I know that normally this structure, preposition+which, is used because the preposition is bound with a verb phrase or noun phrase in the following relative clause, but I came across a sentence in the New York Times which I find does not follow the rule:

> 
> We looked carefully at the physical phenomena by which exposure to such fields might perturb biological systems and lead to cancers
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> 
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Why is "by which" used here? I cannot find any phrases in the relative clause including "by".