If subducting plates can penetrate the 670 kilometer boundary and sink all the way to the base of the mantle..

Question

why are there no earthquakes deeper than 670 kilometers?

Answer 1

Because a phase change occurs around that level that makes it impossible for the rocks to fail in a way that makes earthquakes. Technically this is termed no "brittle failure." To clear up some of the other confusion--this is not the base of the mantle and the mantle rock is solid below this level. I'm a bit shocked by what Jim M. has to say--first because it's untrue, this comes from real data, not models, and second, because my name happens to be Jim M. also, and I'm surprised a namesake would put out wrong information.

Answer 2

Thank you pegminer. There are competing theories as to whether oceanic plates make it to the base of the mantle or not, and this is an area of active research. When I first heard of deep earthquakes my feeling was that it was due to changes in mineralogy (pyroxene assuming the perskovite structure, I believe). A possible reason for no earthquakes deeper than 670 km is that there are no mineral phase changes available after 670 km.

Answer 3

Roberto,

Charlie Bucket is right on target. By the time you get that deep, material is behaving plastically. Thus, the material is not rigid enough to get "snagged" and let any stress build up, so you can't generate any earthquakes.

pegminer and mountaingym: I'm not sure who thumbs-downed my answer, but I'm uncertain as to why you feel my answer is incorrect. In fact, I do not debate anything that pegminer says, I think my answer is very much in line with his. If you could provide me with some info as to why you think my answer is incorrect or inadequate, I would appreciate it. Also, as a mineralogist, I am well aware of the phase changes that accompany the increasing pressure as you go deeper into the mantle, and I am also well aware of the fact that the vast majority of the mantle is solid. But the generation of earthquakes deals with the friction between two plates producing stress, and then the ability for those plates to undergo brittle failure. Earthquakes occuring on a strike-slip fault like the San Andreas occur in rocks with a very different mineralogy than those of earthquakes occuring at depth in a subduction zone, so why would changes in mineralogy make that big a difference in an ability to generate earthquakes? I'm not trying to belittle what you say, and if the two of you have more experience in structural geology than I do, then of course I defer to you on these matters, but I fail to understand why a change in the structure of MgSiO3 from a pyroxene to a perovskite structure would automatically mean no more earthquakes.

Answer 4

If 670 K is the base of the mantle, then this area would be more plastic than solid. In order for the energy from an earthquake to propagate, you neee a solid matrix to transmit that energy. If the mantle is not really solid, this energy will not get transferred. Additionally, the earthquake would not likely happen in the first place as they are caused by a release of energy because of the rigid nature of the crust, which would not be rigid at this depth.

Answer 5

The plates don't really penetrate that deep so much as become "absorbed" the heat from the mantle softens the rock and it becomes maliable. Earthquarkes occur in the rocks on the surface of the earth.

Answer 6

This is just more nonsense derived from models. There is no data and maybe there will never be data but thats no reason to believe BS from modelers. Better to just say I don't know what happens within the mantle.