The 2011 earthquake and tsunami that hit the Pacific coast of Tohoku, Japan ranks among the most powerful and destructive naturally occurring events in recent years.
In an effort to better understand what caused this devastating tsunami, a team of scientists has published a set of studies that shed light on what triggered the dramatic displacement of the seafloor off the northeastern coast of Japan. The findings also suggest that other zones in the northwest Pacific may be at risk of similar huge earthquakes.
Prof. Christine Rowe, of McGill’s Department of Earth & Planetary Sciences, was one of 27 scientists from 10 countries who participated in an expedition to the Japan Trench in 2012.
The joint where the Pacific and North American plates meet forms what is known as a subduction zone, with the North American plate riding over the edge of the Pacific plate. The latter plate bends and plunges deep into the earth, forming the Japan Trench.
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.
Until 2011, the largest displacement of plates ever recorded along a fault occurred in 1960 off the coast of Chile, where a powerful earthquake displaced the seafloor plates by an average of 20 meters. In the Tohoku earthquake, the slip amounted to 30 to 50 meters — and the slip actually grew bigger as the subterranean rupture approached the seafloor. This runaway rupture thrust up the seafloor, touching off the horrifying tsunami.
The results of the expedition reveal several factors that help account for this unexpectedly violent slip between the two tectonic plates.
For one thing, the fault, itself, is very thin — less than five meters thick in the area sampled. To our knowledge, its the thinnest plate boundary on Earth, Rowe says. By contrast, Californias San Andreas fault is several kilometers thick in places.
The scientists also discovered that the clay deposits that fill the narrow fault are made of extremely fine sediment. The discovery of this unusual clay in the Tohoku slip zone suggests that other subduction zones in the northwest Pacific where this type of clay is present — from Russias Kamchatka peninsula to the Aleutian Islands — may be capable of generating similar, huge earthquakes, Rowe adds.