The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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SPECIAL ISSUE: Recent progress on the three-dimensional geological modeling - Part 2-
Preface
Articles
  • Haruko Sekiguchi, Kimiyuki Asano, Tomotaka Iwata
    2019 Volume 125 Issue 10 Pages 715-730
    Published: October 15, 2019
    Released: January 10, 2020
    JOURNALS FREE ACCESS

    We constructed a three-dimensional (3-D) velocity structure model of the Nara basin, Japan, combining geologic knowledge, geophysical surveys, and seismological observations to contribute to the improvement of ground motion prediction for future earthquakes. Few geophysical surveys have been conducted on the underground structure in the Nara basin. However, geologic studies suggest that the sediments filling the Nara basin and those filling the Osaka basin have commonality. Therefore, we utilized the methods and knowledge acquired through modeling the velocity structure of the Osaka basin to construct a velocity structure model of the Nara basin. First, an isochronous surfaces depth model was constructed by combining the bedrock-depth distribution estimated from gravity anomalies and the geologic profile at a borehole near the center of the Nara basin. Then, the isochronous surfaces depth model, which contained the 3-D structure of depositional age, was converted to a model of the 3-D structure of seismic velocity and density. Microtremor observation results were used to constrain the 3-D velocity structure model with respect to the seismological response of the ground. The ability of the velocity structure model to reproduce ground motion was assessed through a ground motion simulation for a small earthquake.

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Regular Section
Articles
  • Shin'ichi Mori, Yujiro Ogawa
    2019 Volume 125 Issue 10 Pages 737-757
    Published: October 15, 2019
    Released: January 10, 2020
    JOURNALS FREE ACCESS

    A series of outcrops, 250 m long and 20−40 m high, were exposed during construction of a factory at Sajima, Yokosuka City, Kanagawa Prefecture, central Japan. Before these outcrops were covered with concrete, we described the stratigraphy and structures in detail. The upper part of the Misaki Formation (Aburatsubo Member) of the Miura Group (roughly 3−5 Ma in age) dips gently northeastward and is 60 m thick. The sequence consists mainly of marine pyroclastic sediments, including mafic scoria, lithic fragments, tuffaceous sandstones, and felsic pumice−ash beds, which are centimeters to several meters thick. Several characteristic tephra layers are determined as key beds and numbered from Sj 1 to Sj 21, in stratigraphically ascending order, among which the tephra key bed Sj 20 is correlated with the previously known key bed So. Several thrust-duplex-like structures were recognized in parts of the outcrop, and we discuss here the formation processes of four representative duplex-like structures. One group of duplex structures developed along thrust faults that are orientated oblique to the bedding and are spatially associated with cataclasites. These duplex structures were tectonically induced and of late-stage origin. The other three duplex-like structures developed along the bedding surfaces and were associated with submarine landslides. These gravitationally induced structures are associated with smaller duplex-like structures and show slump-like folds. The directions of thrusting and submarine sliding are different from the modern attitude of depositional beds in the Aburatsubo Member, and suggest an original transport direction from NW to SE.

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  • Satoshi Nakano, Kuniaki Makino, Izumi Yoshida, Kanae Maniwa, Kazuhiko ...
    2019 Volume 125 Issue 10 Pages 759-773
    Published: October 15, 2019
    Released: January 10, 2020
    JOURNALS FREE ACCESS

    The international color parameters (L*, a*, and b*) of four granite alkali feldspars, determined by a new method using a scanner and personal computor system show a successive variation of reddish colors. Small reddish particles and whitish micropores were observed using a binocular microscope. Raman spectroscopy showed that the common reddish particles are hematite in all four feldspar samples, and showed maghemite in the G175 feldspar. Distributions of Fe-oxide particles and micropores were assessed by electron microprobe mapping, and their area percentages in the feldspars were obtained using image processing. The results suggest that the size distribution and number density of both hematite and micropores control the reddish color variation in the alkali feldspars, with some additional influencing factors such as the presence of maghemite.

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Report
  • Masaya Miyoshi, Kyoko Saito, Takashi Sano, Junko Fujii
    2019 Volume 125 Issue 10 Pages 775-779
    Published: October 15, 2019
    Released: January 10, 2020
    JOURNALS FREE ACCESS

    We report whole-rock chemical compositions and the K-Ar age of obsidian gravel in a Miocene conglomerate in the Mikuni coastal area, Fukui prefecture, central Japan. The obsidian gravel samples are generally aphyric, and some contain crystallites. The estimated K-Ar age of the obsidian gravel sample is 13 Ma, within the age range of Miocene volcanic rocks (basalt to rhyolite) present in the Mikuni coastal area (14-12 Ma). The obsidian gravel samples plot at the SiO2-rich end of the Miocene volcanic rocks when comparing major elements versus SiO2. The MORB-normalized trace-element pattern and K-Ar age of the obsidian gravel are similar to those of the Miocene volcanic rocks around the Mikuni coastal area. These observations indicate that the origin of the rhyolite magma that produced this obsidian is associated with that of the Miocene volcanic rocks in the Mikuni coastal area. The rhyolitic magmatic activity occurred synchronously with the formation of these Miocene volcanic rocks, after the opening of the Japan Sea.

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