Question: How do you predict whether a tsunami will occur after an earthquake?
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rying to predict whether a tsunami will occur after an earthquake is both simple and extremely difficult.
If there is a large fault movement under water that displaces the seafloor (basically, if seismographs point to a large offshore earthquake in an area where we know such events occur) then you are going to get a tsunami.
The size of the tsunami will vary depending upon the magnitude of the earthquake, how deep it was, and various details concerning the extent of seafloor displacement.
That's the easy one, but if it is a "moderate" earthquake – say between a magnitude 6.5 and 8 – it may or may not generate a large tsunami that may or may not be locally significant or a more region-wide event, and that again very much depends upon the type of fault rupture.
In general, we use a magnitude 6.5 as a sort of cut-off – above that and it is more likely that some type of tsunami could be generated. However, this does not mean that smaller earthquakes don't generate tsunamis.
To make matters more confusing, it's not just the earthquake itself that can generate a tsunami. A fault rupture causes an earthquake, and that quaking is basically a lot of shaking, which can lead to landslides just like we saw in the recent New Zealand event.
If those landslides fell into the sea, or occurred under water, then those – not the earthquake – can generate very large, locally significant tsunamis.
This was always a possibility at Kaikoura (at the north end of New Zealand's South Island) where the steep and deep Kaikoura Canyon comes in close to the shore.
Landslides down that canyon are quite capable of generating huge tsunamis, but for some reason it did not happen ... this time.
Then, of course, we have volcanoes. Who needs an earthquake when a simple underwater eruption can generate a tsunami?
Predicting whether a tsunami will occur after an earthquake is difficult, but while we are thinking about that we could be surprised by one that comes without any warning.
We find earthquakes difficult to understand, but predicting tsunamis just takes that problem to a whole new level.
Response by: Professor James Goff
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