Delayed Peaks of Tsunami Waveforms at Miyako from Earthquakes East off Hokkaido

Abstract

In the records of tsunami waves observed at Miyako, Sanriku Coast, Japan, there sometimes appears a tendency that after the initial wave arrival, tsunami waves once become attenuated, and the eminent peak suddenly appears in the cases of tsunamis generated in the sea region east off Hokkaido. This eminent peak is called the delayed peak of the tsunami wave. To clarify the causes of this delayed peak, we carried out numerical experiments on tsunami propagation. As a result, we found out that the cause of the delayed peak of the tsunami is the superposition of waves traveled two different routes. One is the route in which the tsunami wave directly reflects at Shimokita Peninsula from the tsunami source, and transfers into an edge wave mode. The other is the route in which the direct wave reflects at south coast of Hokkaido, propagates to Shimokita Peninsula, and transfers to the edge wave mode at Shimokita Peninsula. Because these two components, which run along quite different routes, arrive at Miyako at almost the same time, the heights of the delayed peaks sometimes exceed those of the initial waves.