Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
Volume 62, Issue 1
Displaying 1-7 of 7 articles from this issue
ARTICLE
  • Masayoshi TAJIKARA, Yasutaka IKEDA, Tsuyoshi NOHARA
    2009 Volume 62 Issue 1 Pages 1-11
    Published: August 31, 2009
    Released on J-STAGE: March 26, 2012
    JOURNAL FREE ACCESS
    The Iwate-Miyagi Nairiku Earthquake in 2008 occurred at June 14, 2008 in the area where active faults have not been mapped. We mapped fluvial terraces and measured the relative height between L1 and M1 surfaces around the source region of the Iwate-Miyagi Nairiku Earthquake, for the purpose of estimating location and nature of the source fault. Relative height between L1 and M1 surfaces is equivalent to amount of incision in the period between ages of L1 and M1 surfaces. We showed that amount of incision varies markedly around the focal region and that the variation of amount of incision is related to the activity of the Iwate-Miyagi Nairiku Earthquake in 2008. Difference of amount of incision between both sides of the source faults is about 60-65 meters. Assuming that incision and uplift are in balance, difference of incision is approximate to vertical displacement of the source fault. Supposing that L1 and M1 surfaces were formed in Marine Oxygen Isotope Stage 2 and 6, respectively, we can estimate the vertical displacement rate of the source fault at about 0.5 mm/yr. This value is similar to that of the Kitakami Lowland Western Margin Faults. We concluded that the location and nature of (blind) active fault can be estimated roughly even in areas with no clear fault scarp, by clarifying the distribution of amount of incision using heights of fluvial terraces.
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LETTERS
Dedicated Section : High strain rate region in the eastern margin of the Japan Sea and related seismic activities
ARTICLES
  • Masahiro KOSUGA, Satoshi IWABUCHI, Kazunori MURATA
    2009 Volume 62 Issue 1 Pages 27-46
    Published: August 31, 2009
    Released on J-STAGE: March 26, 2012
    JOURNAL FREE ACCESS
    We have investigated a stress field in the source region of the 2004 Mid-Niigata Prefecture (Niigata-Chuetsu) earthquake (M=6.8) by using P-wave polarity data obtained by a dense temporal seismic observation. The earthquake took place in central Japan near the eastern border of the Niigata-Kobe Tectonic Zone characterized by E-W contraction. The purpose of this study is to get insight into the stress heterogeneities in the source area including quite complex fault system with two parallel, westerly dipping fault planes and one conjugate plane. About two-thirds of well-constrained 514 focal mechanisms are reverse-fault type with WNW-ESE trending P-axes, which is consistent with the regional stress field in the area. Minor strike-slip events are distributed in the northern and central part of aftershock zone. Ten normal-fault events that are rare in the northeastern Japan are scattered mostly outside the major earthquake faults and occurred in a short period, suggesting that they reflect local stress field. Stress tensors derived from the focal mechanisms exhibit the compressional stress field characterized by nearly horizontal maximum principal stress that is consistent with the average direction of P-axes in the whole aftershock zone. Horizontal rotation of maximum principal stress axis is evident from WNW-ESE in the northeastern part of aftershock zone to E-W in the southwestern part. The direction of intermediate principle stress axis also changes from NE-SW in the northeastern part to NNE-SSW in the southwestern part. The fold axes in the area show similar strike variation. An interesting feature of stress distribution found in this study is the change in dip angle of the maximum principle stress axes. The axes through the aftershock zone dip gently to WNW, but those for earthquake clusters of mainshock fault dip steeply in a similar direction. Judging from the stress ratio, the horizontal compression is dominant near the mainshock hypocenter while the state in the surrounding areas is relatively close to isotropic stress. Though the difference in dip angle may not significant by considering the estimation error of stress axes, the combined anomalies with the stress ratio suggest that the mainshock occurred in an area of local stress inhomogeneity. We further examined the spatial distribution of earthquakes that have inconsistent focal mechanisms with the estimated stress tensors. Such earthquakes are distributed in the northeastern edge of the aftershock zone, in the southern edge of easterly dipping fault plane, and in a shallow earthquake cluster that is located between the faults of mainshock and the largest aftershock. The location of these events indicates small-scale stress heterogeneities in the source area superposed on the larger-scale stress variation mentioned above.
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  • Yosuke YAMAMOTO, Hiroshi TAKENAKA
    2009 Volume 62 Issue 1 Pages 47-59
    Published: August 31, 2009
    Released on J-STAGE: March 26, 2012
    JOURNAL FREE ACCESS
    The 2007 Niigataken Chuetsu-oki earthquake (MJMA6.8) occurred on July 16th, 2007. In this study, we estimate a source model of this earthquake through forward modeling of local strong motion records using the empirical Green's function method. The target frequency range of this modeling is broadband (0.3 Hz to 10 Hz). As the result, the first asperity was estimated that it was located on the NW-dipping fault plane including the hypocenter, and the rupture then transferred to the conjugated fault plane (SE-dipping plane). On the SE-dipping plane, two asperities are identified: one is located just near the first asperity (the second asperity); the other is off the coast of Kashiwazaki city (the third asperity). The estimated three asperities are located in the gaps of the aftershock distribution. Our source model can successfully reproduce strong motion in a wider area than the modeling area selected for source estimation. The estimated stress drop from our source model is almost 15 to 20 MPa, and it is similar to the stress drop empirically expected from the seismic moment and area of asperities.
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LETTER
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