The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 118, Issue 9
Displaying 1-7 of 7 articles from this issue
SPECIAL ISSUE The 2011 off the Pacific coast of Tohoku Earthquake: Towards an integrated understanding vol. 3
Articles
  • Shuichi Kodaira, Toshiya Fujiwara, Takeshi Nakamura
    2012 Volume 118 Issue 9 Pages 530-534
    Published: September 15, 2012
    Released on J-STAGE: January 26, 2013
    JOURNAL FREE ACCESS
    The large tsunami that followed the 2011 Tohoku-oki earthquake is thought to have been caused by a fault rupture that reached the Japan Trench, causing a large displacement in the seafloor. However, the actual cause of this tsunami is still unknown, as an exact up-dip limit of displacement associated with this fault rupture has not as yet been determined. In order to determine the seafloor displacement around the trench axis, we acquired multibeam bathymetric data in a region seaward of the epicenter soon after the earthquake, and compared these newly acquired data with bathymetric data acquired before the earthquake. Based on this comparison, we estimate that the seafloor on the landward side of the trench was displaced some 50 m horizontally towards the SE–ESE and 10 m upward by the earthquake. This large horizontal movement within steep slopes near the trench axis caused an effective up-lift of around 10–20 m near the trench, and numerical modeling indicates that this significant effective up-lift was the main cause of the large tsunami that followed the Tohoku-oki earthquake.
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Review
  • Yuki Sawai
    2012 Volume 118 Issue 9 Pages 535-558
    Published: September 15, 2012
    Released on J-STAGE: January 26, 2013
    JOURNAL FREE ACCESS
    After the 2011 Tohoku tsunami, demand increased greatly for geologists with expertise in the geology of tsunami deposits, who could study prehistoric tsunami deposits. This paper reviews, for those new to tsunami geology, how geologists have studied tsunami deposits during the last two decades. Preliminary identification of a paleotsunami deposit is based on the identification of anomalous sand deposits in peat or mud that was deposited in a low-energy environment. Anomalous deposits can then be evaluated using several criteria, such as lateral extent of the deposit, changes in thickness and grain size, sedimentary structures, single or multiple vertical grading, floral and faunal fossils within the deposits, synchroneity of environmental changes, and correlation with historical records of earthquakes. In this process, a multi-proxy approach is used to discount non-tsunami processes such as floods and storms. Identified tsunami deposits are dated using radiocarbon, lead-210, cesium-137, tephrochronology, and optically stimulated luminescence dating methods. For radiocarbon dating, appropriate selection of materials for analysis is especially crucial for constraining the age of a tsunami. Plant macrofossils or fossil insects within samples from sediments immediately over- or underlying a tsunami deposit will constrain its age well, whereas a bulk peat sample will not. The lateral extent of continuous tsunami deposits allows estimation of a minimum area of tsunami inundation. A meaningful reconstruction of the area of inundation, however, also requires adjustments for changes of the shoreline over the last few thousand years. In studies of paleotsunami deposits, we need to be aware of the uncertainties inherent in geological records. For example, geologic dating at a particular site rarely has the precision to distinguish between a single giant tsunami and a series of small ones.
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Articles
  • Hiroyuki Tsutsumi, Shinji Toda
    2012 Volume 118 Issue 9 Pages 559-570
    Published: September 15, 2012
    Released on J-STAGE: January 26, 2013
    JOURNAL FREE ACCESS
    The devastating 11 March 2011 Mw 9.0 Tohoku-oki earthquake ruptured a 500-km-long and 200-km-wide plate interface between the North American and Pacific plates along the Tohoku coast of northeast Japan. The large source area and huge amount of slip (up to 50 m) of this earthquake caused a drastic change in the crustal stress of northeast Japan and triggered widespread seismic activity. Iwaki City at the southeast corner of Fukushima Prefecture is an example of an area that has experienced significantly elevated seismic activity since the 11 March 2011 earthquake. The most striking characteristic of seismicity in this area is that the majority of the earthquakes have normal faulting mechanisms, contrasting significantly with the geodetically, seismologically, and geologically identified E–W compressional stress field of northeast Japan. This suggests that the dominant E–W tensional stress regime in this area may be transient and related to coseismic and post-seismic eastward movement of the northeast Japan Arc. The largest Mw 6.6 normal faulting event (JMA magnitude 7.0) in this area occurred on 11 April 2011 and ruptured two previously mapped faults: the NW-trending Yunodake fault and the NNW-trending Itozawa fault. A clear ∼15-km-long surface rupture of the Yunodake fault and ∼14-km-long rupture of the western trace of the Itozawa fault appeared during the earthquake, with a normal down-to-the-west sense of slip. The maximum vertical offset on the Yunodake fault is 0.9 m, with the western trace of the Itozawa fault having a maximum vertical offset of 2.1 m. We suggest that the 11 April 2011 earthquake was caused by reactivation of these two sub-parallel normal faults, caused by transient extensional stress related to the 11 March 2011 mega-thrust earthquake. Paleoseismic trenching across the western trace of the Itozawa fault revealed that the penultimate surface-rupturing earthquake occurred sometime between 12,500 and 17,000 cal. yr BP, suggesting that the fault was not activated by the 869 Jogan earthquake, which may have been the last mega-thrust earthquake before the 2011 event.
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Regular
Articles
  • Hideo Takagi, Shigenori Miwa, Tetsushi Yamada, Kei Nishijima, Satoshi ...
    2012 Volume 118 Issue 9 Pages 571-581
    Published: September 15, 2012
    Released on J-STAGE: January 26, 2013
    JOURNAL FREE ACCESS
    Measurements of temporal changes in the three-dimensional (3D) orientation and density of intracrystalline healed and sealed (open) microcracks in quartz grains have enabled the reconstruction of paleostress fields and determination of the history of microcracking in the Late Cretaceous Nojima Granodiorite of northern Awaji Island. This in turn has allowed the determination of the relationship between these features and the Nojima Fault, which was seismically reactivated during the 1995 Kobe Earthquake. The 3D orientations of healed microcracks (HC) indicate that σ3 stress directions had N–S orientations at almost all locations, whereas sealed microcracks (SC) preserve evidence of NW–SE and N–S σ3 orientations. K–Ar biotite ages for the Nojima Granodiorite and fluid inclusion formation temperatures (c. 320–370°C) suggest that the healed microcracks formed at around 80 Ma. Fission-track ages of associated pseudotachylytes indicate that initial movements on the Nojima Fault probably occurred at around 56 Ma, with spatial variations in microcrack density indicating that initial faulting occurred after HC formation but before SC formation. This indicates a NW–SE σ3 orientation during HC formation (after restoration incorporating the clockwise rotation of SW Japan at c. 15 Ma), with the subsequent formation of NE–SW and E–W trending high-angle SCs filled with limonite or iron oxides. These SCs probably formed at shallower crustal depths than the HCs. The fact that the σHmax SC orientation is not consistent with the current NW–SE or WNW–ESE regional stress field means that SC formation must have ceased by the Quaternary. In addition, the HC σ3 orientation is nearly normal to the general trend of the Median Tectonic Line, suggesting the existence of a significant NW–SE extensional stress field prior to the clockwise rotation of SW Japan, rather than a NW–SE regional compressive field that would be conformable with oceanic plate movement directions during the Late Cretaceous.
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Note
  • Go-Ichiro Uramoto, Kazuma Seike
    2012 Volume 118 Issue 9 Pages 582-587
    Published: September 15, 2012
    Released on J-STAGE: January 26, 2013
    JOURNAL FREE ACCESS
    Straub et al. (2009) suggest a metric that allows the quantitative detection of compensational stacking in channelized deposits based on a model whereby the standard deviation of channelized deposit sedimentation rate obeys a power-law function within a given measurement time window. Here, we outline this Straub method and use it to analyze channel deposits stacked within ancient rocks exposed in terrestrial outcrops. A power-law exponent, termed the compensation index κ, can be an indicator of the degree of compensational stacking, where κ = 1.0 for perfect compensational stacking, κ = 0.5 for random stacking, and κ = 0 for anti-compensational stacking. The κ values estimated from stratigraphic cross-sections across submarine-fan successions of the lower Kiyosumi Formation of central Japan can account for the stacking patterns of the channel deposits, and indicate that a combination of the Straub method with the examination of stratigraphic cross-sections allows for an improved understanding of the compensational stacking of channel deposits.
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Report
  • Tohru Ohta, Tomofumi Imai, Naoto Ishida, Yukiyasu Saka
    2012 Volume 118 Issue 9 Pages 588-593
    Published: September 15, 2012
    Released on J-STAGE: January 26, 2013
    JOURNAL FREE ACCESS
    This study re-examines the depositional age, lithostratigraphy, and distribution of Mesozoic sedimentary formations in the section of the Kurosegawa Belt that outcrops in the Toba area, eastern Shima Peninsula, Mie Prefecture, Japan. In this area, Mesozoic successions are divided into the Jurassic Shiranezaki Formation and the Cretaceous Matsuo Group. We collected Bajocian and Callovian–Oxfordian age radiolarians from the Shiranezaki Formation and Valanginian–Barremian age radiolarians from the Matsuo Group. The lithology and age constraints provided by these collected radiolarians suggest that part of the previously defined Matsuo Group actually forms part of the Shiranezaki Formation, with this formation being stratigraphically overlain by the Matsuo Group across a disconformity.
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