7 －９

2007 年新潟県中越沖地震の余震のメカニズム解の特徴
Focal mechanism solutions of aftershocks of the 2007 Niigataken Chuetsu-Oki
2007 ᐕᣂẟ⋵ਛ⿧ᴒ࿾㔡ߩ૛㔡ߩࡔࠞ࠾࠭ࡓ⸃ߩ․ᓽ
Earthquake
Focal mechanism solutions of aftershocks of the 2007 Niigataken Chuetsu-Oki Earthquake

産業技術総合研究所↥ᬺᛛⴚ✚ว⎇ⓥᚲ
Geological Surveyof Japan, AIST
Geological Survey of Japan, AIST2007 年新潟県中越沖地震の余震のメカニズム解の特徴を調べるため，P 波初動の押し引きに加えᐕᣂẟ⋵ਛ⿧ᴒ࿾㔡ߩ૛㔡ߩࡔࠞ࠾࠭ࡓ⸃ߩ․ᓽࠍ⺞ߴࠆߚ߼㧘P ᵄೋേߩ᛼ߒᒁ߈ߦട߃ߡて2007
P 波とSH 波の振幅値を用い，気象庁マグニチュード 3 以上の余震のメカニズム解を推定した．

－365－

P解析には気象庁，防災科学技術研究所，東京大学地震研究所，東北大学の定常観測点の波形データ
ᵄߣ SH ᵄߩᝄ᏷୯ࠍ↪޿㧘᳇⽎ᐡࡑࠣ࠾࠴ࡘ࡯࠼ 3 એ਄ߩ૛㔡ߩࡔࠞ࠾࠭ࡓ⸃ࠍផቯߒߚ㧚⸃ᨆߦߪ᳇⽎ᐡ㧘㒐ἴ⑼ቇᛛⴚ⎇ⓥᚲ㧘᧲੩ᄢቇ࿾㔡⎇ⓥᚲ㧘᧲ർᄢቇߩቯᏱ᷹ⷰὐߩᵄᒻ࠺࡯࠲ࠍ೑↪ߒߚ㧚
を利用した．
（今西和俊・桑原保人）╙ 1 ࿑
7 ᦬ᧃ߹ߢߦ⊒↢ߒߚએ਄ߩ૛㔡7676
୘ߩࡔࠞ࠾࠭ࡓ⸃㧔ਅඨ⃿ᛩᓇ㧕㧚ฝ࿑ߦߪ㧘ฦᷓߐ
第1図
7 月末までに発生したM3
M3 以上の余震個のメカニズム解（下半球投影）．右図には，各深さごとのメカニズム解を示す．メカニズム解のタイプを視覚的に判断しやすくするためにFlohlich
Flohlich(1992)ߏߣߩࡔࠞ࠾࠭ࡓ⸃ࠍ␜ߔ㧚ࡔࠞ࠾࠭ࡓ⸃ߩ࠲ࠗࡊࠍⷞⷡ⊛ߦ್ᢿߒ߿ߔߊߔࠆߚ߼ߦ
(1992) の三角ダイアグラムを使い，逆断層成分，正断層成分，横ずれ成分のそれぞれの比率にߩਃⷺ࠳ࠗࠕࠣ࡜ࡓࠍ૶޿㧘ㅒᢿጀᚑಽ㧘ᱜᢿጀᚑಽ㧘ᮮߕࠇᚑಽߩߘࠇߙࠇߩᲧ₸ߦᔕߓߡ⦡ಽߌࠍ
応じて色分けを行った．本震と最大余震のメカニズム解については，気象庁による P 波初動解を示す．灰色の丸は、当該地域に適切な速度構造と観測点補正値を用いて再決定した余震の震源でⴕߞߚ㧚ᧄ㔡ߣᦨᄢ૛㔡ߩࡔࠞ࠾࠭ࡓ⸃ߦߟ޿ߡߪ㧘᳇⽎ᐡߦࠃࠆP ᵄೋേ⸃ࠍ␜ߔ㧚Ἧ⦡ߩਣߪ‫ޔ‬ᒰある．Flohlich (1992) の定義に従うと，逆断層タイプが 35 イベント，横ずれタイプが 10 イベント，正断層タイプが 6 イベント，その他が 25 イベントである．横ずれタイプや横ずれ成分を⟵ߦᓥ߁ߣ㧘ㅒᢿጀ࠲ࠗࡊ߇
35 ࠗࡌࡦ࠻㧘ᮮߕࠇ࠲ࠗࡊ߇ 10 ࠗࡌࡦ࠻㧘ᱜᢿጀ࠲ࠗࡊ߇ 6 ࠗࡌࡦ࠻㧘
多く含む余震が本震の震源の南部域に集中して発生している．
Fig. 1 Focal mechanism solutions of 76 aftershocks are plotted on map (lower hemisphere of equal-area projecߘߩઁ߇ 25 ࠗࡌࡦ࠻ߢ޽ࠆ㧚ᮮߕࠇ࠲ࠗࡊ߿ᮮߕࠇᚑಽࠍᄙߊ฽߻૛㔡߇ᧄ㔡ߩ㔡Ḯߩධㇱၞߦ㓸ਛtion), where different colors are used to differentiate reverse (green), strike-slip (red), and normal (blue)ߒߡ⊒↢ߒߡ޿ࠆ㧚
faulting mechanisms based on the method developed by Flohlich (1992). It should be noted that strike-slip
events mainly occurred at the south of the mainshock hypocenter.

⹥࿾ၞߦㆡಾߥㅦᐲ᭴ㅧߣ᷹ⷰὐ⵬ᱜ୯ࠍ↪޿ߡౣ᳿ቯߒߚ૛㔡ߩ㔡Ḯߢ޽ࠆ㧚Flohlich (1992) ߩቯFig. 1 Focal mechanism solutions of 76 aftershocks are plotted on map (lower hemisphere of equal-areaprojection), where different colors are used to differentiate reverse (green), strike-slip (red), and normal (blue)
faulting mechanisms based on the method developed by Flohlich (1992). It should be noted that strike-slip events
mainly occurred at the south of the mainshock hypocenter.

－365－╙㧞࿑ ࡔࠞ࠾࠭ࡓ⸃ߩⓨ㑆ಽᏓ߆ࠄᦼᓙߐࠇࠆ
ᢿጀࡕ࠺࡞ߩ 1 ଀㧚ᧄ㔡ߢߪᦨೋߦർ⷏௑ᢳߩᢿጀԘ߇⎕უߒ㧘ߘߩᓟ㧘ධ᧲௑ᢳߩԙߩᢿጀ߇⎕უߒ╙㧞࿑ ࡔࠞ࠾࠭ࡓ⸃ߩⓨ㑆ಽᏓ߆ࠄᦼᓙߐࠇࠆ
ߚ㧚ᮮߕࠇဳߩ૛㔡߇㓸ਛߔࠆߩߪ‫ޔ‬ᢿጀԘߣԙߩ
ᢿጀࡕ࠺࡞ߩ 1 ଀㧚ᧄ㔡ߢߪᦨೋߦർ⷏௑ᢳߩᢿጀႺ⇇ઃㄭߢ޽ࠆ‫ᦨޕ‬ᄢ૛㔡ߪԚߩ㕙ౝߢ⊒↢ߒߚ㧚
メカニズム解の空間分布から期待される断層モデԘ߇⎕უߒ㧘ߘߩᓟ㧘ධ᧲௑ᢳߩԙߩᢿጀ߇⎕უߒ

第２図ルの 1 例．本震では最初に北西傾斜の断層①が破壊し，その後，南東傾斜の②の断層が破壊した．
ߚ㧚ᮮߕࠇဳߩ૛㔡߇㓸ਛߔࠆߩߪ‫ޔ‬ᢿጀԘߣԙߩ
Fig.
2 One possible fault model deduced from the
横ずれ型の余震が集中するのは、断層①と②の境Ⴚ⇇ઃㄭߢ޽ࠆ‫ᦨޕ‬ᄢ૛㔡ߪԚߩ㕙ౝߢ⊒↢ߒߚ㧚
spatialdistribution of aftershock mechanisms. The界付近である。最大余震は③の面内で発生した．
Fig. rupture
2 One possible
fault model deduced
spatialplane
dis- 1,of the mainshock
started fromfromthea fault
tribution of aftershock mechanisms. The rupture of theFig.
2 Onepossible
faultmodeldeduced
from thethen
propagatedto froma fault
plane
2. 1,Mostof strike-slip
mainshock
starteda fault
planethen propagatedto a fault
plane 2.
of strike-slip
eventsoccurredspatialdistributionofaftershockmechanisms.
eventsoccurredat Mostaroundthe boundaryof the atfaultThearound the boundary of the fault plane 1 and 2.

ruptureplane
1 andof2.the mainshock started from a fault plane 1,
then propagated to a fault plane 2. Most of strike-slip
events occurred at around the boundary of the fault－366－

plane 1 and 2.

╙ 3 ࿑ ർㇱߣධㇱߢ௑ᢳߩ⇣ߥࠆᢿጀࡕ࠺࡞ߦࠃࠆࠢ࡯ࡠࡦᔕജᄌൻ㧚ᄖㇱᔕജߪ‫ߩࡦࡠ࡯ࠢޔ‬⎕第3図

北部と南部で傾斜の異なる断層モデルによるクーロン応力変化．外部応力は、クーロンの破壊基準、被り圧、新潟県中越地域で得られた応力比（Imanishi et al.,2006）を参考に設定した。計算されるクーロン応力変化を見ると、北部と南部の断層の間で横ずれの余震の発生が促進され，ࠇࠆࠢ࡯ࡠࡦᔕജᄌൻࠍ⷗ࠆߣ‫ޔ‬ർㇱߣධㇱߩᢿጀߩ㑆ߢᮮߕࠇߩ૛㔡ߩ⊒↢߇ଦㅴߐࠇ㧘᷹ⷰ⚿ᨐ
観測結果と良く一致することがわかる。計算には，Coulomb3.0（Lin and Stein,2004; Toda et╙ 3 ࿑ ർㇱߣධㇱߢ௑ᢳߩ⇣ߥࠆᢿጀࡕ࠺࡞ߦࠃࠆࠢ࡯ࡠࡦᔕജᄌൻ㧚ᄖㇱᔕജߪ‫ߩࡦࡠ࡯ࠢޔ‬⎕ߣ⦟ߊ৻⥌ߔࠆߎߣ߇ࠊ߆ࠆ‫ߪߦ▚⸘ޕ‬㧘Coulomb3.0㧔Linand Stein, 2004; Toda et al., 2005㧕ࠍ೑↪ߒal.,2005）を利用した
al., 2006㧕ࠍෳ⠨ߦ⸳ቯߒߚ‫ߐ▚⸘ޕ‬
Fig. 3 Coulomb stress change caused by the fault model shown on the left. et
Regional
stress field are assumed basedߚ უၮḰ‫ࠅⵍޔ‬࿶‫ޔ‬ᣂẟ⋵ਛ⿧࿾ၞߢᓧࠄࠇߚᔕജᲧ㧔Imanishi
onCoulomb
fracture
criteria,
overburdenpressure,
andstress
ratio
determinedin Mid Niigata area (Imanishiࠇࠆࠢ࡯ࡠࡦᔕജᄌൻࠍ⷗ࠆߣ‫ޔ‬ർㇱߣධㇱߩᢿጀߩ㑆ߢᮮߕࠇߩ૛㔡ߩ⊒↢߇ଦㅴߐࠇ㧘᷹ⷰ⚿ᨐ
et al., 2006). Strike-slip aftershocks are promoted around the area between two faults, which is consistentߣ⦟ߊ৻⥌ߔࠆߎߣ߇ࠊ߆ࠆ‫ߪߦ▚⸘ޕ‬㧘Coulomb3.0㧔Linand2004;wasToda
2005㧕ࠍ೑↪ߒwith stressobservational(Linand Stein,2004;
et al.,Regional
2005)
usedettoal.,calculateFig.
3 Coulombchangeevidence.
caused Coulomb3.0
by the fault
modelshown
onTodathe Stein,left.
stressfield
are theassumed
Coulomb
stresschange.
ߚუၮḰ‫ࠅⵍޔ‬࿶‫ޔ‬ᣂẟ⋵ਛ⿧࿾ၞߢᓧࠄࠇߚᔕജᲧ㧔Imanishi et al., 2006㧕ࠍෳ⠨ߦ⸳ቯߒߚ‫ߐ▚⸘ޕ‬

based on Coulomb fracture criteria, overburden pressure, and stress ratio determined in Mid Niigata area

(Imanishi et al., 2006). Strike-slip aftershocks are promoted around the area between two faults, which isFig. 3 Coulomb
stress change
caused by
the fault modelshown
on theleft.Toda
Regional
field
assumedconsistentwith observational
evidence.
Coulomb3.0(Lin andStein,2004;et al.,stress2005)
wasareusedtobased theon Coulomb
Coulomb stressfracture
criteria, overburden pressure, and stress ratio determined in Mid Niigata area
calculatechange.
(Imanishi et al., 2006). Strike-slip aftershocks are promoted around the area between two faults, which isconsistent with observational evidence. Coulomb3.0 (Lin and Stein, 2004; Toda et al., 2005) was used to
calculate the Coulomb stress change.

－366－参考文献１）Flohlich, C.（1992）, Triangle diagrams: ternary graphs to display similarity and diversity of earthquake
focal mechanism, Phys. Earth Planet. Interiors, 75, 193-198.
２）Imanishi, K., Y. Kuwahara, T. Takeda and Y. Haryu（2006）, The Seismicity, Fault structures and stressfield in the seismic gap adjacent to the 2004 mid Niigata earthquake inferred from seismological observation, Earth Planets Space, 58, 831-841, 2006.
３）今西和俊・長郁夫・桑原保人・平田直・Yannis Panayotopoulos（2006）, 糸魚川－静岡構造線活断層系中・南部域における微小地震の発震機構解 , 活断層・古地震研究報告 , 6, 55-70, 2006.
４）Lin, J. and R.S. Stein（2004）, Stress triggering in thrust and subduction earthquakes, and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults, J. Geophys. Res., 109,
B02303, doi:10.1029/2003JB002607.
５）Toda, S., R. S. Stein, K. Richards-Dinger and S. Bozkurt（2005）, Forecasting the evolution of seismicity in southern California: Animations built on earthquake stress transfer, J. Geophys. Res., B05S16,－367－

doi:10.1029/2004JB003415.

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