Emile Okal: Scientists did not expect 9.0 magnitude earthquake in Japan

The March 11 undersea earthquake off the coast of Japan – which ultimately measured 9.0 on the Richter scale – was a more powerful earthquake than expected for that region, according to geophysicists. That is why the Sendai area of Japan, near the earthquake’s epicenter, was not prepared for such a large quake and the subsequent tsunami. EarthSky’s Beth Lebwohl spoke to Emile Okal of Northwestern University in Evanston, Illinois. He is an expert in large earthquakes and their tsunamis. He said:

Essentially, the feeling was that – if you had a very, very old ocean floor – when it was eventually recycled into the mantle of the Earth, you wouldn’t get these mega-earthquakes.

In other words, although it’s true the March 11 quake happened along the Ring of Fire around the Pacific Ocean, where earthquakes are common, such a large quake had not been anticipated. The March 11 quake happened in a place where Earth’s crust is very old – 140 million years old according to scientists’ reckoning. Old crust is cooler and denser than younger crust. The older crust in this region was believed by geophysicists to dive more smoothly beneath the land plate that carries northern Japan.

History seemed to confirm the idea that colossal earthquakes like the 9.0-magnitude March 11 event would not happen in and around this region. This part of the world had had 8.0-magnitude earthquakes in recent centuries, but not a magnitude 9, which releases 30 times more energy. Dr. Okal told EarthSky that the March 11 event is causing scientists to reassess what they understand about earthquakes.

We need to take a cautious attitude that any area which has a long enough fault zone is capable of very large earthquakes. That means we should be very careful in assessing the potential for very large earthquakes in Tonga, north of new Zealand, perhaps the Antilles, the Caribbean and Puerto Rico, where we have very old oceanic floor. In the past, we haven’t measured very large earthquakes in those areas. It could be they don’t happen very frequently, but they can occur.

The Sendai area of Japan, near the epicenter of the earthquake, was considered to be one of Japan’s best-prepared areas for tsunamis, but it was not prepared for a 9.0-magnitude quake and subsequent tsunami. The high seawalls along much of that part of the Japanese coast were not built to stop the huge waves that battered the coastline. Dr. Okal said these waves were more than 50 feet high.

The mitigation aspects – what was done to try to minimize the danger of the next tsunami – were designed with the expectation of about a 6-meter wave (about 20 feet). The waves that came were anywhere between 15 and 20 meters (between 50 and 65 feet). There is a question of why they built walls of that height when, in their history, they knew they could face larger waves. We’ve all seen the pictures. These walls were completely overridden. I think there’s going to be a reassessment of the kind of tsunami mitigation that’s put in place, and a symbol would be to raise the height of the walls.

He added that, moving forward, scientists who study earthquakes and tsunamis would do well to look to history more frequently. He spoke of the devastating Japanese earthquake and tsunami of 1896 – during Japan’s Meiji era – which caused over 25,000 deaths. Okal said it should be possible to use present-day scientific investigative techniques to study this historical earthquake, to learn more about how and when earthquakes might occur.

One would think that the study of the old historical events should be exercised. That does not mean only historical records, which have been written by historical seismographers, but perhaps going much farther into history – looking at tsunami deposits in stratigraphy [a branch of geology that studies rock layers and layering] to try to figure out the history of very large tsunamis.

While there are lessons to be learned from prior earthquakes, said Dr. Okal, he emphasized that, when it comes to future tsunamis – and earthquakes – the science is not entirely certain. As of now, no one has a precise mechanism for predicting earthquakes of any size. In part for that reason, he said, it’s important, during natural disaster emergencies, to follow the instructions of local authorities.

An educated resident or visitor to a coastline is the best possible survivor. Any persons who find themselves on the beaches – who feel a very long and strong tremor – should evacuate as quickly as possible.

Scientists will continue to study the tragic 9.0-magnitude earthquake and tsunami near Japan – which was not anticipated for that region – to find out how these events can be more clearly anticipated in the future.

April 4, 2011

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