Why was the Chilean Tsunami so Small?


For the people of the Juan Fernandez Islands and the coastal town of Talcahuano in Chile, Saturday morning's tsunami certainly didn't feel small. On the heels of a titanic magnitude 8.8 earthquake, surges of ocean water over 7 feet high crashed ashore, flinging large boats and shipping containers inland and leaving piles of wreckage along the coastline.

As of the time of writing, 5 people are dead and 11 still missing as a result of the tsunami. Some 711 people are confirmed dead from the earthquake itself, and that number is expected to rise. There is no question that this was a grave disaster for the people of Chile.

But compared to the 2004 tsunami that killed 230,000 people across the Indian Ocean, people living on the Pacific coasts of Mexico, California, Hawaii, Japan, New Zealand, the Philippines, and numerous other places recorded wave heights of 1 meter (3.3 feet) or less. In short, they dodged a bullet.

The tsunami was spawned by the 5th largest earthquake ever recorded, so why was it so small?

The short answer is that scientists aren't sure yet. As I wrote back in December, tsunami experts are just starting to figure out why the 2004 waves were so big — it turns out there were effectively two different tsunamis formed from two different faults during that event, one of which was a 30-meter (100-foot) high wave aimed directly at Banda Aceh. Close to 130,000 people are thought to have perished in that city alone, accounting for the majority of casualties in the disaster.

It may be several months or years before the Chilean tsunami is precisely understood.

But there are several other reasons why so many fewer people lost their lives this weekend. One is preparedness. Having suffered a devastating tsunami following a magnitude 9.5 earthquake in 1960 — the largest earthquake ever recorded — Chileans knew what was coming. Reports indicate that alarms were sounded in the town of San Juan Bautista on Robinson Crusoe Islands, for example, possibly saving hundreds of lives.

That preparedness is also reflected in Chile's building codes, another lesson of the 1960 quake. Strong buildings (along with Chile's relative affluence and robust infrastructure) likely saved thousands of lives and prevented this disaster from approaching the magnitude of the continuing horrors in Haiti.

The size and depth of the Pacific Ocean may also have been a factor. Tsunami experts point out that if a fault ruptures in relatively shallow water, the energy pulse may weaken as it emanates out into the deep ocean. This may have been the case with the Chile quake. It's also possible that the fault moved somewhat horizontally, lessening the amount of vertical displacement in the water.

Time — and a good deal of future investigation into the area — will tell.

Videos: Channel 4, Steve Ward (via Andrew Revkin) 

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