Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
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Volume 60, Issue 2
Displaying 1-6 of 6 articles from this issue
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ARTICLES
  • Characteristics of the Short-Period Seismic Wave Radiation from the Historical Megathrust Earthquakes along the Nankai Trough
    Masayuki TAKEMURA, Katsuhisa KANDA
    2007 Volume 60 Issue 2 Pages 57-69
    Published: October 25, 2007
    Released on J-STAGE: August 20, 2013
    DOIhttps://doi.org/10.4294/zisin.60.57
    JOURNAL FREE ACCESS
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    Geographical relationships between short-period seismic wave radiation zone (SPRZ) and large slip area (called asperity in this study) on the fault plane were examined for the 1944 Tonankai and the 1946 Nankai earthquakes in the Showa era, and reappearances of the SPRZs from the Ansei and the Hoei events were also investigated. The SPRZs were evaluated from the inversion analysis of seismic intensity data in the previous study. Most of the centroids of the SPRZs were found at almost the same areas in every event and they were located at the edge of corresponding asperities of the Showa Nankai and Tonankai earthquakes in the forward direction of fault rupture. This suggests that the short-period seismic waves were radiated from the terminal slip points of asperities of the events also in Ansei and Hoei eras on assumption of the same rupture propagation process as those of the Showa events. The sequence of these megathrust events along the Nankai trough included both single-segment rupture events and the multiple-segment rupture events. Magnitude of the short-period seismic wave radiation was deflned and calculated for each SPRZ of the analyzed earthquake in the previous study. We used the result to discuss an influence of multiple-segment rupture for the excitation of the short-period seismic waves from asperities. It was found that the sum of the magnitude values of the SPRZs for a segment of the multiple-segment rupture event was larger than that for the single-segment rupture event occurring from the same segment.
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  • The Age and Offset of Faulting Events on the Sekidosan Fault in the Southeastern Margin of the Ouchigata Plain, Central Japan
    Nobuhiko SUGITO, Takashi AZUMA, Kiyohide MIZUNO, Hiroyuki TSUTSUMI
    2007 Volume 60 Issue 2 Pages 71-83
    Published: October 25, 2007
    Released on J-STAGE: August 20, 2013
    DOIhttps://doi.org/10.4294/zisin.60.71
    JOURNAL FREE ACCESS
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    The Sekidosan fault is a northeast-trending active reverse fault that extends for about 33 km along the southeastern margin of the Ouchigata Plain, central Japan. We constrained the age and offset of the recent faulting events of the fault by re-interpreting the results of trenching and drilling surveys at three sites, namely Mijiro, Hongo, and Nakagawa-Udono sites. We identified three surface-rupturing events that had occurred during the last 5000 years: 4830-3580, 3690-1990, and 850-250 cal yrBP, and estimated the amounts of net slip during individual events at Mijiro and Nakagawa-Udono sites to be about 1.9-2.2 m and 3.6-3.8 m, respectively. The amount of vertical displacement estimated at Mijiro site is discordant with the slip distribution previously reconstructed based on tectonic geomorphological investigations. This is probably due to the lack of a tectonic scarp associated with only the latest event around the site. On the other hand, the amount of vertical displacement estimated at Nakagawa-Udono site is consistent with the slip distribution. So, we suggest that the previous slip distribution is appropriate to a certain extent. The length of surface rupture calculated from the amounts of the net slip using empirical relationships is at least 25-30 km, indicating that at least three quarters of the entire length of the Sekidosan fault rupture during individual events. Recurrence interval and slip rate of the fault are estimated at 1400-2300 years and 1-3 mm/yr, respectively.
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  • A Prior Distribution of the Parameters in the Renewal Model with Lognormal Distribution Used for Estimating the Probability of Recurrent Earthquakes
    Masami OKADA, Hiroyuki TAKAYAMA, Fuyuki HIROSE, Naoki UCHIDA
    2007 Volume 60 Issue 2 Pages 85-100
    Published: October 25, 2007
    Released on J-STAGE: August 20, 2013
    DOIhttps://doi.org/10.4294/zisin.60.85
    JOURNAL FREE ACCESS
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    We determined the preliminary prior distribution of parameters in the renewal model with lognormal distribution for time intervals between large/moderate recurrent earthquakes in the world and between small repeating events with identical waveform occurring in the east off northeastern Japan. Using the prior distribution for parameters has great advantage to estimate the probability of the oncoming earthquake in a given period by a Bayesian approach from the dates of a few recurrent events in a sequence. It was supposed that n+1 recurrent events had occurred on a fault or in a certain source area, separated by n time intervals, Ti, of which logarithm, xi=ln (Ti), followed a normal distribution, N (μ, σ2). We supposed that the prior, π(μ, σ2)was uniform for the mean, μ,and inverse gamma with the shape- and scale- parameters, φ and ζ,for the variance, σ2,respectively. Then it was shown that the random variable φV/ζ obeyed F-distribution with degree of freedom (n-1, 2 φ), where V was the unbiased variance of xi. We got preliminary results as φ=2.5 and ζ=0.23 from 29 sequences with five or more large/moderate events in the world and 24 sequences with 4 events, respectively, using the maximum likelihood method for the variances, Vk, of samples, and by fitting the theoretical distribution of unbiased standard deviation to the observed one. On the other hand, φ=2.5 and ζ=0.44 were obtained from 70 sequences of small repeating events with identical waveform. The means, ζ/(φ-1), of the prior distribution for σ2 were 0.15 for large/moderate events in the world and 0.29 for small repeating events. Large variance for small repeating events suggested that they were more influenced by stress changes in near field relating to larger earthquake occurrences than large/ moderate events. The expectation of standard deviation, σ, calculated from the prior distribution, was 0.36 for large/moderate events, which was fairly close to the mean of two values, 0.24 and 0.5, given by the Earthquake Research Committee of Japan in 2001 and by the Working Group on California Earthquake Probabilities in 1995, respectively.
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