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[S09-12] The present status of the seismicity off the southeast coast of Hokkaido, where the relative quiescence started more than a decade ago
松浦(2014)は、2011年東北地方太平洋沖地震の前の東北太平洋側沖合と同様に、北海道のえりも岬から択捉島南半分の沖合にかけて、2008年末辺りからETASモデルからの相対的静穏化が見られることを報告した。松浦・岩佐(2018)・松浦(2019)では、その静穏化が2015年後半辺りで完了している可能性を報告した。2011年以降の東北地方太平洋沖地震の影響が、東北地方に於いては一応の収束を見せつつある現時点で、北海道南東沖での地震活動度がどうなっているか、今回5年ぶりの定期的チェックとして定量的な検討を行ったので、ここに報告する。 北海道南東沖の当該領域では、相変わらず地震活動度は高くない。特にM6.5以上の地震は2016年以降2023年7月末までに僅か4個しか発生しておらず、地震活動度が回復していても、ETAS 効果による地震数の上昇が望めない。そのため、Kumazawa et al. (2010)の厳しいチェンジポイント導入に伴うペナルティを含めると、数理的に有意な回復という判断ができない状態である。その半分である理論的ペナルティであれば、2016年頃から地震活動度は有意に回復している、と判断できる。データは以前と同様1965年7月以降2023年7月末までの気象庁震源カタログを用いたが、2021年7月末までのISCカタログでも震源分布は多少異なるが、同様の結果となる。 約7年間継続した相対的静穏化の、全てが次の北海道南東沖の超巨大地震の準備過程を反映しているのだろうか。静穏化の定量解析だけでは限界がある。隣接域で2011年に発生した超巨大地震は、当該地域のプレート境界型地震の発生に対してどのような変化を与えたのか、試みに弾性変形による影響も検討する必要がある。このような多様な方向からの検討によって、2009年から2015年までと、2016年以降とで、当該地域で異なる指標がないかを、多種類の検討で実施する契機になればと考える。
本研究は、文部科学省による「地震調査研究推進本部の評価等支援事業」の一環として実施された。
参考文献
Kumazawa, T., Ogata, Y., Toda, S. (2010) Precursory seismic anomalies and transient crustal deformation prior to the 2008 Mw = 6.9 Iwate-Miyagi Nairiku, Japan, earthquake, J. Geophys. Res., 115, B10312, doi:10.1029/2010JB007567.
松浦律子(2014) 根室沖を含む北海道東部沖広域で継続中の大きな相対的地震活動度の静穏化 について，日本地震学会講演予稿集，2014 年度秋季大会，D21-03．
松浦律子・岩佐幸治(2018) 北海道東部沖合の地震活動静穏化の現状，日本地震学会講演予稿 集，2018 年度秋季大会，S23-02, 77.
松浦律子(2019) 北海道東方沖の相対的地震活動度の静穏化と現状，予知連会報，101，476-483．
Matsu'ura (2014) reported a relative quiescence from the ETAS model around the end of 2008 in the offshore area from Cape Erimo to the southern half of Iturup Island in Hokkaido, which is similar to the quiescence in the offshore area off the Pacific side of Tohoku before the 2011 off the Pacific coast of Tohoku earthquake. Matsu'ura and Iwasa (2018) and Matsu'ura (2019) reported the possibility that the quiescence was completed around the latter half of 2015. The effects of the 2011 off the Pacific coast of Tohoku earthquake are now appearing to have subsided in the Tohoku region. Here we report a quantitative examination of the seismicity level off the east coast of Hokkaido. The seismicity level remains not high in the studied area. In particular, only four earthquakes of M6.5 or larger have occurred since 2016 until the end of July 2023, and even if the seismicity level has been recovered, the number of earthquakes cannot be expected to increase due to the ETAS effect. Therefore, including the severe penalty associated with the introduction of change points [Kumazawa et al. (2010)], we are still unable to declare that the recovery is statistically significant. If the theoretical penalty that is a half of theirs', the recovery of seismicity is significant since about 2016. As before, the JMA hypocenter catalog from July 1965 to the end of July 2023 were used for the analysis. The ISC catalog up to the end of July 2021 also yields similar results, although the hypocenter distribution is slightly different. Does the relative quiescence for about seven years solely reflect a preparatory process for the next giant earthquake off the east coast of Hokkaido? Quantitative analysis of the quiescence alone can only detect a dangerous area within a decade. It is necessary to examine how the 2011 giant earthquake that occurred in the adjacent area affected the occurrence of interplate earthquakes in the region. We will examine the effects of elastic deformation as the trial. We believe that studies of various directions on the studied area will provide an opportunity to find some distinguished indicators for the term between 2009 and 2015 from the term after 2016.
This study was supported by the Headquarters for Earthquake Research Promotion (HERP) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.
本研究は、文部科学省による「地震調査研究推進本部の評価等支援事業」の一環として実施された。
参考文献
Kumazawa, T., Ogata, Y., Toda, S. (2010) Precursory seismic anomalies and transient crustal deformation prior to the 2008 Mw = 6.9 Iwate-Miyagi Nairiku, Japan, earthquake, J. Geophys. Res., 115, B10312, doi:10.1029/2010JB007567.
松浦律子(2014) 根室沖を含む北海道東部沖広域で継続中の大きな相対的地震活動度の静穏化 について，日本地震学会講演予稿集，2014 年度秋季大会，D21-03．
松浦律子・岩佐幸治(2018) 北海道東部沖合の地震活動静穏化の現状，日本地震学会講演予稿 集，2018 年度秋季大会，S23-02, 77.
松浦律子(2019) 北海道東方沖の相対的地震活動度の静穏化と現状，予知連会報，101，476-483．
Matsu'ura (2014) reported a relative quiescence from the ETAS model around the end of 2008 in the offshore area from Cape Erimo to the southern half of Iturup Island in Hokkaido, which is similar to the quiescence in the offshore area off the Pacific side of Tohoku before the 2011 off the Pacific coast of Tohoku earthquake. Matsu'ura and Iwasa (2018) and Matsu'ura (2019) reported the possibility that the quiescence was completed around the latter half of 2015. The effects of the 2011 off the Pacific coast of Tohoku earthquake are now appearing to have subsided in the Tohoku region. Here we report a quantitative examination of the seismicity level off the east coast of Hokkaido. The seismicity level remains not high in the studied area. In particular, only four earthquakes of M6.5 or larger have occurred since 2016 until the end of July 2023, and even if the seismicity level has been recovered, the number of earthquakes cannot be expected to increase due to the ETAS effect. Therefore, including the severe penalty associated with the introduction of change points [Kumazawa et al. (2010)], we are still unable to declare that the recovery is statistically significant. If the theoretical penalty that is a half of theirs', the recovery of seismicity is significant since about 2016. As before, the JMA hypocenter catalog from July 1965 to the end of July 2023 were used for the analysis. The ISC catalog up to the end of July 2021 also yields similar results, although the hypocenter distribution is slightly different. Does the relative quiescence for about seven years solely reflect a preparatory process for the next giant earthquake off the east coast of Hokkaido? Quantitative analysis of the quiescence alone can only detect a dangerous area within a decade. It is necessary to examine how the 2011 giant earthquake that occurred in the adjacent area affected the occurrence of interplate earthquakes in the region. We will examine the effects of elastic deformation as the trial. We believe that studies of various directions on the studied area will provide an opportunity to find some distinguished indicators for the term between 2009 and 2015 from the term after 2016.
This study was supported by the Headquarters for Earthquake Research Promotion (HERP) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.