㧠㧙㧣  ᐕ  ᦬  ᣣᚱ✚ඨፉධ᧲ㇱߩ࿾㔡
Earthquake below the southeastern part of the Boso Peninsula in February 17, 2009
㒐ἴ⑼ቇᛛⴚ⎇ⓥᚲ
National Research Institute for Earth Science and Disaster Prevention
2009 ᐕ 2 ᦬ 17 ᣣᚱ✚ඨፉධ᧲ㇱߩᷓߐ 29 km ߢM4.3 ߩ࿾㔡߇⊒↢ߒᦨᄢ㔡ᐲ 4 ߩំࠇ߇᷹ⷰߐࠇߚ㧚ᚱ✚ඨፉᴒߢߪࠬࡠ࡯ࠬ࡝࠶ࡊࠗࡌࡦ࠻(SSE)߇߅ࠃߘ 6 ᐕ㑆㓒ߢ➅ࠅ㄰ߒ⊒↢ߔࠆߎߣ߇⍮ࠄࠇߡ߅ࠅ㧘࿾Ზᄌേ࠺࡯࠲߆ࠄࡈࠖ࡝ࡇࡦᶏࡊ࡟࡯࠻(PHS)਄㕙ઃㄭߦᢿጀࡕ࠺࡞߇ផ
ቯߐࠇߡ޿ࠆ 1, 2)㧚ᚱ✚ SSE ߩ㔡Ḯၞߩ᧲஥ߢߪ SSE ߦ઻ߞߡ࿾㔡߇⟲⊒⊛ߦ⊒↢ߔࠆ߇㧘⷏஥ߢߪ࿾㔡߇ቯᏱ⊛ߦ⊒↢ߒߡ޿ࠆ 3)㧚৻ᣇ㧘ᚱ✚ඨፉ᧲ጯߢߪ 1987 ᐕජ⪲⋵᧲ᣇᴒ࿾㔡(M6.7)ߥ
ߤߩࠬ࡜ࡉౝᄢ࿾㔡߽⊒↢ߒߡ޿ࠆ 4)㧚ߘߎߢ㧘੹࿁ߩ࿾㔡ߦߟ޿ߡ⹦ߒߊ⺞ߴߚ㧚੹࿁ߩ࿾㔡ߩ㔡ᄩ૏⟎ࠍ 1987 ᐕජ⪲⋵᧲ᣇᴒ࿾㔡߅ࠃ߮ᚱ✚ SSE ߩ㔡ḮၞߣᲧߴࠆߣ㧘ߎࠇࠄࠃࠅ⷏஥ߦ૏⟎ߔࠆ (╙ 1 ࿑)㧚ࡊ࡟࡯࠻Ⴚ⇇ߣߩ㑐ଥࠍ⷗ࠆߚ߼㧘੹࿁ߩ࿾㔡ߩᷓߐࠍ⋧ૃ࿾㔡ࠃࠅផቯߐࠇߚ PHS ߣᧄᎺፉᒐ࿾Ზߩ㑆ߩࡊ࡟࡯࠻㑆ߔߴࠅߩ⿠߈ࠆ㕙ߣߒߡߩࡊ࡟࡯࠻Ⴚ⇇ߩᒻ⁁ࡕ࠺࡞ 5)ߣᲧߴࠆ(╙ 2 ࿑)㧚੹࿁ߩ࿾㔡ߩᷓߐ 29 km ߦᲧߴߡߎߩ๟ㄝߢߩࡊ࡟࡯࠻ᒻ⁁ࡕ࠺࡞ߩᷓߐߪ߅ࠃߘ 22.5 km ߢ޽ࠅ㧘੹࿁ߩ࿾㔡ߪߎࠇࠃࠅᷓ޿㧚ᰴߦࡔࠞ࠾࠭ࡓ⸃ࠍᬌ⸛ߒߡߺࠆ㧚੹࿁ߩ࿾㔡ߩࡔࠞ࠾࠭ࡓ⸃ߪ㧘Hi-net ߦࠃࠆೋേ᛼ߒᒁ߈߅
ࠃ߮ F-net ߦࠃࠆᐢᏪၞ࿾㔡ᵄᒻߩ⸃ᨆ⚿ᨐߣ߽ർ⷏- ධ᧲ᣇะߦ T ゲࠍ᦭ߔࠆᱜᢿጀဳߦ᳞߹ߞߡ޿ࠆ(╙ 2 ࿑)㧚ㆊ෰ߩਥߥ࿾㔡ߩࡔࠞ࠾࠭ࡓ⸃ࠍ⷗ࠆߣ㧘PHS ߅ࠃ߮ᄥᐔᵗࡊ࡟࡯࠻(PAC)਄
㕙ઃㄭߢߪߘࠇߙࠇߩࡊ࡟࡯࠻⋧ኻㆇേᣇะߦ P ゲࠍ᦭ߔࠆࠬ࡜ࠬ࠻ဳߩࡔࠞ࠾࠭ࡓ⸃߇ථ⿧ߔ
ࠆߩߦኻߒߡ㧘PHS ౝㇱߢߪਥߥࠗࡌࡦ࠻ߩࡔࠞ࠾࠭ࡓ⸃ߪߔߴߡ㕖ࠬ࡜ࠬ࠻ဳߦ᳞߹ߞߡ޿ࠆ㧚੹࿁ߩ࿾㔡߽㕖ࠬ࡜ࠬ࠻ဳߢ޽ࠅ㧘㔡Ḯߩᷓߐߣࡔࠞ࠾࠭ࡓ⸃߆ࠄ PHS ౝㇱߢ⊒↢ߒߚߣផ᷹ߐࠇࠆ㧚੹࿁ߩ࿾㔡ߩ⊒↢႐ࠍᮨᑼ⊛ߦ␜ߔߣ╙ 3 ࿑ߩࠃ߁ߦߥࠆ㧚੹࿁ߩ࿾㔡ߪ 1987 ᐕජ⪲⋵᧲ᣇᴒ࿾㔡ߣห᭽ߦ PHS ࠬ࡜ࡉౝㇱߩ࿾㔡ߢ޽ߞߚ߇㧘㔡ᄩߪߎࠇࠃࠅධ⷏ߦ૏⟎ߒᷓߐߪᵻ޿㧚߹ߚ㧘ࡔࠞ࠾࠭ࡓ⸃ߪ 1987 ᐕජ⪲⋵᧲ᣇᴒ࿾㔡ߩฝᮮߕࠇဳߦኻߒߡ㧘੹࿁ߩ࿾㔡ߪᱜᢿጀဳߢ޽ࠅ㧘ࡔࠞ࠾࠭ࡓ⸃߽⇣ߥࠆ㧚(ᧁ᧛ዏ♿࡮ᱞ↰ື਽)
Hisanori Kimura and Tetsuya Takeda- 174 -ෳ ⠨ ᢥ ₂1) Sekine, S., H. Hirose, H. Kimura, and K. Obara (2007) The 2007 Boso Slow Slip Event and the associatedearthquake swarm, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract T21A-0363.
2) Hirose, H., S. Sekine, H. Kimura, K. Obara (2008), Source Process of the 2007 Boso Slow Slip Event
With an Earthquake Swarm From Tiltmeter Data, Eos Trans. AGU 89 (53), U32A-02.
3) ᧁ᧛ዏ♿࡮ᱞ↰ື਽ (2009), 2008ᐕ10᦬12, 14, ߅ࠃ߮16ᣣ ᚱ✚ඨፉ᧲ጯߩ࿾㔡㧘ㅪ⛊ળႎ㧘81㧘
303-307.
4) Okada, Y., and K. Kasahara (1990), Earthquake of 1987, off Chiba, central Japan and possible triggeringof eastern Tokyo earthquake of 1988, Tectonophysics, 172, 351-364.
5) Kimura, H., K. Kasahara, T. Igarashi, and N. Hirata (2006), Repeating earthquake activities associatedwith the Philippine Sea plate subduction in the Kanto district, central Japan: a new plate configuration
revealed by interplate aseismic slips, Tectonophysics, 417, 101-118.
6) Seno T., S. Sakurai, and S. Stein (1996), Can the Okhotsk plate be discriminated from the North Americanplate?, J. Geophys. Res., 101, 11305-11315.
7) Kimura, H., T. Takeda and K. Obara, K. Kasahara (2008), Configuration model of the Pacific plateestimated from repeating earthquakes at the Kanto region, central Japan , 7th General Assembly of Asian
Seismological Commission and Seismological Society of Japan, 2008 Fall meeting, B41-11.

- 175 -╙ 1 ࿑ (a) ੹࿁ߩ࿾㔡ߩᵴേၞࠍ␜ߔ㧚⍫ශߪ㑐᧲࿾ᣇߦኻߔࠆࡈࠖ࡝ࡇࡦᶏࡊ࡟࡯࠻(PHS)߅ࠃ߮ᄥᐔᵗࡊ࡟࡯࠻(PAC)ߩㆇേᣇะࠍ⴫ߔ 6)㧚(b)੹࿁ߩ࿾㔡๟ㄝߩ㔡ᄩಽᏓ(৻ㇱ⥄േಣℂ㔡Ḯࠍ฽߻)㧚M4 ࠢ࡜ࠬએ਄ߩ࿾㔡ࠍᤊශߢ␜ߔ㧚✛ਣߪ⋧ૃ࿾㔡ࠍ㧘⚡✢ߪ Kimura et al. (2006)5) ߦࠃࠆ PHS ਄㕙ߩࡊ࡟࡯࠻㑆ߔߴࠅߩ⿠߈ࠆ㕙ߣߒߡߩࡊ࡟࡯࠻Ⴚ⇇ߩᒻ⁁ࡕ࠺࡞(ᢙሼߪᷓߐ(km)) ࠍ⴫ߔ㧚ᚱ✚ࠬࡠ࡯ࠬ࡝࠶ࡊࠗࡌ
ࡦ࠻ߩߔߴࠅၞ(Hirose et al., 2008) 2)߅ࠃ߮ 1987 ᐕජ⪲⋵᧲ᣇᴒ࿾㔡ߩ૛㔡ၞ࡮ࡔࠞ࠾࠭ࡓ⸃ 4)ࠍ޽ࠊߖߡ␜ߔ㧚Fig. 1 (a) Map showing activity area of the earthquakes below the southeastern part of the Boso Peninsula in February 17, 2009. Arrows denote directions of platemotion of the Philippine Sea plate (PHS) and the Pacific plate (PAC) relative to the Kanto region6). (b) The distributions of hypocenters around this earthquake(Automatically determined hypocenters are partially included). Stars denote M4-class or larger earthquakes. Green circles and purple lines denote small
patches of repeating earthquakes and configuration model of plate boundary where interplate shearing occurs (numbers denote depths (km)) 5), respectively.
Source region of the Boso slow slip event2) and aftershock region and a focal mechanism of the Earthquake of 1987, off Chiba4) are also shown.

- 176 -╙ 2 ࿑ ╙ 1 ࿑ ߩධർᢿ㕙࿑㧚ਥߥࠗࡌࡦ࠻ߩ Hi-net࡮F-net ࡔࠞ࠾࠭ࡓ⸃ࠍਅඨ⃿╬Ⓧᛩᓇ࿑ߢ␜ߔ㧚⚡✢ߪ PHS ߅ࠃ߮ PAC ਄㕙ߩࡊ࡟࡯࠻㑆ߔߴࠅߩ⿠߈ࠆ㕙ߣߒߡߩࡊ࡟࡯࠻Ⴚ⇇ࠍ⴫ߔ 5,7)㧚
Fig. 2 The cross section of Fig. 1 along N-S direction. Focal mechanisms determined by Hi-net and F-net are
shown by the equal-area projection onto the lower hemisphere for major earthquakes. Purple lines
denote upper interfaces of the PHS and the PAC where interplate shearing occur5,7).

╙ 3 ࿑ ᚱ✚ඨፉධ᧲ㇱ๟ㄝߩ PHS ᧲┵߅ࠃ߮ PAC ߩᮨᑼ࿑(Okada and Kasahara, 1990 4) ߦട╩)㧚ᤊශߪ 1987 ᐕජ⪲⋵᧲ᣇᴒ࿾㔡ߩ㔡Ḯ૏⟎ࠍ⴫ߔ㧚Fig.3 Schematic figure of the eastern edge of the PHS and the PAC (modified from Okada and Kasahara,19904)). A star denotes the hypocenter of the Earthquake of 1987, off Chiba.

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