Thread: SAC APK parameters

Started: 2006-02-20 21:04:28
Last activity: 2006-02-20 21:04:28
Topics: SAC Help
Matthew R. Agius
2006-02-20 21:04:28
Hi again,
I have went through Rex V. Allen, but i still haven succeed to get
control of APK.
If I have in memory a 24hour sac file will it pick all events? Can
someone share with the parameters used at their station?
Which sample rate is best for APK?



George Helffrich wrote:

Dear Matthew -

c3 and c4 are pure numbers, and serve as weights. Here is the part of
the algorithm that they act in:

* - Compute first difference of filtered data.
fdfd = fdnew-fdold
* - Compute characteristic function.
chf = fdnew**2 + c2*fdfd**2
* - Compute the short term average.
chfsta = chfsta + c3*(chf-chfsta)
* - Compute the long term average.
chflta = chflta + c4*(chf-chflta)

You should look at the reference for more details on how to modify
these factors:
Rex V. Allen, Automatic Earthquake Recognition and Timing from Single
Traces, BSSA, Vol. 68, No. 5, Oct. 1978.

On Wednesday, February 1, 2006, at 07:45 AM, Matthew R. Agius wrote:

Hi Everyone,

I am trying to find my way with the SAC funciton APK.
I would like to alter the STA and LTA constants

The default values as qouted from online manual are:

APK C1 0.985 C2 3.0 C3 0.6 C4 0.03 C5 5.0 C6 0.0039 C7 100. C8
-0.1 D5 2. D8 3. D9 1. I3 3 I4 40 I6 3 VALIDATION ON

and that :

3. C3 is the timing constant, used to compute the short term average
of the characteristic function.

4. C4 is the timing costant used to compute the long term average
of the characteristic function.

My questions is what are the values of c3 and c4, are they minutes
or seconds?? 0.6 means 60 seconds? and 0.03 is 3 seconds?


M.Sc Studnet
University of Malta

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George Helffrich

06:13:46 v.eb79165e