The asthenosphere consists of many varieties of rock - mostly in a semi-molten condition. The temperatures reach levels high enough for most of the rock to turn soft and almost plastic-like. This soft, malleable (vocab word!) layer supports the lithosphere - consequently creating earthquakes when the plates above it shift. Just imagine a few pieces of play-do on top of a thing of slime (like the type you would get at a carnival). If you moved even one of the play-do pieces, it would move and push into another one of the "plates" - therefore creating an earthquake. (That's a whole different lesson, so I won't go into that.) I hope this answered your question fully!
