Tensor components, time-resolved

By analyzing complete seismic waveforms, it is often possible to resolve a multiple event into subevents with different mechanisms. Doing this requires use of algorithms that allow the moment tensor to vary with time in a general way. Algorithms that assume that all the moment-tensor components have identical time functions can produce spurious non-DC mechanisms, even if the subevents have identical DC mechanisms. 

If earthquakes occur close together in space and time, observed seismic waves may not be able to resolve them, and they may be misinterpreted as a single event The apparent moment tensor of the composite event is then the sum of the true moment tensors of the earthquakes, and because the sum of two DCs is not, in general, a DC, shear faulting can produce non-DC mechanisms in this way. Combining DCs cannot ever produce mechanisms with isotropic (volume change) components, because the trace of the moment tensor is a linear function of its components, so multiple shear-faulting mechanisms lie on the horizontal axis of the source-type plot. 

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