The Brune (1970) classic theory's suggestion that the source radius be determined from the corner frequency of earthquake spectra is based on a number of insufficiently constrained assumptions that make the result virtually a guess. Viewing earthquakes as displacement-discontinuity sources radiating ω-2 spectra indicates that the two other parameters can be accurately resolved from the corner frequencies, without reliance on any additional assumptions. These parameters are the source duration (rise time) and the maximum slip velocity on the fault; their determination is rooted in the exact formulas of radiation from dislocation sources. These directly observable parameters can serve as important constraints on dynamic theories of friction and faulting.

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