A natural rock block, which was visited by he famous Black Ship visited Japan in 1853, exists in Nakagusuku Village. This natural rock structure and its vicinity has been recently designated as a touristic spot and the authorities are concerned with its static and dynamic safety. The authors have been asked to investigate the stability of this natural monumental rock block. For this purpose, a physical model of this block with a similar density was prepared and its stability was investigated using a shaking table. The block become unstable when the base acceleration exceeds 350 gals. The authors also studied how to increase the dynamic resistance of the block against large earthquakes using an agent to bond the block to its base. The experimental results clearly indicated that the increase of the resistance was possible and the overall seismic resistance of the block increases. The outcomes of this experimental study has been described and their implications are presented and discussed.

1 Introduction

The recent earthquakes have recently shown that the monumental rock structures may be quite vulnerable to collapse besides other engineered structures. A natural rock structure exists in Nakagusuku Village of Okinawa Island. There is a drawing depicting the banner on the rock structure at the time that the famous Black Ship visited Japan in 1853 (Fig. 1a). The vicinity of the rock block has been now designated as a touristic spot. The elevation of the base of the rock structure is about 155 m and the rock itself is about 10 m high. The rock structure is locally known as Taachi (twin rock) and it was officially named as "Perry Banner Rock" by the Nakagusuku Village in 1997.

The authors visited this natural rock structure in April 2, 2011 and its vicinity was like a jungle. It was designated as a touristic spot in Nakagusuku Village and a project was initiated to arrange the site as a park. Geologic and rock mechanics investigations were carried out to check the static and dynamic stability of the natural rock structure and its vicinity.

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