Tsunami in Open Ocean

Tsunami in open ocean have a very small wave height (10cm to 1 m) with very long wavelengths, to the point where they are visually undetectable in open ocean. Which leads to a question: what is up with this footage? ([alternate version])

GoogleEarth bathymetry off the coast of Matsushima, Japan

GoogleEarth bathymetry off the coast of Matsushima, Japan.

The footage is clearly in the ocean, yet the tsunami wave is equally clearly visible. The BBC did not fall for a hoax, and I didn’t lie to you about tsunami behaviour. The resolution is in the caption: this is recorded a mere 5 km offshore of Matsushima. That sounds like quite a bit of distance, but even with the most sloppy use of bathymetry in GoogleEarth, water depth is easily under 50 m for the first 20 km off shore. The tsunami is shoaling, with wave height dramatically increasing (and will keep increasing as the water depth drops farther). The tsunami period for this event is roughly 12-15 minutes, so if the video footage continued longer, we’d expect another crest, and another, and another…

 

GoogleEarth bathymetry farther off the coast of Matsushima, Japan

GoogleEarth bathymetry farther off the coast of Matsushima, Japan.

The continental shelf (the area of relatively shallow water) off the east coast of this part of Japan extends for roughly 200 km before abruptly dropping off, with water depths going from 1,000 m to more than 5,000 m in the next 100 km off shore. A tsunami is in open ocean (and only detectable through pressure changes recorded by the DART buoy system) once it’s in this much deeper water. If we had footage from a coast guard ship 300+ km off shore, it would be thoroughly undramatic as the tsunami passed by without notice.

 

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