Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
Volume 55, Issue 3
Displaying 1-9 of 9 articles from this issue
  • Spatial and Temporal Distributions of b-value East off NE Japan
    Fuyuki HIROSE, Ayako NAKAMURA, Akira HASEGAWA
    2002 Volume 55 Issue 3 Pages 249-260
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    We found a clear b-value variation associated with the rupture of asperities based on investigations of spatial and temporal distributions of frequency-magnitude relation for earthquakes in the northeastern (NE) Japan subduction zone. We used the Tohoku-University earthquake catalogue in the period from January 1, 1981 to October 3, 2001 after correcting artificial magnitude shift and removing artificial events. Spatial distribution of b-value is compared with the locations of asperities estimated on the plate boundary east off NE Japan. Estimated b-value decreases with increasing depth. Anomalously high b-values are observed in two areas off-Iwate and off-Fukushima regions, and asperities do not intrude into them. Temporal variation of b-value for presently analysed six asperities shows a remarkable increase in b-value after the rupture of each asperity. Then the b-value decreases gradually. These observations suggest that b-value reflects the stress change occurring in the vicinity of the asperities.
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  • Hiroyasu MABUCHI, Masakazu OHTAKE, Haruo SATO
    2002 Volume 55 Issue 3 Pages 261-273
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    We expect the existence of upper limit in the size of earthquakes, which may vary from region to region under the influence of the tectonic environment, stress distribution, structural heterogeneity and so on. In this paper, we investigated the possibility of estimating the maximum possible magnitude M* based on a modified G-R model of frequency distribution of earthquake magnitude.
    The modified G-R model we used was proposed by Utsu (1971) as
    logn (M) =a-bM+log (Mc-M),
    where Mc is the maximum magnitude estimated from the magnitude-frequency distribution model. We estimated Mc by using 9734 globally distributed earthquakes with moment magnitude equal to or larger than 5.5 that are listed in the Harvard Centroid Moment Tensor Catalogue for the last 25 years.
    We estimated three model parameters a, b and Mc by the maximum likelihood method. The error in Mc was also estimated by taking the maximum range of 1σ error ellipsoid on the assumption that the error of maximum likelihood estimates follows a bivariate normal distribution. We estimated Mc=8.61±0.12 for all the data. Dividing the whole data into data subsets by focal depths and seismic regions, we found depth dependence and remarkable regional dependence for shallow earthquakes (h≤60km) in the Mc value. The comparison with the earthquake catalogue for the past about 100 years showed that Mc estimated from the data of 25 years can predict the maximum magnitude in the longer period with an uncertainty of ±1-2. These results indicate that there exists a strong regionality in Mc Mc may be used, to some extent, for estimating the maximum earthquake magnitude inherent to the region.
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  • Sadaki HORI
    2002 Volume 55 Issue 3 Pages 275-284
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    To examine availability of moment tensor data for local earthquakes obtained from broadband seismic observation, we made a comparison of earthquake mechanisms between moment tensor solutions and first motion solutions for earthquakes occurring beneath the Kanto-Chubu district, central Japan. Agreement of these two solutions is good as a whole. Large difference between the first motion and moment tensor solutions is, however, observed for a considerable number of earthquakes.
    No systematic relationship is found between the quality of the solution and the difference between them. The difference thus cannot be explained by uncertainty in determination of the mechanism solutions. Difference of first motion and moment tensor solutions seems to become weak for deeper earthquakes. The direction of principal axes (P- and T-axes) of first motion solutions is much scattered than that of moment tensor solutions among neighboring events. It is also recognized that earthquakes with large difference in mechanism solutions are occurring in the two clustered regions; off Ibaraki prefecture, and near the Miyake-jima island. There would be a number of small faults in a well-ruptured region such as the Miyake-jima island region. It is possible that the orientation of these small faults is considerably scattered because of the local heterogeneity of strength in the crust. Suppose that a slip along one of them triggers an earthquake occurrence along a different main fault. The first motion solution of this event must be dominated by the orientation of the starting small fault, but the moment tensor solution represents the main fault, which is dominated by the surrounding tectonic stress. In this case the first motion solutions are different from event to event. In contrast the moment tensor solutions are almost the same among neighboring events. From the present study it is concluded that the moment tensor solutions are more suitable to discussion of local seismotectonics.
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  • Tomiichi UETAKE, Tomonori IKEURA
    2002 Volume 55 Issue 3 Pages 285-290
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
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  • Shigekazu KUSUMOTO, Toshiyasu NAGAO, Masao KOMAZAWA, Takemi ISHIHARA
    2002 Volume 55 Issue 3 Pages 291-294
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
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  • Hiroyuki TAKAYAMA, Akio YOSHIDA
    2002 Volume 55 Issue 3 Pages 295-300
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
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  • Toshikazu YOSHIOKA, Yasuo AWATA, Koichi SHIMOKAWA, Hiromi ISHIMOTO, Mi ...
    2002 Volume 55 Issue 3 Pages 301-309
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
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  • Ichiro NAKANISHI
    2002 Volume 55 Issue 3 Pages 311-316
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
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  • Toshiki KAKUTA, Kazuhiko GOTO
    2002 Volume 55 Issue 3 Pages 317-336
    Published: December 15, 2002
    Released on J-STAGE: March 09, 2010
    JOURNAL FREE ACCESS
    We reviewd characteristics of seismic activities in and around Kyushu and the northern half of Nansei Shoto (Ryukyu Islands), mainly on the basis of hypocenter distributions and focal mechanisms. Interplate and intraplate earthquakes, inclusive of intermediate-depth events, considerably differ in seismicity from region to region along the arc. Most of the boundaries between the seismic regions correspond to left-lateral tectonic lines geologically inferred and/ or narrow zones of crustal earthquakes of left-lateral strike slip types across the arc. Thus we surmise that the Philippine sea plate spiitted into several segments subducts with low coupling under the Kyushu-Ryukyu arc.
    Not only rifting along the Okinawa trough but also southeastward spreading from the Shanshi graben with left-lateral movements is probably important for crustal earthquakes of normal and strike-slip fault types dominant in an extensional stress field in the marginal sea and inland areas.
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