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Focal mechanisms and stress field in the Nobi fault area, central Japan

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Title: Focal mechanisms and stress field in the Nobi fault area, central Japan
Authors: Katsumata, Kei Browse this author →KAKEN DB
Kosuga, Masahiro Browse this author
Katao, Hiroshi Browse this author
Yamada, Takuji Browse this author
Kato, Aitaro Browse this author
Keywords: Nobi fault system
Nobi earthquake
Focal mechanism
Stress tensor inversion
Intraplate earthquake
Active fault
Issue Date: 25-Jun-2015
Publisher: Springer
Journal Title: Earth planets and space
Volume: 67
Start Page: 1
End Page: 10
Publisher DOI: 10.1186/s40623-015-0275-2
Abstract: In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P- and T-axes were N97 degrees +/- 23 degrees E and N6 degrees +/- 32 degrees E, and the averages of the dips of the P- and T-axes were 11 degrees +/- 10 degrees and 32 degrees +/- 25 degrees, respectively. These variations in the P- and T-axes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (sigma(1)) is oriented E-W over almost the entire study area. The stress ratio R, which is defined as R = (sigma(1) - sigma(2))/(sigma(1) - sigma(3)), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating sigma(2) approximate to sigma(3), and thus the dominant stress in this region is a uniaxial compression in the direction of sigma(1). The direction of the sigma(1)-axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released.
Type: article
Appears in Collections:理学院・理学研究院 (Graduate School of Science / Faculty of Science) > 雑誌発表論文等 (Peer-reviewed Journal Articles, etc)

Submitter: 勝俣 啓

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