The sand boils induced by the 1995 Southern Hyogo Prefecture earthquake (M7.2) occurred in many locations along the Nojima earthquake surface rupture zone. Some concentric circles of sand boils that occurred along the surface rupture zone were displaced by surface rupturing at the Ogura location in the central part of the rupture zone. This indicates that liquefaction was caused by instantaneous, strong ground shaking before surface rupturing. The grain-size distribution patterns of sand boils show that the sand boils are in the range of grain size to generate liquefaction. Liquefaction induced by earthquakes is generated by the horizontal vibration of S-waves in noncohesive soil under increased pore pressure and induced effective stress. In the Ogura location, however, field evidence and seismic data indicate that the liquefaction induced by the 1995 Southern Hyogo Prefecture earthquake formed during 3 s to 4 s after the main shock, mainly by the vertical vibration of P-waves rather than the horizontal vibration of S-waves; we define this as instantaneous-shaking liquefaction.

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