Journal: News & Comment

How far did the tsunami go inland?

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Six months later, I find a lot of people coming here trying to figure out how far the tsunami of last December 24 went inland. Here's one:

Derek, I apologize for asking this question but I find there are over 9 million entries on the Asian disaster. I have a friend who does not have a computer and she asked me how far the tsunami went inland. I cannot find the information. She has relatives there and they were not harmed so I'm not sure why she wanted to know. I "thought" I saw on the news when this first happened that it went 1 mile inland. Do you know if that is the approximate? Thanks for your help.

Following up on my earlier consolidated article about the Indian Ocean tsunami, here's my answer:

There were several places where the tsunami went a lot further than a mile inland, and many, many where it did not. But it's not a simple relationship of how close those places were to the earthquake that caused the tsunami.

How far inland the tsunami went (or any tsunami would go) varies widely depending on:

• How close the land location you're talking about was to the quake (it would be different for Sumatra than Somalia) and in what direction (Sri Lanka was in the east-west path, Bangladesh wasn't).
  
  
• What sorts of intermediate objects (islands, channels) lie between it and the quake (Thailand and the west coast of Sri Lanka and India were hit even though they weren't in the direct path, because the waves diffracted around channels and headlands; much of Malaysia and the rest of Indonesia was protected by intermediate islands, and outside the Indian Ocean there was little effect because of intervening land masses).
  
  
• The shape of the seafloor near land (shallower and steeper slopes lead to very different kinds of tsunami waves once they hit shore). Watch the simulation at the Seed tsunami page to see what I mean.
  
  
• The shape, slope, and composition of the land itself where the tsunami hit (a cliff would be very different than a gentle shore slope) and how far that shape, slope, and composition reach inland (a steep slope that ends in a wall wouldn't let the tsunami get far; a shallow slope that extends a long way would).
  
  
• What kinds of buildings, plants, and other material were on the seashore (dense forest or habitation can slow or redirect or channel the tsunami's waves).

So, in some places (like where my friend Mark was in Malaysia), there were just higher than normal surf waves (and he was able to walk out of chest-high water), while only a few hundred metres away people were swept out to sea by massive surges. Similarly, in places with shallow, gradual slopes both under and above the sea, close to the quake, the tsunami traveled several miles inland, as in Banda Aceh in Sumatra and in some places in Sri Lanka.

In other places nearby, it may have gone hardly anywhere because of steep cliffs, or because it was a long way and perhaps also in an off-axis direction (Somalia, Bangladesh). The local topography (above and below the sea) determines almost everything.

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