1974年伊豆半島沖地震と石廊崎地震断層

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The Off Izu Peninsula (Izu-Hanto-oki) earthquake of May 9, 1974, of which magnitude was 6.9, occurred at southern tip of the Izu Peninsula, central Japan, accompanied with surface faults. The most distinct fault, named Irozaki fault, runs from Irozaki village to Yoshida village in NW-SE direction displaying right-lateral strike-slip (up to 45 cm) with vertical component (north-side diown, up to 25 cm). Before the earthquake occurred, topographical study had revealed many active faults at southem part of Izu Peninsula. The Irozaki fault coincides with one of these active faults in its locality and scnse of displacement. The major parts of aftershocks were distributed in the elliptical area stretched in NW-SE direction. This direction, NW-SE, was also obtained as the strike of one of the nodal planes by P-wave first motion analysis. After-seismic movements of the Irozaki fault were surveyed at two small quadrilaterals. During one year since the earthquake occurred, lateral displacements along the fault have attained to 20 mm (at quadrilateral A) and 12 mm (at B), in amounts, while vertical displacements have attained 5.4 mm and 1.7 mm, respectively. The sence of thesc displacement is concordant with that at the time of the mainshock. The relation between the after-seismic displacements (S) and the time after the mainshock (T) is shown as follows approximately; S=a_0+a_1log T+a_21/T. Many research studies on these phenomena by various organizations and staffs are summarized in this paper.
1976-03-25

1974年伊豆半島沖地震と石廊崎地震断層

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