Australasian Science Magazine

By Stephen Luntz

The astonishing size of the tsunami produced by the Tohoku earthquake in March 2011 has been attributed to the slipperiness of the clay that fills the boundary between the Japanese and Pacific plates.

Japan’s tsunami defences proved inadequate. It had not been expected that the fault could move so far and thus produce such a large tsunami.

The Japan Trench Fast Drilling Project, an international collaboration, drilled into the fault to explain why. Their work has produced a series of papers in Science.

“What the core samples show, for the first time, is that the fault, particularly near the sea floor, is composed of less than 5 metres of very fine volcanic sediment, highly altered to a special type of clay (smectite), which acted as an incredibly slippery lubricant and allowed the huge quake to occur,” says Dr Virginia Toy of Otago University’s Department of Geology.

“One of the big revolutions in earthquake mechanics is that slippery sections can be associated with larger earthquakes,” says Toy. “Earthquakes were thought of being like an elastic band where pressure was applied until something broke, while more slippery sections of faults were thought to creep along, avoiding jerkiness.”

While this model fits the San Andreas Fault, it is now understood that an earthquake can start where friction is strong and propagate to nearby areas where movement is easier. The magnitude 9 quake caused 50-metre displacements in the sea floor, and consequently the sea above it.

The clay that makes this possible is blown by the prevailing winds from Japan’s volcanos out to sea, where currents bring it back to settle in the trench and be converted to smectite. Toy says that the build-up of pressure from the heating of liquids within the fault also contributed.

The drilling operation was named “fast” because great efforts were made to do the research before the frictional heat had fully dissipated. Heat measurements were consistent with models placing trench clay samples between appropriate rocks under high pressure.

The location of New Zealand’s volcanoes and the Kermadec Trench suggests that circumstances there may be similar. Drilling in an unprecedented 7 km of water, as was needed for Tohoku and would be required off New Zealand, is a considerable challenge. Toy is hopeful that the same team may soon do a study of the contents of the fault, “but only if the New Zealand government keeps contributing to the drilling program. Finding out what is in the trench will help us predict the size of disturbance we might face in our lifetimes, but it will not enable us to predict the timing of a quake.”