Scientists Identify Cause of Japan's Devastating 2011 Tsunami

In March 2011, a devastating tsunami struck Japan's Tohoku region, leaving destruction in its wake. Now, researchers have uncovered the cause of this tsunami, shedding light on what displaced the seafloor off the northeastern coast of Japan.

Learning more about the 2011 tsunami and its causes is an important step for monitoring future events. This could help researchers provide earlier warnings to coastal areas, which could help reduce the number of fatalities in a region.

During their study, the scientists underwent a 50-day expedition on the Japanese drilling vessel Chikyu. They then drilled three holes in the Japan Trench area in order to study the rupture zone of the 2011 earthquake, a fault in the ocean floor where two of Earth's major tectonic plates meet deep beneath the surface of the Pacific Ocean.

The conventional view among geologists has been that deep beneath the seafloor, where rocks are strong, movements of the plates can generate a lot of elastic rebound. Closer to the surface of the seafloor, where rocks are softer and less compressed, this rebound effect was thought to taper off. In fact, the largest displacement of plates before the 2011 tsunami occurred in 1960 off the coast of Chile. That's when a powerful earthquake displaced seafloor plates by an average of 20 meters. The Tohoku earthquake, in contrast, displaced its own plates by 30 to 50 meters.

So what caused this unexpectedly violent slip between two tectonic plates? It turns out that the fault itself is very thin--less than five meters thick in the area sampled. This makes it the thinnest plate boundary on Earth. In addition, clay deposits that fill the narrow fault are made of extremely fine sediment, which makes it extremely slippery. These traits in particular caused the massive slip that resulted in the major tsunami that devastated the region.

Yet these findings don't just show researchers a bit more about the past; they also have implications for the future. Other subduction zones in the northwest Pacific where this type of clay is present--from Russia's Kamchatka peninsula to the Aleutian Islands--may also be capable of generating similar, huge earthquakes.

The findings are published in a series of studies here, here and here in the journal Science.