地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
論説
強震動の継続時間から見た平成15年(2003年)十勝沖地震とその最大余震の破壊伝播特性
青木 重樹吉田 康宏勝間田 明男干場 充之
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ジャーナル フリー

2012 年 65 巻 2 号 p. 163-174

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The strong motion duration of the short-period accelerogram depends on the direction of rupture propagation due to the directivity effect: it is short at seismic stations located along the forward direction of rupture propagation but long along the opposite side. A larger earthquake tends to have a longer strong motion duration. In this study, we examined the characteristics of rupture propagation: rupture direction θ, rupture duration L/VR, and fault length L of the 2003 Off-Tokachi Earthquake and its largest aftershock from the azimuthal dependence of strong motion duration, where VR is rupture velocity. A filtered (5 to 10 Hz band-pass) accelerogram revealed clear azimuthal dependence of the duration for the main shock, which is consistent with the distribution calculated for unilateral faulting. We obtained θ=N27°W, L/VR=42s, and L=91km for the main shock. These results are comparable to the fault parameters estimated by the slip inversion using the long-period waveforms in previous studies. The largest aftershock could be regarded as unilateral faulting in the view of the azimuthal distribution of the duration as well as the main shock. We obtained θ=N70°W, L/VR=40s, and L=98km for the aftershock. The large L means that the rupture of the aftershock extended to off Urakawa, Hokkaido. The estimation is consistent with the observation that peak ground accelerations of the aftershock were greater than those of the main shock in western Hokkaido and northern Tohoku district. Consequently, the rupture extent of the aftershock was as great as that of the main shock despite a magnitude difference of 1.0.

Rupture directions and lengths of the faults estimated by strong motion durations of seismic stations around the 2003 Off-Tokachi Earthquake and its largest aftershock. Fullsize Image
 
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© 2012 公益社団法人 日本地震学会
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