Lens effect makes Chilean tsunami a threat in Japan

Lens effect makes Chilean tsunami a threat in Japan

More than half a day had passed before the Japan Meteorological Agency lifted tsunami advisories it issued for Pacific coastal regions of eastern Japan in the wake of a magnitude-8.1 earthquake that occurred off Chile at 8:46 p.m. on April 1 (8:46 a.m. JST on April 2).

Because of the positional relationship between Japan and Chile, tsunami waves continued to hit Japan for a long time.

In the wake of the earthquake, tsunami waves began reaching the Pacific coast of eastern Japan at about 7:00 a.m. on April 3. Chile, which is on the opposite side of the globe, is about 15,000 kilometers from Japan.

But the deeper the water, the higher the speed of tsunami tends to be. In the central part of the Pacific Ocean, where the depth is 5,000 meters, tsunami waves move at 800 kilometers per hour, or as fast as a jet plane.

Thus, the waves reached Japan less than a day after the earthquake occurred.

The JMA issued the tsunami advisories at 3:00 a.m. on April 3 to prepare for the arrival of the waves and lifted them 15 hours later.

The reason why the advisory remained in place for more than half a day is because a series of tsunami waves continuously reached Japan.

"Tsunami waves that started off Chile spread into all directions, first reaching the coasts of the North American continent, islands in Southeast Asia and other places," said Fumihiko Imamura, director of the International Research Institute of Disaster Science of Tohoku University, who is an expert on tsunami engineering research. "Then the waves were deflected and many of them headed toward Japan."

The phenomenon in which tsunami waves become concentrated within a specific region is called a lens effect, as the mechanism is similar to how light can be focused on a single point through the use of a lens.

"Japan is located on the opposite side of the Pacific Ocean from Chile," said Imamura. "This relationship, in terms of geographical position, is what caused the lens effect."

Due to the lens effect, tsunami waves combine and overlap, making it possible for the highest waves to arrive long after the first wave.

In the wake of the April 1 quake, the highest recorded tsunami was in Kuji, Iwate Prefecture, 60 centimeters. Tsunami waves were first observed in the city at 6:50 a.m. on April 3, but the highest came about 5½ hours later.

Tsunami coming from faraway places are called distant tsunami.

Yohei Hasegawa, director of the JMA's Earthquake and Tsunami Observation Division, emphasised: "With distant tsunami, the arrival of waves occurs over a longer period, and in many cases the biggest waves arrive later. This type of tsunami should not be underestimated."

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