[Skywise]

(05-31-2015, 12:14 PM)Island Chris Wrote: The M7.8 very deep quake is nothing like this. It was a stretching focal mechanisms and USGS suggested it was within the subducting slab (as probably all major to great very deep quakes are).

My bad. You're correct. I didn't give enough thought to the depth. And, I failed to realize just how quickly the slab turns and dives downward. The quake was at 678km deep, but only about 270km from the trench. The slab has to dive pretty quick. I always thought the pictures of such quick diving were due to vertical exaggeration of the diagrams.

Then after doing a bit of digging I find an excellent cross section diagram of the Izu-Bonin-Mariana trench system on Wikipedia.


from: http://en.wikipedia.org/wiki/Izu-Bonin-Mariana_Arc

After a little georeferencing I found the quake occurred almost exactly on cross section B-B', and looking at it's chart the quakes at 140 longitude are indeed this deep. There seems to be a gap in depths in the middle range, so I wonder if these could also be due to phase changes of the minerals?

BTW, I estimate the vertical exaggeration in the above cross sections to be about 8.4 to 1. Even after compensating for that in image editing software, it is clear that the slab dives nearly vertically in some locations. Of course, the text accompanying the image on Wikipedia says exactly that.

Brian