Numerical Analysis on Tsunami Reduction Factor – A Study in Matsushima Bay during the Great East Japan Earthquake Tsunami

The Great East Japan Earthquake Tsunami caused severe damage in various areas; however, the damage was relatively less in the coastal area of Matsushima Bay. The high water marks resulting from the tsunami around the bay were noticeably smaller than those in other nearby regions. It has been suggested that the islets at the mouth of the bay may have reduced the tsunami height, but the specific factors that caused the topography of Matsushima Bay to demonstrate a tsunami reduction effect in the surrounding area of the bay were not clear. Elucidating these factors can provide valuable information for future tsunami measurements. In this study, we evaluate the factor by numerical analysis in which the topographic details of the islands are removed in stages, and the water depth at the entrance to the bay is increased. A comparison of the results show that the relatively shallow water depth as well as the islets in the bay contributed to a reduction in the tsunami height and caused a delay in tsunami arrival to the coastal areas around the bay. In addition, we show that an appropriately sized breakwater at the mouth of the bay can obtain the same tsunami reduction effect as the one given by the actual topography in the bay.