TOKYO: Mt. Fuji erupted 43 times in 2,000 years

Jul 27, 2013 Full story: Asia News Network

Mt. Fuji has erupted and discharged lava at least 43 times in the past 2,000 years, according to a study by Japan's National Institute of Advanced Industrial Science and Technology .

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“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Jul 29, 2013
This is an important finding in other ways.

The phenomenon of rebound/long duration mercalli X (r/lx) during major megathrust earthquakes (as seen in Tohoku, 2011) relaxes the crust of the overriding plate.

The result of r/lx is that it opens subvertical fractures @ the base of the crust which fill with magma from discs @ the base of the initially tight & compressing crust which form from magma extracted from the descending plate. These fractures, generally perpendicular to the vector of the r/lx, will then squeeze magma sideways along the pressure gradient along the fracture once the megathrust seals & locks in the months following r/lx.

The recompression phase lasts for a long time, typically 500 to 800 years @ Izu where Fuji happens to be, as the r/lx was typically a backarc runout. Fuji as a volcano is fed by individual magma filled fractures where they travel sideways until they hit a wall.@ that point they can only go in the direction of least compression (up). Fuji is a classical stratovolcano of cone shape as a result of the base of the volcano being (now) warm & relatively soft with the resulting product erupting generally out of a narrow pipe.

The past eruption history of Fuji indicates how far into the Izu backarc basin the previous r/lx series decompressions reached.

Fuji is currently dormant for a particularly good reason. The penultimate r/lx in 1703 gave a backarc runout in the south of the Izu arc (Tori shima etc.) and an arc runout in the north (Fuji). The recompression cycle in the north since 1703 resulted in another r/lx in the northern sector during the Kanto earthquake (1923) which did nasty things to Tokyo. Again it appears that the r/lx did not reach Fuji.

Look sideways to the SSE from Fuji and you will see that Izu-Tobu reactivated after the last r/lx and the magma now emerges there instead.

Also note that Oshima is still very busy, last r/lx there may have been 1703 or Kanto the segment boundary is probably N. of Oshima & S. of Izu-Tobu.

Have a nice day: Ag

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