Tohoku megaquake shows big tremors make volcanoes sink
Five Japanese volcanoes are a bit stouter than they were just a few years ago. Within a day of the 2011 megaquake, they shrank by up to 15 centimetres.
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“Geologist [I'm Climate Change]”
Since: Mar 07
#1 Jul 1, 2013
About time too.
The "megaquakes" are the closing point of the recompression cycle typical of converging island arcs due to subduction convergence. The megaquake is a Rebound/ Long duration mercalli X event (r/lx) which as the acronym states relaxes the crust of the overriding plate. The same r/lx will also cause a significant plate lurch of the descending plate in the region where the megathrust breaks.
One interesting side effect of the plate lurch & crustal relaxation is that the crust around the edges of the broken megathrust where the fault remains locked, tightens significantly. This crustal tightening incidentally results in compression of the forearc & also volcanoes adjacent to the r/lx in the arc. This has been commented on several times by volcanologists in the recent past and an example of an adjacent volcano is Copahue just N. of the central Chile 2010 r/lx.
The other interesting side effect of plate lurch & relaxation of the overriding plate, has just been describe in this article. Crustal relaxation GPS data has noted outward lurch & subsidence @ the forearc, and this outward flowage extends also to the arc which in the case of Tohoku has been remarkably aseismic both during the "megaquake" & bloody great big tsunami, and also during the aftershock sequence in the forearc & descending plate.
The r/lx in relaxing the crust in a sudden lurch decompresses the overriding plate which causes subvertical fractures to form parallel to the arc & particularly perpendicular to the vector of r/lx. These fractures tap discs of magma built up under the solid crust by dewatering of the descending plate, and magma being liquid is aseismic. The volcanoes respond to the widening of the arc by sinking but being buffered by magma injection into the base of the crust & solid upper mantle they do not sink very far in a typical arc runout r/lx (2010 & 2011).
The aftershock series compressing the forearc also compresses the base of the arc which has pinched out the base of the magma filled fractures and has now sealed them off from the magma discs.
The expectation now is that recompression @ the arc will transfer magma sideways along the new base crustal fractures until the magma hits a wall & can go only upward in response to further arc recompression. It is only then (about a decade or so after r/lx) that the newly injected magma erupts through a volcano in the area affected by crustal relaxation during the r/lx.
Keep watching that space.
Have a nice day: Ag
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