Japan's quake updated to magnitude 9.0

From New Scientist.

Seismologists at the US Geological Survey in Menlo Park, California, have just revised their calculations regarding the magnitude of today's quake. They now say it was magnitude 9.0. Already one of the top 10 recorded earthquakes in history, the revision suggests the quake was even more powerful than first thought.

Harold Tobin of the University of Wisconsin-Madison told New Scientist that this figure will probably change again. This is typical in the hours after a large seismic event, as more information becomes available.

Earlier today, it was suggested that the tremor may have occurred along a splay fault - a branch off the main megathrust fault which runs through this area of the Japan trench. If so, that could mean the fault was previously unknown to geologists.

Splay faults tend to break at steeper angles than megathrust quakes, making them highly likely to lead to a large uplift of the seafloor that produces damaging tsunamis such as the one that crashed ashore in Sendai and the Honshu coast earlier today.

But the new set of calculations indicate that the giant quake ruptured at an angle of 14 degrees below horizontal. Such a shallow slip suggests the earthquake did in fact occur along the main megathrust fault.

Tobin said that in the next few days, seismologists will be working feverishly to come up with a new set of calculations known as an "inversion" to determine over what area the fault slipped and how great the slip was. It's likely to have been several metres for an earthquake of this magnitude.

Once that's established, geologists will begin to look at how this earthquake may have transferred stress onto - or away from - faults in the Tokyo bay and Nankai trough regions, to the south-west.

This will help get a handle on the big question that will be on everyone's mind: when, and where, will it happen again?


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