Hi Steve and Geoffry,

I have been wondering about your filtering.  I assume that you are looking 
at filtered data.  There is a type of very effective digital filter 
described as IIR, Infinite Impulse Response, which is in a class of filters 
called non-causal.  If that is what Larry, or the writer of your filter 
program is using, then it might be possible to get false precursors to a 
large, sudden signal.

In effect these filters "look into the future", using as part of their 
input, recorded data which is later in time than the output point being 
calculated.  In that way it is possible for them to generate the 
precursors, though how much earlier and how large those might be will 
depend on the details of the filter routine.  Or then again, maybe it's 
something real.

There is one wave that could be described as a pre-P wave, called the Pn, 
which is only seen in the nearer quakes, in which part of the P wave takes 
the fast lane along the top of the Moho, arriving before the regular P 
phase would.  The USGS travel time program usually includes the Pn time 
when the quake is close enough.  The Pn was quite clear from one quake in 
Mexico as seen in Los Angeles, though I don't think is could precede the P 
for one as close as 25km.

Regards,
Brett

At 10:06 PM 9/2/2009 -0700, you wrote:

>I also noticed an anomaly in local events within 20-25km south of my old 
>San Jose location when I lived in south San Jose. My house was located 
>between the San Andreas and Calaveras faults and I had data from both 
>faults that had something going on just before the P wave arrived. I 
>showed John Lahr the data and asked if there was such a thing as  a  pre-P 
>wave and he told me there was not. I finally figured that it was a just a 
>flaw in the sensor.
>
>Regards, Steve Hammond
>
>From: psn-l-request@.............. [mailto:psn-l-request@............... 
>On Behalf Of Barry Lotz
>Sent: Wednesday, September 02, 2009 6:54 PM
>To: psn-l@..............
>Subject: Re: ARRIVAL TIMES: Quiet Before The Storm
>
>
>
>Geoffrey
>I've also noticed this for quite some time. I thought it was my sensors. 
>Maybe not.
>Regards
>Barry
>
>
>--- On Wed, 9/2/09, Geoffrey  wrote:
>
>
>From: Geoffrey 
>Subject: ARRIVAL TIMES: Quiet Before The Storm
>To: "PSN-LIST" 
>Date: Wednesday, September 2, 2009, 5:50 PM
>
>I have noticed in some Earthquakes you seem to notice
>a short increase in quietness just before the impulse arrives.
>
>Is this a real phenomena or simply a coincidence ?
>
>Does this mean the real arrival time is
>the beginning of the noticeable quiet ?
>
>WHY THE QUIET ??
>What physics would possibly muffle the noise ?
>
>arrival of Possibly eP instead of simply P ?
>
>Also,
>
>What is the preferred format for a PSN files data ?
>FLOAT or INTEGER
>16 bit or 32 ?
>
>My data is all 8 bit So should I expand it to
>meet the new format ?
>
>Comments ?
>
>geoff
>__________________________________________________________

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