In early 2011, Japan was rocked by a massive earthquake. Known as the Tohoku quake, it caused a tsunami that devastated the east coast of the country, killing over 15,000 people and throwing the Fukushima nuclear power plant into a meltdown. Scientists believe that this event brought to an end a series of massive earthquakes in the region that happen in a chain—known as a “supercycle."
Whilst earthquakes can be caused in a number of different ways, supercyles have only been observed in regions where one tectonic plate is being pushed under another, areas known as ‘subduction zones.’ The plate boundary off the coast of Japan is one such zone. It was the energy that built up as the two plates passed over each other that was eventually released, causing the destruction.
But, curiously, earthquakes don’t occur all along the plate boundary, only in certain regions. It’s been suggested that in these earthquake-prone regions, there is a large amount of friction that builds up over a long period of time until—crack—an earthquake occurs, releasing it. This in itself causes another build up of friction that causes another quake, and this happens again and again until eventually a superquake—like the one seen in Japan—releases all the energy along the whole section of the plate at once.
The conventional explanation for this is that different sections of the fault line have different ‘stickiness,' or frictional properties. “This... results in a kind of 'patchwork rug',” says Robert Herrendörfer, co-author of the paper on these supercycles. “To begin with, earthquakes rupture individual smaller patches, but later a 'superquake' ruptures several patches all at once.”
The new study, published in Nature Geoscience, has now offered a different explanation as to what it is that causes these supercycles. Instead of different patches of stickiness, the researchers suggest that it’s the width of these earthquake-prone regions that increase the probability of a supercycle occurring. The wider the zone, the higher the probability.
Herrendörfer and his team ran computer simulations of how the two plates move over each other at certain angles, and how at certain places along the fault line, the two plates can become linked. It was at these points where the friction started to build, and this happened most rapidly at the edges of the active zones. As an earthquake occurred at the edge of the region, it set off others further along in a sequence. This is what they found spurred the supercycles.
The team of researchers from ETH Zurich, however, warn against suggestions that this could lead to aiding in earthquake prediction. “Our theoretical models represent nature only to a limited extent, and aren't suitable for predicting earthquakes,” emphasizes Herrendörfer. “Our efforts were aimed at improving our understanding of the physical processes at work in an earthquake cycle. In [the] future, this knowledge could be used for generating long-term estimates of the risk of earthquakes.”
