Triggering

Triggers is the OSOP implementation of Carl Johnson’s carlstatrig STA/LTA trigger algorithm in C.

To help Sixaola users tune their O-triggers algorithms, we have created an interactive GUI available at: https://gitlab.com/osop/otriggers-tuning-gui

Configuration

Configuration parameters include:
  • Long-term average time (LTA, seconds)
  • Short-term average time (STA, seconds)
  • Ratio - an empirical coefficient (unitless)
  • Quiet - This is a trigger-level adjustment (unitless)
  • Trace length (seconds) - The total trace length written to disk
  • Pre-event length (seconds) - The pre-trigger trace length- This is only used when we write an event to disk. The trace written to disk starts with this many seconds back in time of the total ‘trace length’.
_images/trigger1.png

How it works

The OSOP implementation in C is similar to the Earthworm module carlstatrig: (note that below paragraph is cut and paste directly out of the carlstatrig manual and the default values cited are not the same as are delivered on the Sixaola)

Note

The station trigger algorithm works as follows. For each configured station, the short-term average (STA) of the trace is calculated. The time length of the averaging window (STAtime) is one second by default, but can optionally be specified by the user. A weighted average of previous STAs is then taken to get the long-term average (LTA) of the trace. The number of STAs used to calculate the LTA (LTAtime) is eight by default, but can optionally be specified by the user. LTAtime is the number of STAs over which the time-dependent weighting factor used in the averaging decreases by a factor of approximately 1/e. In addition to these two `straight’ averages, there are corresponding rectified averages. The short-term rectified average (STAR) is taken from the absolute value of the difference between the trace and the LTA, averaged over one second (by default) or optionally over the user-specified window length described above. The long-term rectified average (LTAR) is a weighted average of previous STARs, calculated in a manner analogous to LTA.

These four averages are combined to determine the station trigger status, using a modified version of Carl Johnson’s magic formula:

eta = STAR - Ratio * LTAR - | STA - LTA | - Quiet

In its original form this calculation was done in integer arithmetic. In that case, the value of Ratio was in the form Enumer / Edenom. The current calculations are performed in floating-point arithmetic. This obviates the need for some subtle adjustments used in the earlier code, and seems to make the algorithm more sensitive.

If eta is greater than 0.0, the station is considered triggered, the trigger turns off when eta <= 0.0.

Source: Earthworm Trac Wiki