#jfast

Heat pulse from 2011 Tohoku earthquake found

A few weeks ago, we brought you the story of the JFAST project, a project which went to sea in 2012, a year after the Tohoku great earthquake off the coast of Japan (http://tinyurl.com/lze4l5d).

The task was simple; go out after the Earthquake, drill deeper into the ocean floor than anyone had ever drilled before, sample the fault itself, install sensors, leave them there for a year, retrieve them in another cruise without having them damaged by the fault or a landslide, and try to measure a heat pulse in the fault that no one had ever successfully measured before.

Ok, maybe not that simple.

For understanding the behavior of faults, this data really is important. Earthquakes should generate heat from friction. Everyday behavior illustrates this property; just rub sandpaper up against something and it will start heating up. But prior to now, no one had ever successfully measured the heat generated along a fault, despite repeated efforts to do so.

The heat generated on a fault is an important parameter to understand because it gives information about the friction on the fault. Friction is the force that resists motion, so it is the force that keeps a fault locked, and it is the force that must be overcome for an earthquake to start and to expand. The fact that the heat from a fault had never been measured meant that scientists who wanted to understand how earthquakes start and expand had to do so with virtually no data on the fault itself.

The cruise to install the sensors took place in 2012 and they were successfully recovered a few weeks ago. Here you see members of the drilling team examining part of the core in 2012; there is a story in the article linked below of how the Japanese crewmembers of the boat were actually able to see and touch part of the fault which devastated their country in 2011. That was something of a humbling read.

Finally, the temperature data is now available, and success! A whopping 0.5°C temperature anomaly was measured due to the 2011 earthquake. You might think “that doesn’t sound like much”. You’d be correct.

Fault models since the 1970’s have used a generally-agreed upon number for the friction on a fault during an earthquake, but that number had never been measured. The researchers took this data, projected back in time to when the earthquake happened, and produced an estimate of the friction necessary to produce that heat. The friction on this fault was an order of magnitude less than has been used in virtually every fault model for 40 years, and this low friction might well be why no one has ever pulled off this measurement before.

This low coefficient of friction suggests that during an earthquake, or at least during a large earthquake, the fault begins to slide easier than would be predicted just from rocks grinding against each other. Something must lubricate the fault; allow it to slide easier than predicted from the models. One obvious candidate is clay minerals trapped in the fault surface; clay minerals hold water that can be released by rapid heating, water which could serve to lubricate a fault during an earthquake.

Researchers now are moving to the laboratory and computers to try to simulate the properties actually observed. Thanks to this data, collected from the fault that created the 2011 disaster, we’ll hopefully wind up understanding vastly more about how faults operate and what happens when one of them begins moving.

-JBB

Image credit: JAMSTEC/JFAST project website: http://www.jamstec.go.jp/chikyu/exp343/e/gallery.html

Update from Nature: http://blogs.nature.com/news/2013/05/seismic-faults-temperature-implies-deadly-earthquake-involved-low-friction.html