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Comment by cossatot | Hacker News Reader

Accroding to the USGS's page (linked to by civilian), the earthquake was a strike-slip earthquake, where two blocks of crust slide laterally relative to eachother on a vertical fault, with no real uplift of subsidence of the seafloor. It's unlikely that there will be a major tsunami, as these are caused by rapid displacement of lots of water by the seafloor. However, given the right topography along the fault, it is possible.

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When the Japan Meteorological Agency issues a tsunami warning, do not disregard it thinking you've got a better handle on the factors in play. The Japanese have their country monitored thoroughly and take disaster preparedness more seriously than anything I've seen in the US.

http://www.jma.go.jp/jma/indexe.html (currently has warning)

With historical tsunami information:

http://www.jma.go.jp/en/tsunami/list.html

The JMA is top notch, no argument here. I didn't 'disregard' their warning, I just made a claim that the tsunami was unlikely to be major. This probably wasn't evident when the JMA's warning was made because it takes a while for the seismic waves to reach a broad-enough distribution of stations to confidently determine the type of earthquake.

In reply to many of the comments about tsunami height: A 1-2 m tsunami is a moderate tsunami, not a major tsunami. It can be very damaging to coastal infrastructure depending on the tide, the incidence angle of the waves, and other local factors, but it's simply not in the same magnitude as the major tsunamis that accompany M8+ subduction zone earthquakes.

Also, the measured amplitude of a tsunami will vary locally depending on the distance from the source and the geometry of the submarine topography: some areas focus the waves and the tsunami will be much higher, while some areas can spread the tsunami waves out. It's very similar to how a sound may be louder or quieter based on how far one is from the source of the sound and what's in between (walls of certain orientations, trees, etc.).

You were right. There was a 90cm tsunami. https://vosocc.unocha.org//GetFile.aspx?xml=4248gjer_l1.html...

Initial predictions had wave height of 3m. That's high enough to wash a person away were they to stay on the seaside. I would suggest that JMA information be a primary reference in the early hours after a Japan natural disaster instead of the USGS. They have a better handle on event measurements and worst case predictions than the USGS will.

Thankfully your assessment turned out to be more in line with observation.

Just note here so that people unfamiliar of tsunamis won't get misconception. 50cm of tsunami is enough to wash people away. It's totally different from 50cm-high wave[1]. 1m of tsunami is deadly if you got swallowed. 3m can easily sweep two-story houses.

[1] This illustration depicts the difference: https://twitter.com/_KE_N_/status/295134747603107840/photo/1 Calculate the inertia of the water. Also mind the actual tsunami carries lots of debris so you can't easily swim in it.

Very good point.

ありがたい、本当にいいポイントですね。　未来に日本へ帰りたい！

Authorities are warning of at most 1m above high tide levels.

http://ptwc.weather.gov/ptwc/text.php?id=pacific.TSUPAC.2016...

For reference, the tsunami from the Tohoku earthquake from 2011 (which caused the Fukushima disaster) was as high as 40m.

I was going to ask for a source for that 40m figure, but I just found it.

That is absolutely terrifying.

Edit: To put in context, even if your were at the top of a ten story building, the wave (and ensuing mass of water) would still reach you.

And if you were at the top of an eleven story building, you'd find yourself without 10 stories to support you!

1.4m has been recorded so far.

The warning in Japan is for as high as 3m tsunamis:

http://www3.nhk.or.jp/nhkworld/en/news/20161122_12/

Highest was 3m.

Interesting thanks for the details!

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