Had a story published in New Scientist about research at the quake subduction zone revealing weak geological factors at the fault zone being behind the massive quake that hit Japan’s northeast in 2011. It sounds as though this is one of those “duh” moments, but methods used to come to this conclusion are truly groundbreaking, involving deep-sea drilling more than
800 meters below the seabed at the subduction site, which itself is over 7 km below the water.
The research was conducted aboard the Chikyu deep-sea research vessel (pictured right), which I recently boarded when it docked a couple of months ago — a fascinating experience in itself and one I shall be writing about.
Using the data collected from the site during research that began in early 2012, three papers were published in the peer-reviewed Science journal by a team of international scientists.
The first is about the geology of the fault zone and the main result found there is that the fault zone is only about 5 meters or less thick. “That’s fairly unusual and different from what we have observed in other subduction zones. Usually in the 10s of meters or more,” researcher James Mori told me.
The second paper is about measuring the friction of the fault using the core sample of the fault zone material and putting it in a machine that simulates an earthquake and measures the friction. The main result from that paper is the friction is very low due to a huge presence of called smectite — a a slippery clay that is often found to be at the centre of large landslides in Europe. “That means the fault slips very easily during the big earthquake,” Mori added. “Essentially this is the first time such material has been taken from a big subduction zone earthquake so it’s really a brand new result.”
The third paper relates to temperature measurement taken using devices that were placed in the boreholes earlier this year. This too was designed to measure friction. Essentially the same results were found as from other core sample experiment, meaning researchers had two very different ways of estimating the friction. Both gave about the same level of what is called the “coefficient of friction”. In this case the COF was about 0.1, which is very low — most rocks slip at a COF of about 0.5 or 0.6.
“So one of the ways of applying this is to say that subduction zones that have especially thin fault zones with a lot of smectite potentially can produce these very large slips of 50 meters and potentially could produce very large tsunamis,” said Mori. “That’s not to say that … if there is no smectite there is not going to be big tsunamis — that’s not a good conclusion.”
A separate report utilised data taken from the European Space Agency’s GOCE satellite that showed the March 2011 quake had been “felt” in space and distorted gravity over time.