This is what you will see when a pulse goes through a filter.

Without knowledge of the exact nature of the filter it is difficult to know 
exactly how it will behave, but in general you will get a time delay and often 
extreme phase effects.

So while the chirp may appear to precede the signal, it may be the signal which 
is delayed by the filter, and the chirp may be a filter artefact aligned with 
the initial arrival.

It could also be a local electrical pulse (anything being switched on or off) 
making the filter ring, so you could try plugging something noisy into the same 
mains outlet and switching it on and off to see if you can induce the same 
behaviour.

On 24/07/11 08:36, Larry Conklin wrote:
> Well, whatever it is that I'm seeing, I'm very sure it has nothing to do with
> the quake. The signal isn't natural looking at all. It is a nearly sinusoidal
> wave that begins pretty abruptly, sweeps down in frequency from 2 hz to 0.4 hz
> and then stops fairly abruptly. I have a history of similar artifacts that have
> showed up from time to time, but I don't think they have shown this sort of
> frequency sweep before. That is what caught my interest this time. In past
> occurrences I would see a minute or so of a very pure looking tone that would
> show up in the FFT plot as a very narrow spike around 2 hz. I'd love to know
> what's going on, but don't have much of an idea of how to run it down.
>
> Larry
>
>
>> both of my Leymans show a sharp spike about 25 seconds before S arrival here
>> in Evansville, IN...largest spike occurred on my E-W unit it is visible on
>> the N-S and vertical AS-1 to lesser extent.

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