An unprecedented drilling expedition has revealed that the devastating 2011 tsunami off the coast of Tohoku, Japan, was due to a particularly weak and slippery fault zone.
Starting in July 2012, a team of 27 scientists from 10 different countries measured the fault zone where the tsunami-triggering earthquake occurred in March 2011. The expedition, known as the Japan Trench Fast Drilling Project (JFAST), drilled about half a mile (800 meters) into rock covered by more than 4 miles (7 kilometers) of water, the deepest drilling expedition ever, Emily Brodsky, one of the team scientists, told weather.com. The Deepwater Horizon rig, for comparison, was drilling into rock at about the depth of a mile (1.5 kilometers). The results of the expedition are published in a series of three papers today in Science.
The magnitude-9 earthquake occurred in what is known as a subduction zone, meaning one plate of the Earth’s crust is diving down below another. The plates are generally held back by friction, but sometimes slip, causing an earthquake.
We know a lot about what friction is prior to earthquakes but we didn’t actually know what friction was during earthquakes, and that’s kind of important, Brodsky, a University of California, Santa Cruz geophysicist and co-author on one of the papers, said. Because how far it’s going to go is in some sense governed by what’s holding it back.
In this case, the plate went far about 50 meters, the largest slip ever recorded. Because friction creates heat like what happens when you rub your hands together the team decided to measure the temperature of the fault zone’s rock in relation to the surrounding area, as a way of measuring the friction during the quake.
Using 55 instruments during nine months, they found that the temperature around the earthquake fault zone was one-third of a degree higher than the surrounding rock, a significant difference, but not as extreme as the two-degree difference predicted by laboratory projections based on models. This indicated that friction in the fault zone was unusually low, allowing the plate to slip so far and trigger such a massive tsunami.
Another of the papers described rock samples brought up from the sea floor. The fault turned out to be filled with a slippery clay that feels like a lubricant, geologist Christie Rowe, co-author of that paper, said in a release. The third paper detailed laboratory studies on the rocks.
What’s important is that we have found a reason why this earthquake might have moved so much, which is a direct cause of the extreme size of the tsunami, Brodsky said.
Japan has an early-warning system in place for earthquakes, which was quite effective during the Tohoku earthquake, Brodsky said. The United States has no such system, though there are efforts underway to build one.
Something to bear in mind is that what happened off of Japan is very similar to something that we expect [could] happen off the Pacific Northwest, off of Seattle and Washington, Brodsky said. That’s a very similar system and could easily have a similar-sized earthquake any day.
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