Why Was the February 27, 2010 Tsunami Smaller than Expected?

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While a huge earthquake off the coast of Chile triggered a tsunami that moved at the speed of a jet aircraft across the Pacific Ocean on Feb. 27, the tsunami event – thankfully — was smaller than scientists expected. Some experts forecasted the event would produce 9-foot tall tsunami waves slamming coastlines along the Pacific Rim, which did not materialize; it would have been one of the biggest tsunami on record. At magnitude 8.8, this earthquake was among the largest seismic activity ever recorded. So, why was the resulting tsunami not a “mega event” as well?

“It is too early to know for sure,” said Anne Sheehan, a geologist from the University of Colorado – Boulder.

“It was a truly enormous “megathrust” earthquake, shallow and offshore,” Sheehan told Universe Today. “That kind of earthquake can generate a large tsunami if it displaces a large area of seafloor vertically (either up or down). It could be that more of the earthquake displacement was at depth below the seafloor, and did not rupture the seafloor surface as much as was expected given the size and depth of the earthquake.”

Sheehan said the Chilean earthquake released more than 400 times the energy of the recent Haiti earthquake. “It was truly an enormous earthquake in terms of energy release, the largest in the world since the 2004 Sumatra earthquake and the fifth largest since 1900,” she said.

The earthquake that generated the great Indian Ocean tsunami of 2004 is estimated to have released the energy of 23,000 Hiroshima-type atomic bombs, according to the U.S. Geological Survey (USGS).
The 8.8 quake in Chile released the energy equivalent of 20 billion tons of TNT, or 400 times the largest nuclear weapon ever detonated, the Tsar Bomba, a 50 megaton test done by the USSR in 1961.

Chile is along the “Ring of Fire” that stretches north from South America to the Aleutian Islands, then south through Japan, Indonesia and to New Zealand. The fault zone of the Chilean earthquake was extremely long — several hundred miles — signaling the potential for further large earthquakes in the region.

“These large earthquakes in the Southern Hemisphere have the potential to cause tsunamis all over the Pacific Rim,” Sheehan said in a press release. “Fortunately, people have a much greater understanding of the phenomenon today. Before 2004, a lot of people didn’t even know what a tsunami was,” she said.

Sheehan said she believes that lessons learned by Chilean experts following a world-recording setting magnitude 9.5 quake there in 1960, and subsequent quakes in the next several decades, resulted in stricter building codes, saving many lives. “The death toll is expected to be far smaller than in Haiti, an example showing that mitigation efforts really can be effective.”

As of this writing, the death toll stands of the Chile earthquake stands at 711. The quake in Haiti killed over 200,000 people.

Lessons learned from the 2004 Indian Ocean event also allowed officials to send out effective early warnings and initiate the evacuation of tens of thousands of people living on Pacific islands. Now, even more is being learned about the nature of tsunamis from this latest event, and future predictions should improve.

Sources: Email interview with Anne Sheehan, CU-Boulder press release