2011 Japanese tsunami can be blamed on weak clay

The deep-drilling ship Chikyu, which collected the cores of the Tohoku-Oki fault zone.

Seismologists have to sort of compartmentalize their emotions about big earthquakes. They present exciting opportunities to study the details of earthquakes, but they can also result in tremendous human suffering. The massive magnitude 9.0 quake off Japan in March of 2011 was one such occasion—truly remarkable yet also infamous. Of course, a seismologist’s scholarly pursuits are not just academic. It’s critical to learn about these events in order to reduce the potential for future calamities.

That 2011 Tohoku-Oki earthquake was nothing if not colossal, but the size of the deadly tsunami that it triggered was still a surprise. Faults along these subduction zones, where one tectonic plate dives beneath another, extend at an angle from the surface near the seafloor trench to deep beneath the overriding plate. We often picture faults as simple planes marking the boundaries between two, distinct slabs of rock, but they are far more geometrically complex in reality. The shallow portion of the fault runs through contorted layers of sediment and rock that have been squished between the bulk of the two plates, and it behaves differently from the deep portion of the fault during an earthquake.

The Tohoku-Oki earthquake was centered at a depth of 20 to 30 kilometers (12.5 to 18.6 miles), where the rock is under greater pressure. As the motion on faults like this propagates towards the shallow end, the amount of sliding between the two plates normally decreases. Down deep, the frictional resistance along the fault weakens the faster the plates slide. Closer to the surface, the opposite is true—the faster the slipping motion, the greater the friction to dampen it. That didn’t seem to happen in this case, as the seafloor moved an astounding 50 meters (164 feet) or so, displacing the water above it and creating the tsunami wave.