Understanding why Japan's recent quake defied expectations

[n2doc]

By John Timmer

Last week, Science released three papers and a perspective, all focused on understanding what happened during the March earthquake that struck Japan. Now officially termed the Tohoku-Oki quake, the event is estimated as a magnitude 9 quake—one of the biggest in recorded history—and it has triggered significant aftershocks. But it's not the size alone that has people worried; it's the fact that something this size occurred on a segment of fault that we didn't think was capable of producing a quake of this magnitude (an estimate that has had disastrous consequences at the Fukushima nuclear reactors). Understanding what happened and why can potentially tell us a lot more about risks elsewhere along this fault.

The quake occurred along a segment of fault that creates the Japan Trench, where the Pacific plate slides underneath the one that plays host to Japan. This subduction zone gives rise to Japan's volcanoes, and the pressure helps push Japan upwards, creating more of its topography. As with many faults, the two plates sporadically stick as they slide past each other, triggering large earthquakes when the strain is released. All told, the earthquakes have to release a strain that results from a relative motion of the plates that's estimated at about 8.5cm every year.

Historic events have suggested that this strain is generally released along relatively narrow segments of a fault. During a large quake, one or two of these segments would typically shift, releasing much of the strain and transferring the rest to the flanking segments. This process made Japan the site of frequent earthquakes, many of them quite large, but few reaching anywhere close to the magnitude seen during Tohoku-Oki. Most of these occurred deep in the fault, closer to Japan than the trench.

That's one of the reasons that the March event was so unexpected. The other is the fact that, historically, this segment of the fault appeared to be relatively inactive. There were two ways to think about that. Either it was completely stuck, making quakes rare but building up excessive pressure, or it was moving with relatively little resistance, releasing the remaining tension through lots of small earthquakes.

more
http://arstechnica.com/science/news/2011/05/scrambling-to-understand-why-japans-recent-quake-defied-expectations.ars



Prior to the quake, the edge of the plate was pushed down (orange), while its interior buckled upwards (blue). The quake (bottom) released these, allowing the plate edge to rise and the interior to drop, a change that was accompanied by significant horizontal motion (black).

[meow mix]

1. good article ty.. wonder if the nukies will read that last part

"Globally, the quake has also told us that it's possible for a relatively small fault segment to spawn a giant earthquake. A perspective quotes the US Geological Survey's David Wald as saying, "If you can get a 9 that is this compact, it increases the number of places you can a 9 where you may not have expected one." So, coastal nations around the globe might want to start re-evaluating the risks they face from nearby subduction zones."

[Igel]

2. It certainly upset some settled science, that's for sure.

More than one bit of settled science.

Although, for the life of me, I can't imagine why anybody should hold TEPCO or anybody else responsible for some pretty weighty decisions untaken on the basis of settled science. If it's settled, it's settled, and that's that, good as gold.

Until, of course, it isn't settled.

[truedelphi]

3. Thank you for posting this n/t

[denbot]

4. We have the same type of earthquatke mechanics in the Juan de Fuca Plate

I remember a lecture discussing sedimentry evidence of huge tsunamis roaring over the coasts of Washington and Oregon.

http://en.wikipedia.org/wiki/Juan_de_Fuca_Plate

The Juan de Fuca last produced a 9 Richter scale event in 1700.

http://en.wikipedia.org/wiki/1700_Cascadia_earthquake

[bananas]

5. And yet seismologists warned about the problem and resigned from a nuclear safety board in protest

The nuclear industry has tried to deflect blame by claiming that nobody could've expected this to happen.
That's wrong, a number of seismologists spoke out warning about this.
One even resigned in potest from a government panel because it was using junk science:

http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=439x871004

Katsuhiko Ishibashi, a seismology professor at Kobe University, has said Japan’s history of nuclear accidents stems from an overconfidence in plant engineering. In 2006, he resigned from a government panel on reactor safety, saying the review process was rigged and “unscientific.”

There were many others, such as:

http://www.democraticunderground.com/discuss/duboard.php?az=show_mesg&forum=102&topic_id=4860251&mesg_id=4860310

Masanobu Shishikura: The Man Who Predicted the Tsunami in 2009.
...
Toshiaki Sakai: Utility Engineer Warned of Tsunami Threat at Japanese Nuclear Plant in 2007