Guralp Systems CMG-DM24 - Manual (Page 67)

Operator’s Guide DM-24 Digitiser
Issue G January 2003 65
In order to capture all of a seismic event, some seconds of buffered data from before the
trigger is declared are recorded. This facility is particularly useful for emergent type
signals where the system triggers on a phase after the first arrival. Furthermore, some
seconds of data after the system stops triggering are also recorded to ensure the coda of
an event is not missed.
There are 5 parameters directly associated with the STA/LTA trigger algorithm:
sta
defines the length of the Short Term Average window in seconds. Takes 3
arguments which are the values to use for each of the three seismometer
components (Z, N/S, E/W)
Example: 1 1 1 sta
lta
defines the length of the Long Term Average window in seconds. Takes 3
arguments which are the values to use for each of the three seismometer
components (Z, N/S, E/W)
Example: 1 1 1 lta
ratios
defines the STA/LTA ratio above which the system will declare an event.
Takes 3 arguments which are the values to use for each of the three
seismometer components (Z, N/S, E/W)
Example: 4 4 4 ratios
pre-trig
specifies the amount of time in seconds for which data prior to the trigger
will be retrieved from the buffer and output with the triggered data.
Because of the block nature of the data format and compression algorithm
this time is only approximate.
Example: 20 pre-trig
post-trig defines the amount of time in seconds after the trigger has ceased during
which triggered data will continue to be output. Because of the block nature
of the data format and compression algorithm this time is only approximate.
Example: 40 post-trig
An example illustrating the various trigger parameters is shown overleaf.