Subduction zones: the primary evidence

In this thread
http://tinyurl.com/25y9fm
D. Findlay/ J. Harshman wrote

D.F.
I say the primary evidence for subduction is earthquakes and that's
all. What do you say that is different?

J.H.

What about seismic scans of mantle density that show descending plates?
What about all the patterns of volcanic island arcs and mountain ranges
adjacent to every trench? Why is the historical pattern of volcanic
activity on the North American west coast coincident with the history of
convergent plate movement -- when subduction ends, so do active
volcanoes? Why are the Cascades conicident with the region of the
currently subducting Juan de Fuca plate, and there are no volcanoes
active in southern California, though there were a few million years ago
before the spreading center was overriden?

I'll answer this later ( D.F.) Maybe others have something they would
like to add?
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