2018
    JOURNAL FREE ACCESS

    Extensive research has been conducted on tsunamigenic deposits along the eastern margin of the Japan Sea since the 2011 Tohoku Earthquake and tsunami. This paper reviews the characteristics of the large known tsunamis and the possible tsunamigenic deposits identified in this region. We have also reconstructed the spatio-temporal distribution, and a provisional correlation, of the tsunamigenic deposits in order to estimate the tsunami recurrence intervals and their wave source regions.

    Most of the tsunamigenic deposits were recognized in peaty and marshy deposits found on swales of coastal dunes, flood plains, and small valley plains. Large numbers of tsunamigenic sandy layers were also identified in lagoonal deposits. In addition, some gravelly deposits of possible tsunamigenic were recognized on top of low-lying marine terraces and slope talus.

    Although several historical tsunamis in the 18-19th centuries have been recorded along the eastern margin of the Japan Sea, there are few localities where deposits can be correlated to these records. Tsunamigenic deposits suggest that four tsunami events have occurred during the 9-14th centuries, despite there being limited historical records of tsunamis older than the 18th century. The distribution of these four deposits is as follows: 14th century = Aomori-northern Yamagata; 12th century = southwestern Hokkaido; 11th century (?1092 AD) = Sado/Niigata-southern Yamagata; 9th century (?850 AD) = (Sado)-Yamagata-Aomori. These events are also recorded in the deep-sea Japan Basin as seismo-turbidites. Most tsunamigenic deposits older than the 9th century have only been identified at remote islands (Okushiri, Tobishima, and Sado).

    Several unanswered research questions on these tsunamigenic deposits remain, such as robust identification of the origins and precise ages of the deposits. Our work provides a framework for future screening of potential tsunamigenic deposit data and an improved understanding of paleotsunami events along the eastern margin of the Japan Sea.

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Pictorial
Special Issue Recent Progress and Future Perspectives of Metamorphic Studies in Japan. Part 2
Review
  • precise analyses on ultrahigh-temperature metamorphism
    Yasuhito Osanai, Nobuhiko Nakano, Tatsuro Adachi
    2017 Volume 123 Issue 10 Pages 879-906
    Published: October 15, 2017
    Released on J-STAGE: January 25, 2018
    JOURNAL FREE ACCESS
    Supplementary material

    Ultrahigh-temperature (UHT) and ultrahigh-pressure metamorphism indicate extreme metamorphic conditions in regional metamorphic settings. In particular, the metamorphic temperature of UHT metamorphism is the highest limit of crustal metamorphism, exceeding 900°C and is equivalent to the temperature of basaltic magma. Typical UHT metamorphic rocks have Mg-Al-rich bulk chemical compositions and are easily identified by a characteristic mineral assemblage of sapphirine + quartz±garnet, orthopyroxene + sillimanite±quartz, or garnet + osumilite + quartz. Precise reconstruction of the metamorphic evolution of UHT metamorphic rocks on the basis of metamorphic reaction textures, mineral inclusion studies, pressure-temperature estimations, and micro-geochronological analyses is a powerful tool for understanding orogenic processes. In the last 25 years, Japanese geoscientists have highly activated for the research works on the UHT metamorphic rocks worldwide, including in the Napier Complex of East Antarctica, southern Indian peninsular, Highland Complex of Sri Lanka, Kontum Massif of Vietnam, Inner Mongolian Suture Zone in north China, Limpopo Belt in Africa, Lewisian Complex in Scotland and other terranes. These geological and petrological (including experimental petrology) investigations have led to the publications more than 150 research papers. In this review paper including small amount of our new data, we outline our research activities on four different types of Archean-Proterozoic to Cretaceous UHT metamorphism, as well as dehydration melting experiments on F-bearing biotite under UHT conditions.

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Articles
  • Hirokazu Maekawa, Keiko Murata
    2017 Volume 123 Issue 10 Pages 907-919
    Published: October 15, 2017
    Released on J-STAGE: January 25, 2018
    JOURNAL FREE ACCESS

    A zone of dome-shaped diapiric seamounts occurs along the trench axis in the Mariana forearc. The seamounts mainly comprise blocks of serpentinized peridotite of varying sizes and fine-grained serpentinite matrix. The serpentine minerals are mostly chrysotile and/or lizardite, but antigorite is also found in peridotites recovered from the Raijin, Conical, Twin Peaks, Big Blue, Celestial, and South Chamorro seamounts. Antigorite is commonly associated with iron-rich secondary olivine (Fo86–89) as rim overgrowths or along the cleavage traces of primary olivine (Fo90–92). The assemblage is indicative of high-temperature serpentinization at ~500°C. Chrysotile and/or lizardite veins, both predating and postdating antigorite formation, are also present in antigorite-bearing samples. This may reflect a complex serpentinization history, suggesting the tectonic migration of serpentinized peridotites from the shallow mantle wedge to greater depths, and then back to the surface. Blueschist-facies rocks recovered from the Conical, Twin Peaks, and South Chamorro seamounts have a variety of mineral assemblages and chemistries, suggesting a wide range of metamorphic conditions, but all formed under low-temperature, blueschist-facies conditions (150–300°C, 0.5–0.7 GPa). The protolith of some blueschist-facies clasts from the Twin Peaks Seamount was amphibolite. The clasts contain metamorphic pargasite, Al-rich epidote and rutile, which are indicative of higher-grade epidote-amphibolite-facies metamorphism prior to blueschist-facies metamorphism. It is likely that these samples were initially metamorphosed at depths of ca. 40 km within the subduction zone, and then ascended along the subduction boundary to depths of 20–25 km, where they were metamorphosed under blueschist-facies conditions. Although the progressive metamorphic evidence cannot be detected from metamorphic rocks, the P-T histories obtained from the serpentinized peridotites and metamorphic rocks are broadly consistent, implying that both the serpentinized peridotites and metamorphic rocks experienced similar transport histories within the subduction zone.

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