[SSS16-P07] Drifted huge boulders by storm surges since 1976 derived from the dike of quartz porphyry in Hashigui-iwa facing the Nankai Trough, Pacific side of central Japan
Keywords:Nansai Trough, tsunami boulder, storm surge
Hashigui-iwa is located seven kilometers northeast of Cape Shiono-misaki facing the Nankai Trough, which is the subduction zone between the Eurasian Plate and the Philippine Sea Plate. It is Neogene dike of quartz porphyry intruded into Neogene sedimentary rocks mainly composed of shale and sandstone. It also seems like a bridge pier composed of 25 individual slab or pillar rocks aligned in a straight line. In the western side of Hashigui-iwa dike, wave-cut benches are well developed on sedimentary rocks during the late Holocene. A few thousands of boulders are scattered on the benches. All boulders are composed of volcanic rock derived from Hashigui-iwa dike. Possible mechanism for transporting the boulders more than several tens of meter from their original position must be inferred as strong wave action due to typhoons or tsunamis. We clarified the main cause for horizontal movement of scattered boulders should have been tsunamis generated by faulting in the Nankai Trough region from numerical analysis on water velocity (Maemoku et al.,2010). This study aims to constrain the maximum dimensions of boulders to be drifted by storm surge using multiple aerial photographs taken in-between 1976 and 2019. We found that at least 19 boulders were moved during the period. The maximum length of the boulders is about 1.85 m (estimated mass is 2.6 ton), and it is classified into light mass of all the boulders. This indicates that the boulders with length larger than 2 m are difficult to be move due to storm surges, but are moved due to large tsunamis.