March 2011 Japan Killer Quake and Tsu...

March 2011 Japan Killer Quake and Tsunami Triggered by Slippery Clay at Ultra-Thin Fault Zone

There are 1 comment on the International Business Times story from Dec 6, 2013, titled March 2011 Japan Killer Quake and Tsunami Triggered by Slippery Clay at Ultra-Thin Fault Zone. In it, International Business Times reports that:

Around 27 scientists from 10 countries said it was a fine sediment clay within the fault that caused the destructive earthquake two years ago.

Join the discussion below, or Read more at International Business Times.

“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Dec 9, 2013
Interesting, but only partly correct.

The true driver of the tsunami was the enormous runout of that megathrust quake, caused by both the plate lurch of the descending plate, and also the relaxation of the crust in the Tohoku zygomorph.

The region is a zygomorph as a result of it having 2 megathrusts (one on the east, and another on the west). The result is that the arc acts as a range and stores up a lot of energy as a result of it being 2* the normal load capacity of an island arc.

The true driver of this event was the locked megathrust @ Tohoku (which was somewhere between 15 and 100Km deep with all the pings & pongs of the forearc in the previous centuries occurring above the level of the locked megathrust.

Once the megathrust broke off Iwate the break travelled both sideways along the megathrust & more importantly unlocked the megathrust from point break BOTH up to the surface & down to the region of the arc graben @ 115Km, with transmission to depth going faster than the sideways spread. The crustal relaxation terminated N.@ the region of the 1896 megathrust quake (which had already relaxed & had a SE vector). The megathrust terminated S.@ the southern end of the Tohoku megathrust which @ that region was only tightened to classic arc values as a result of the Jogan (869 AD) event relaxing the S. part of Tokoku in full @ that time.

With the megathrust broken in full the descending plate became free to move (The bulk of the 50m runout occurred @ that time @ Iwate as before then, the locked part of the megathrust would have prevented descending plate lurch. The largest lurch was @ the most compressed bit (off the coast of Iwate) Being a zygomorph, the descending plate ALSO stored a lot of energy which would have been impossible to maintain in a single megathrust arc,(a megathrust event would have occurred in the ~ 1/2 time interval with ~ 1/2 runout in that case).

The smectite lubricant along the upper level of the megathrust produces near aseismic slow creep in the toe of the forearc until the depth & pressure enables metamorphism of smectite to chloritic schist & high pressure low T greenstone facies. This is important, the toe of the forearc wedge therefore was near aseismic & deformed en bloc along the megathrust trace. It was not however the causative agent of the tsunami & merely facilitated the upthrusting of the flat lying turbidites in the floor of the trench pushing the water out of the way.

This is the classic mechanism of megathrust tsunami production in mature island arcs subducting sediment poor oceanic crust.
By contrast the tsunami produced by the 2004 megathrust was an unusual event in which the clastic sediment rich bengal fan was not lubricated right up to the trench & instead broke out into the forearc wedge along a splay upthrusting the outer forearc wedge into large scarps. In this backarc event there was ALSO very little associated plate lurch as a result of the descending indo/australasian plate being backed up against the Himalayan main frontal thrust, with the bulk of the runout being backarc driven crustal relaxation with slightly less then usual arc type descending plate lurch (the 2004 tsunami could have been bigger).

Next item of fun & games for Japan is likely to be Hokkaido. Last event there was 1611, and average recurrence time is ~450-500 years. The forearc trench structure there is the same as Tohoku but the environment is backarc with a weaker megathrust & more overriding plate relaxation with less descending plate lurch.

Have a nice day: Ag

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