Poronaysk Russia, 7.7 mag -lower- mantle quake

[dgavin]

There was large quake off the east russia coast, coming in at 7.7 Magniture, at a whopping 626km depth! This likely places this massive quake in the area of the lower mantle.

No Tsunami warning, and no damage to surface structures are expected from this, as it was so deep.

[dgavin]

This quake, which was recived at Mt. Hood Siemographs at 8:09pm PDT last night, Did not have a ring-like a bell reading, but a sustained signature that faded 8:34pm PDT, immediately followed am small avalanch/rock slide signature at Mt. Hood at 8:38pm PDT.

I would have to say it's a good chance the avalanch was triggered by the quakes waves after they passed through the Mt. Hood region.

[dgavin]

I've emailed the USGS CVO on this one to find out that if there was an avalanche, if it might of been partialy triggered by the 7.7quake. I'll keep you posed on the results of that email.

[grapes]

I think the upper mantle can go farther down than 625km. It's pretty much defined by where earthquakes can occur, no?

Looking it up on wikipedia, it says the lower mantle starts at 660km:
http://en.wikipedia.org/wiki/Mantle_(geology)

Still, that's *deep*

[Ara Pacis]

What would cause an earthquake that deep? Is it still from subducting slabs descending toward the core? I would have thought that the plasticity of the mantle would prevent quakes.

[dgavin]

I think you nailed it on the head Ara.

Before this event, there were three quakes along the subsudtion zone 2 at 40km then a third at 138km. Then two more intense quakes at 400km depth further to the west, then the 7.7 even more westward and deeper. The depth of the quakes match the expected angle of the subducting plate.

On a map, it looked looked a connect the dots for an almost perfect triangle. I would guess that a triangle is roughly the shape of that portion of the subducting plate, and that perhaps, the unmelted tip of it broke off completely, or partially.