Abstract
Neogene Shimajiri mudstone is widely distributed in the central and southern areas of Okinawa Island and along the marine terrace in the northeastern part of Miyrko Island. Landslides are apt to occur in those areas controlled by the landform and geological structure of the mudstones. The fist-time slide occurs along the small fault and joint planes of the Shimajiri mudstone which is uplifted and exposed in the ground. In the mudstone which is caprocked by the Ryukyu limestone of Quaternary-age, slides occur in the colluvial deposits. Some distinctive features of the first-time slides in the Shimajiri mudstone are as follows: (1) structurally weak planes become a slip surface; (2) the shear strength along the slickensided surfaces of weak planes has lowered to near the residual strength; (3) fractured mudstone mobilizing the low peak strength exists in a part of the shear zone, and it, together with the structurally weak surface, resists the sliding force of the whole block; (4) the sliding is triggered by the artificial reformation of land and initiated by the rainwater seepage into the fractured shear zone.
