Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
Volume 60, Issue 1
Displaying 1-4 of 4 articles from this issue
ARTICLES
  • Fuyuki HIROSE, Junichi NAKAJIMA, Akira HASEGAWA
    2007 Volume 60 Issue 1 Pages 1-20
    Published: August 25, 2007
    Released on J-STAGE: August 20, 2013
    JOURNAL FREE ACCESS
    We estimated three-dimensional seismic velocity structure in and around the Philippine Sea plate subducting beneath southwestern Japan by applying the double-difference tomography method to arrival time data of earthquakes obtained by the dense nationwide seismic network (Kiban-network). Alow S-wave velocity and high Vp/Vs layer shallowly dipping toward the subduction direction of the slab and several km thick has been clearly imaged at locations immediately above the top of intraslab seismicity in a wide area from Tokai to Kyushu. Comparison with the location of the upper surface of the Philippine Sea slab estimated from seismic refraction surveys at four survey lines shows that this low S-wave velocity and high Vp/Vs layer corresponds to the crust of the Philippine Sea slab. This indicates that many of intraslab earthquakes are occurring in the slab mantle, suggesting that the location of the upper plate interface previously estimated only from spatial distribution of intraslab seismicity is not correct. Based on the presently obtained location of the low S-wave velocity and high Vp/Vs layer and hypocenter distribution of relocated intraslab earthquakes, we estimated the configuration of the upper surface of the Philippine Sea slab in the whole area of southwestern Japan.
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  • Kenji HIRATA, Kenji SATAKE, Shigeru YAMAKI, Yuichiro TANIOKA, Yoshiko ...
    2007 Volume 60 Issue 1 Pages 21-41
    Published: August 25, 2007
    Released on J-STAGE: August 20, 2013
    JOURNAL FREE ACCESS
    We compiled eyewitness accounts of the first tsunami arrivals of the 1952 Tokachi-oki earthquake (Mt8.2), obtained east of Kushiro, Hokkaido, from previously published reports in order to estimate the northeast edge of the tsunami source. Large scatter in reported tsunami travel times obstructed to determine the edge uniquely. A careful evaluation of the reports discriminated reliable tsunami travel times from not-reliable ones. In addition, all the reported tsunami travel times were judged as the first wave peaks rather than the first wave onsets. The reliable travel times indicate that the northeast edge of the 1952 tsunami source extended to at least off Akkeshi Bay, and probably farther east.
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  • —Its Relation with the 1978 Miyagi-Oki Earthquake (M 7.4) —
    Tadashi YAGINUMA, Tomomi OKADA, Akira HASEGAWA, Kenichi KATO, Masayuki ...
    2007 Volume 60 Issue 1 Pages 43-53
    Published: August 25, 2007
    Released on J-STAGE: August 20, 2013
    JOURNAL FREE ACCESS
    On August 16, 2005, a large earthquake (M 7.2) occurred along the plate boundary of off Miyagi Prefecture (Miyagi-Oki), northeastern Japan. In this area, large earthquakes (∼M 7.5) have occurred repeatedly at intervals of about 37 years, and it has passed more than 27 years since this event occurred. To understand the relation between this earthquake and the previous 1978 event, we estimated the coseismic slip distribution of the 2005 Miyagi-Oki earthquake by the seismic waveform inversion method. We used the same structure model used in the analysis of the 1978 event. By comparing the coseismic slip distribution of the 2005 Miyagi-Oki earthquake of this study with those of previous studies, we investigated the reliability and our result seems to be plausible. Then we compared our result with the previous 1978 Miyagi-Oki earthquake and found that the rupture area of the 2005 event overlapped with the southeastern part of the 1978 earthquake’s rupture area. This result supports the idea that there exist plural asperities which cause the sequence of Miyagi-Oki earthquake, and the 2005 event ruptured part of such asperities, while the previous 1978 event ruptured all of the prural asperities at one time.
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LETTER
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