内陸地震の長期評価に関する課題と新たな視点  [in Japanese] Current issues and a prospective view to the next step for long-term crustal earthquake forecast in Japan  [in Japanese]

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Author(s)

    • 遠田 晋次 Toda Shinji
    • 京都大学防災研究所|現所属:東北大学災害科学国際研究所 Disaster Prevention Research Institute, Kyoto University|present address: International Research Institute of Disaster Science, Tohoku University

Abstract

1995年兵庫県南部地震以降,地震発生確率の低い地域で内陸被害地震が続発した.この事実は,活断層の調査結果を反映した確率予測図が必ずしも有効に機能しているわけではないことを示す.そのため,著者1923年以降の内陸地震と地震断層出現率を再検討した.その結果,震源の把握につながる地震断層の出現率はM6.5以上で20%,M7.0以上で44%となった.したがって,地震断層の同一パターンの変位の累積が活断層と等価であると仮定すると,活断層情報のみによる地震評価に過小見積もりが生じる.また,既知の活断層と確率値からの計算でも,観測された地震数の2–7割程度までしか再現できず,多数の活断層が潜在する可能性がある.一方,活断層分布・内陸地震には顕著な地域性や偏在性がある.これは断層成熟度や応力解放システムの違いを反映し,地震発生の時系列的特徴までもコントロールする.地震サイクルの理解が進んでいる海溝型地震では更新過程を考慮した確率算定が有効だが,内帯では活断層情報だけではなく広域地殻変形等を考慮した確率算定が必要であろう.

Several large inland earthquakes in Japan since the 1995 Hygokennambu (Kobe) earthquake have struck the lower probability areas mapped by the Earthquake Research Committee (ERC) of the Headquarters for Earthquake Research Promotion in 2005. To seek the reason why it appears to have failed, here we re-examined the surface-rupturing earthquakes since 1923 when the Japan Meteorological Agency (JMA) officially started their catalog recording. Only 20% of M ≥ 6.5 and 44% of M ≥ 7.0 shallow earthquakes left the surface breaks that correspond to their source fault dimension. We thus speculate that the number of potential destructive earthquakes of M6-7 estimated from the major active faults would be significantly underestimated. Numerical calculations with active fault data and their assigned probabilities in the report also largely underestimate the number of observed earthquakes. Both independent analyses suggest that there would be far more minor active faults hidden beneath Japanese islands. From a viewpoint of elastic rebound in different scales, we discuss efficiency of stress release associated with inland earthquakes and maldistribution of active faults. Simple elastic half-space models qualitatively demonstrate that highly dipping reverse faulting earthquakes such as the 2004 Niigata-ken-Chuetsu earthquake associated with tectonic inversion are unfavorably oriented for the recent stress field and inefficient to release regional differential stress. The models also imply that numerous moderate-size earthquakes due to minor faults cannot compensate one large earthquake caused by a mature fault system. It explains that the inland deformation zone sustains high continuous seismicity, whereas the outer zone facing the Pacific Ocean is significantly influenced by the seismic cycles of subduction mega-thrust events. Regarding to the probabilistic estimates of large earthquakes, the former would be appropriate for more Poisson forecasting taking bulk deformation into account, and the latter would be evaluated from time-dependent conditional probabilities.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 119(2), 105-123, 2013

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130003363920
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    024350030
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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