Core samples from the fault show it was quite slippery.
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.
The obvious question—why did it behave unusually?—isn’t an easy one to answer. Luckily, researchers had already been organizing an ambitious project that was just waiting for a big Japanese earthquake to study. The Japan Trench Fast Drilling Project (JFAST) utilized the incomparable Chikyu deep-ocean drilling ship to drill cores right through the fault while the crime scene was still fresh.
Just over a year after the Tohoku-Oki earthquake, Chikyu sailed about 200 kilometers (124 miles) east of the tsunami-ravaged city of Sendai. In water nearly seven kilometers deep, Chikyu drilled three holes some 850 meters (2789 feet) below the seafloor. Three new papers published in Science detail some results of the work done by bringing up core samples of the fault zone and sending instruments down the boreholes.
Chikyu drilled through lots of layers tilted up by deformation before hitting a horizontal layer sitting on top of the basalt of the subducting plate. That layer was made of clay that accumulated on the deep-ocean floor during the Cretaceous. The fault that allowed the plates to slip past each other ran through this layer, and its properties provide the explanation for the tsunami.
A zone less than five meters (16.4 feet) thick was riddled with the scars of past earthquakes—a chaotic jumble of slick clay violently smeared and split. Normally, fault zones like this are thicker, but the weakness of this rock accommodated all the damage, providing a sort of lubrication between the plates.
To quantify that weakness, the researchers employed a couple different techniques. First, they lowered a string of temperature sensors down one of the holes and left them in place for nine months. During an earthquake, the friction caused by the sliding along the fault generates heat that warms the surrounding rock. One of the ideas behind the JFAST project was to measure the temperature along the fault before it can cool off.
They found that the fault zone was about one-third of a degree Celsius warmer. Given what else we know about the distance the fault slipped and how long that lasted, the researchers were able to calculate the frictional resistance of the fault. The other technique went after the same number, but it did so using laboratory experiments subjecting samples of the rock to earthquake-like torture.
Both studies yielded similar estimates. To compare using the familiar coefficient of friction, the fault zone came in at about 0.08 while most fault materials are closer to 0.6 to 0.8. The laboratory work showed that instead of dampening the sliding by stepping up frictional resistance, the fault was weakest at the high rate of slip during the earthquake. Without anything to put on the brakes, the large amount of motion on the fault extended all the way to the surface, leading to the tsunami.
The immediate obvious question is whether this type of situation exists elsewhere in places where large earthquakes could occur in the near future. The researchers were able to compare the clay to a sample from a fault in the Nankai Trough—the subduction zone along the southwestern part of Japan. The Nankai Trough sample was grittier and contained a much smaller proportion of the incredibly weak clay mineral smectite, which accounted for over 75 percent of the Tohoku-Oki fault rock. Consequently, it provided much more frictional resistance in the lab experiment.
It seems unlikely that this is the only place where faults developed in smectite-rich clay. The 2011 tsunami wrote a cautionary tale about placing too much confidence in our ideas of where tsunamis can occur. Seismologists will be on the lookout for areas along other subduction zones where weak fault zones could exacerbate the damage from strong earthquakes. Unfortunately, it may take one of those earthquakes to reveal that fact.