The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 117 , Issue 2
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Articles
  • Yoshie Nasuhara, Yusuke Kashima, Takashi Nakamura, Tsuneo Yamauchi, Ke ...
    2011 Volume 117 Issue 2 Pages 63-78
    Published: 2011
    Released: June 02, 2011
    JOURNALS FREE ACCESS
    To detect the precursors of deep groundwater changes related to anticipated large earthquakes off Miyagi Prefecture, we selected two deep boreholes located on the hanging walls of active reverse faults, and established an observation network with a real-time, automatic data-acquisition system. After taking into account the hydrological conditions and the effects of barometric pressure, the earth’s tide, and rainfall, as well as the resolution limits of temperature measurements of groundwater for the two boreholes, we identified plausible earthquake-related groundwater changes during the observation period (4 April 2004 to 31 December 2007). One of the events was a pre-seismic 0.003°C fall in groundwater temperature that began 2.7 hours prior to the main shock (MJ 6.6) of the 2 December 2005 off-Miyagi Prefecture earthquake. The second event was a preseismic 0.0012°C rise in groundwater temperature that started 2 hours before a shallow earthquake (MJ 4.5) located beneath Sendai Bay on 12 April 2007. The third event was a coseismic temperature fall by as much as 0.02°C associated with a shallow earthquake (MJ 4) on 29 May 2007, located close to one of the observation sites. The resolution limit of our temperature observation system indicates that it has the potential to detect the precursors of future large earthquakes off Miyagi Prefecture if associated with preseismic slip as large as that for a MJ 6.5 earthquake.
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  • Takeshi Makinouchi, Mai Kato, Yasuo Ohishi, Masayoshi Tsukamoto, Keiji ...
    2011 Volume 117 Issue 2 Pages 79-94
    Published: 2011
    Released: June 02, 2011
    JOURNALS FREE ACCESS
    Anjo City is divided topographically into the Hekikai terrace (which represents the main area of the city), the Yahagigawa lowland (eastern area), and the Aburagafuchi lowland (southwestern area). The city is underlain by Holocene sediments, the Hekikai and Koromo formations, the Tokai Group, and basement rocks, in descending order. The Jomon transgression, which deposited a marine clay bed in the lower half of the Holocene sediments, caused inundation up to the midstream parts of narrow valleys carved into the Hekikai terrace. The Hekikai Formation, which forms the Hekikai terrace, consists of a sand member (upper half of the formation) and marine clay member (lower half), and is 16–36m thick. The marine clay and sand members correspond with the bottomset and foreset beds of a delta, respectively. The age of the Hekikai Formation ranges from about 120 to 70ka. The transgression that deposited the marine clay member of the Hekikai Formation started from Kinuura Bay (southwestern part of the Nishimikawa Plain) and inundated the western half of the city area. The upper part of the Koromo Formation consists of alternating beds of sand and mud, and is well consolidated compared with the Hekikai Formation. The basal surface of the Koromo Formation is located at an elevation of about –100m a.s.l. in the southwestern part of the city, –45m in the central part, and –25m in the northeast. A generalized geological cross-section through the Nishimikawa Plain is proposed as a tentative model of the local geology.
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