The tsunami that never was: Why the Indonesian earthquake didn't make waves

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People riding motorbikes and cars packed the street in Banda Aceh after a strong earthquake struck Indonesia
People riding motorbikes and cars packed the street in Banda Aceh after a strong earthquake struck Indonesia Photo: AP Photo/Ichsan Ivansah

The latest Earthquake just off the coast of Sumatra - and a series of aftershocks - must have struck terror into the people of Aceh Province. Eight years ago, they suffered one of the world worst Earthquakes. Its epicentre was very close to today's quake and, at first sight they seemed about the same magnitude.

But the 2012 quake is no repeat of the disaster eight years ago when 230,000 died in the quake and the the Tsunami that followed.

Why not?

Well first, todays quake was less powerful 8.6 - 8.7 compared to 9.1 in the disaster year. That doesn't sound like much of a difference. But scientists use a special scale for measuring Earthquakes - a change of 0.1 in magnitude is about 1.4 times the energy release. So 9.1 releases (1.4 x 1.4 x 1.4 x 1.4 x 1.4) as much energy as a 8.6 - that's around seven times as powerful.

People riding motorbikes and cars packed the street in Banda Aceh after a strong earthquake struck Indonesia
People riding motorbikes and cars packed the street in Banda Aceh after a strong earthquake struck Indonesia Credit: AP Photo/Ichsan Ivansah

Secondly, since 2004, scientists have set up a chain of monitoring stations around the Indian Ocean (one already existed in the Pacific) using buoys which measure wave height. So they can predict Tsunamis and issue alerts - as they did this time. In fact they never issued a full-blown warning, only a "watch".

That means people were prepared.

People riding motorbikes and cars packed the street in Banda Aceh after a strong earthquake struck Indonesia
People riding motorbikes and cars packed the street in Banda Aceh after a strong earthquake struck Indonesia Credit: Reuters

Thirdly, and perhaps most important, today's event was "the wrong kind of quake". The 2004 quake was what geologists call a Megathrust quake. Nearly all really powerful quakes (over 9) are like this. They happen when one plate in the Earths crust is pushing under another.

There's friction between the two plates and so stress builds up. When that stress is released, that's your earthquake. And in the case of a Megathrust, the plate on top flips upwards. If its undersea, that pushes the water on top upwards too, and that creates the Tsunami.

Today's quake was a different type - geologists called it Strike-Slip. In this case two plates are moving side by side HORIZONTALLY. Again, there's friction and stress builds up until its released in a violent movement that is the earthquake. But there's no (or not much) UPWARD movement so you don't get a Tsunami.

This time it looks as if the Indian Ocean States have been lucky.