Thanks everyone. I appreciate the answers so far.

Based on the answers I've read, I dont get the impression that any of the possible EM signals coming from stressed rock would be of sufficient "quality" to contain enough information to be useful in a predictive sense.

Is that a fair statement?

Another question:

Assuming that perceptible signals were produced from the underlying rock in a fault zone, and that the type of rock would affect that signal, would there be enough variation of rock throughout California so as to be able to pinpoint the location based on that signal?

Thanks again all,

Russell