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,
 
    Thank you for your comment!
 
    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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  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  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=
you=20
  can't use the sub microsecond accuracy of a good GPS 
clock. However, i=
f=20
  your clock only had an accuracy of 1 sec, you would 
have a possible er=
ror=20
  of ~10 km. If it had an error of 10 sec, the 
possible error is ~100 km=
..=20
  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=
l=20
  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.
Cheers
Kevin

Kevin=20
  McCue
Director
Australian Seismological=20
Centre


Dear Dr McCue,
 
    Thank you for your comment!
 
    I am not sure if you have appreciated quite how=
=20
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 =
;6=20
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
please?
 
    Regards,
 
    Chris Chapman