Why is it unlikely to find earthquakes at depths greater then 300km? (something to do with the athenosphere)

According to the principle of isostasy, what will happen to the elevation of a mountain range in the early stages of erosion?

Why are continents unlikely to be destroyed by the process of subduction?

Thanks alot

I think a better subject title for this would be "geology."

Since this is not my area of expertise, I searched Google under the key words "asthenosphere earthquake" to get these possible sources:

http://en.wikipedia.org/wiki/Asthenosphere
http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-246X.2006.02815.x?journalCode=gji
http://www.earthquakecountry.info/10.5/MajorMovieMisconceptions/
http://www.bartleby.com/65/as/asthenos.html
http://www.globalchange.umich.edu/globalchange1/current/lectures/evolving_earth/evolving_earth.html

Under "isostasy mountain erosion" I found:

http://en.wikipedia.org/wiki/Isostasy
http://findarticles.com/p/articles/mi_m0GDX/is_3_76/ai_75434343
http://gpc.edu/~pgore/geology/geo101/mountain.htm
(Broken Link Removed)

Under "subduction continents" I found:

http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Continents.shtml
(Broken Link Removed)
http://en.wikipedia.org/wiki/Plate_tectonics
http://en.wikipedia.org/wiki/Continental_collision
http://www.platetectonics.com/book/page_12.asp

In the future, you can do your own searches using appropriate key words.

I hope this helps. Thanks for asking.