Geoff and Wayne,

The filter artifacts you describe sound like what you get from 
non-causal digital filtering.  Non-causal just means that the usual 
relationship that effects can only occur after their causes, is 
violated; that is, that some change in the filter output can precede 
the input that "caused" it.  I have seen data published before, 
suggesting that someone was seeing earthquake precursors which I 
believe were almost certainly the result of using some type of 
non-causal (IIR) digital filter.  Although such types of filters have 
certain advantages, they probably shouldn't be used when seeking precursors.

Of course you can't really get causes coming before effects.  These 
filters are being applied, a little after the fact, to recorded, 
data.  The filtered output data for a point at, say, 3 seconds are in 
effect, computed from data points recorded both before and after 3 
seconds, so the filter apears to have some knowledge of what is 
coming in the future.

Though the kinds of time intervals the researcher seems to be talking 
about would appear to be greater than you'd expect to get from 
non-causal filtering.

I'd want to see a great many more examples before I'd get too excited 
about this.  Although he apparently saw this phenomenon before one or 
two quakes, how often does it occur with no quake following, and how 
often do quakes occur without seeing it?

Brett

At 06:22 AM 3/25/2012, you wrote:
>On 3/24/2012 3:03 PM, Wayne wrote:
>>BBC video segment on analyzing GPS signals as precursors to equakes.
>>
>>
>>
>I just hope the GPS predictor curve is happening
>on a system not filtering the incoming data signal.
>
>That predictor curve may be a filter artifact
>and not what they think it to be.
>
>I have seen defective precursors to earthquakes
>arrival times using filtered geophone signals.
>The filtering does something to add a undulation
>before the actual signal has arrived.
>But it is only a second or so of time.
>Unfiltered signals do not seem to show this.
>
>geoff


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