The 9.0 quake in Japan was different, huge and released a lot of energy but now we know how different. In our understanding of how quakes work we have always thought they went, as some say, like zippers, starting at one point and ripping along a fault line with the length of the rip dictating the size of the earth quake. It was because of this that the maximum of a 7.5 was thought to be the maximum quake that could happen along the fault line by Japan and why only that size of quake was the design basis for the protection of the nuclear reactors at the stricken Fukusima nuclear plant.
But apparently even though that type of pattern has been the normal type of quake on the worlds fault lines we were wrong. Instead of the quake opening like a zipper along the quake it burst in star burst formations along and around the fault like fireworks over an area of fault line of 40,000 square kilometers. This is a lot less area than what is normally required to produce a quake of this magnitude but it’s not the size of this quakes rupture area that makes the power of the quake. What appears to have caused the large release of energy is the amount of slip between the two plates that make up this fault line.
The two plates are now estimated to have moved as much as 60 meters past each other. Now for instance the Chile quake last year the ground moved 30 meters and that much movement was in only one portion of the slip zone. This Japan quake may have moved 60 meters along 3 different portions of the fault resulting in as much as 10 times the energy released in those first 3 minutes of the quake.
This new type of quake and the way it happened could also happen at any other subduction zone. This means that any of the other subduction zones could also have a quake of this new type. Meaning that any other subduction fault now has the possibility of a quake of 1 to 1.5 magnitude higher that it’s currently projected for that fault.
The japan fault had a quake 2 days before the 9 with a magnitude of 7.2, the expected max for the fault. It seems now that the 7.2 was just setting up the the fault for the 9 to happen. That 7.2 was thought to be the big one. So now we need to apply this new knowledge about how a quake can rupture and apply it to other subduction zones. For instance, the chile quake last year (feb 27, 2010) was an 8.8. It ruptured in the classical style of a zipper. If it had instead ruptured in the style the Japan quake did it could have been as high at 9.8 or 9.9.
Another area that could rupture and create a quake of this style is the cascadia subduction zone. This zone is a single long fault that is thought to have produced a quake of 9.6 and in the standard model is thought to be able to produce up to a 9.8 quake. With the new model of quake this could be pushed into as to now unknown magnitude of 10.1 to 10.5 (or higher, possibly to 10.8) which are just so much energy released that it is unthinkable.
While these magnitude of quakes may not be seen for a while, it doesn’t mean that they can’t happen and so we must prepare with this new maximum quake as our new design basis for things like nuclear plants. This may mean that some places are not suitable for putting nuclear plants where with the old model we would have built one. OR we may have to build tsunami walls higher, since the quake itself did not break the reactors in Japan but the consequences of the tsunami that crippled the reactors.
Both Japan and Chile have had quakes in the last year. Both were really well documented with lots of in place instrumentation so we know exactly how these two different quakes behaved. This new information on how the Japan quake happened and why it was so powerful will have seismologists working for years updating what could happen on all of the worlds subducting plate boundaries.
- Disaster, Japan Earthquake (echlinm.wordpress.com)
- New Scientist article – Starburst megaquake: Japan quake overturns geology (requires login.)