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...
