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, 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 ChapmanIn a message dated 11/04/2005, asc@............... writes:<= FONT=20 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 CentreDear 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