More Earthquakes in the Indian Ocean

There's been another earthquake in the Indian Ocean - this time it was 8.2 on the Richter Scale.

A USGS spokeswoman told Reuters the quake struck 125 miles west northwest off Sibolga, Sumatra or 880 miles northwest of the Indonesian capital of Jakarta, close to where the 9.0 magnitude quake struck in December.

A spokesman for the U.S. Geological Survey told CNN that the earthquake could cause tsunamis.

"Certainly evacuations should be occurring. I hope they are," spokesman Don Blakeman said, saying of the earthquake that "it could also cause some local tsunami activity."

In an attempt to use my meager education to enlighten those unfamiliar with the science of the situation, here is a repeat of my post from Boxing Day 2004, outlining the mechanics of how earthquakes cause tsunamis. Lets pray that the requisite criteria aren't fulfilled for this one. Lets also hope that the countries that could be battered by them again are taking steps to evacuate.

From Dec. 26th 2004

It occurs to me that the general public learn much of what they know about tsunamis from Hollywood movies like The Day After Tomorrow - movies that tend to sacrifice fact for spectacular effects. I'm no expert, but I might as well use my soapbox to straighten out a few facts.

To begin, the tsunamis were a symptom of an earthquake measuring 8.9 on the Richter scale. To give you some sort of idea of the power released during a quake of this magnitude, it's equal to about 32 billion tons of TNT. As residents of LA will be aware, the Richter scale is a base-10 logarithmic scale, which means that an earthquake measuring 9.0 on the scale will release many times more energy than an 8.0 quake.

The quake occurred just off the south coast of the Indonesian island of Sumatra, where the Eurasian and Indo-Australian tectonic plates meet. Those of you who have a firm grasp of plate tectonics can skip over the next paragraph, but for the rest of you...

As you probably know, the surface of the earth is made up numerous sections known as tectonic plates. These plates effectively float on the dense, hot asthenosphere, a layer of the mantle in which the rock is hot enough to become pliable - to be able to deform without fracturing.

Now, these tectonic plates move incredibly slowly - usually less than a few centimetres every year, and it's where they meet each other that we get problems. The Eurasian plate and the Indo-Australian plate meet each other at Sumatra. The Indo-Australian plate is being forced underneath the denser Eurasian plate at a rate of about 6cm each year at the Java Trench (here is a simplified animation of the process). The movements of the two plates are what cause earthquakes. The stresses caused by the two plates scraping together force pressure to build in the rock until it discharges all at once, literally snapping. The faultline slips perhaps 10-15m, causing the displacement of huge volumes of water. 15 metres doesn't sound like much, but you have to realise that this is 15 metres movement over maybe 1,000km of plate margin.

The displaced sea water travels at speeds of up to 500mph, effectively invisible to the naked eye. The crest of the waves will only be a few feet high in open water, the only visible sign of the enormous volume of water speeding along under the surface.

The momentum of the waves means that they can travel vast distances with little loss of energy. The 1960 earthquake off the coast of Chile created a tsunami that had enough energy to travel 10,000 miles in 22 hours until it hit land in Japan, killing about 150 people. Update - the Sumatran tsunami has now crossed almost 3,000 miles of ocean, killing at least 9 people in Somalia.

As the waves approach land, and the sea floor rises, the water slows dramatically as the waves compress like an accordion, forcing them to pile up vertically. They draw water from the coastline, creating powerful undercurrents that can drag swimmers out to sea. The tsunamis don't break like normal waves, but simply hit the coast like walls of water, destroying buildings, tossing boats into the air like toys, and smothering those who have been unable to escape under unimaginable volumes of water.

It's folly to think of a tsunami as just a large wave. It's much more accurate to imagine that it is an extension of the sea, conquering the land and pushing back the coastline, in some cases by hundreds of metres. The 1993 tsunami at Okushiri, Japan reached 32m in height - the same as an 8-storey building. You can't hide from that, and you sure as hell can't try to swim to the surface. All you can do is run.

Some of us have a head start. I wrote my dissertation on the gap in preparedness and aid for earthquakes between the developed and developing world and, tragically, most of the nations affected by today's disaster have laughable warning systems. It's been several years since I've looked into the subject but, last time I looked, India's method of saving lives following tsunamis was to build refuges on stilts. Their warning system consisted of rusting air raid sirens from WW2. I hope they've improved in the last 5 years, but I wouldn't bet on it.

The Pacific, in comparison, is relatively well-prepared for tsunamis. Underwater sensors can alert us to approaching tsunamis, giving people living on the Pacific Rim adequate time to find high ground or flee inland. Unfortunately, such systems are not widely used in the Indian Ocean and Bay of Bengal due to the cost of installing and monitoring the sensors, and the relative rarity of tsunamis.


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