## PSN-L Email List Message

Subject: Re: On timing
From: ChrisAtUpw@.......
Date: Mon, 11 Apr 2005 22:03:12 EDT

```
In a message dated 11/04/2005, asc@............... writes:

Reluctant as I was to get involved in a good story, Chris
Chapman  finally got me in with:

' I agree that you  can't use the sub microsecond accuracy of a good GPS
clock. However, if  your clock only had an accuracy of 1 sec, you would
have a possible error  of ~10 km. If it had an error of 10 sec, the
possible error is ~100 km.  Neither would be particularly helpful when
estimating the depth of a quake  at, say 40 km, or of it's position. I'm
sorry to put it so bluntly, but if  you CAN'T give error limits to your
measurements, you are JUST COLLECTING  GARBAGE !'

You can use S-P intervals to locate the epicentre, travel  time curves
to compute an origin time and depth-phase identification to  sort out
focal depth - one could get away without accurate time at all,  but it
makes life easier.
Cheers
Kevin

Kevin  McCue
Director
Australian Seismological  Centre

Dear Dr McCue,

I am not sure if you have appreciated quite how  erratic computer
software clocks can be? Let's say that we have three amateur  stations which know
their Lat and Long co-ordinates, but are each only 50  km apart in a roughly
straight line in a UK setting. My clock lost 6  sec per hour and the central
station gained 6 sec per hour when  checked last week, but the other end one is
unknown. We are all using  the standard Widows clock update of once per week and
are at the end of the  cycle. We all measure a P / S delay times of the order
of 10 min, but  we have only the one vertical sensor with some cross
sensitivity.

Sure we can put in figures for the average travel  times for a range of
depths, but estimating a 'cocked hat position' and working  back to the time of
origin leaves several minutes unexplained.

How do you suggest that we get an estimate of the  time, location and
depth of the quake and the probable errors,  please?

Regards,

Chris Chapman

In a message dated 11/04/2005, asc@............... writes:
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size=3D2>Reluctant as I was to get involved in a good story, Chris=20
Chapman  finally got me in with:' I agree that=20=
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can't use the sub microsecond accuracy of a good GPS clock. However, i=
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your clock only had an accuracy of 1 sec, you would have a possible er=
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of ~10 km. If it had an error of 10 sec, the possible error is ~100 km=
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Neither would be particularly helpful when estimating the depth of a q=
uake=20
at, say 40 km, or of it's position. I'm sorry to put it so bluntly, bu=
t if=20
you CAN'T give error limits to your measurements, you are JUST COLLECT=
ING=20
GARBAGE !'You can use S-P intervals to locate the epicentre, trave=
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time curves to compute an origin time and depth-phase identification t=
o=20
sort out focal depth - one could get away without accurate time at all=
,=20
but it makes life easier.CheersKevinKevin=20
McCueDirectorAustralian Seismological=20
Centre

Dear Dr McCue,

I am not sure if you have appreciated quite how=
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erratic computer software clocks can be? Let's say that we have three amateu=
r=20
stations which know their Lat and Long co-ordinates, but are each only =
50=20
km apart in a roughly straight line in a UK setting. My clock lost =
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sec per hour and the central station gained 6 sec per hour wh=
en=20
checked last week, but the other end one is unknown. We are all us=
ing=20
the standard Widows clock update of once per week and are at the end of the=20
cycle. We all measure a P / S delay times of the order of 10 min,=20=
but=20
we have only the one vertical sensor with some cross sensitivity.

Sure we can put in figures for the average trav=
el=20
times for a range of depths, but estimating a 'cocked hat position' and work=
ing=20
back to the time of origin leaves several minutes unexplained.

How do you suggest that we get an estimate of t=
he=20
time, location and depth of the quake and the probable errors,=20