A trigger is an algorithm that checks the signal for variations that could indicate an
event. There are several trigger algorithms available, some very sophisticated using pattern
recognition, spectral analysis etc. They are sometimes used in seismic network recorders, but
rarely in portable data loggers currently in the market. In the hands of an expert, they can
significantly improve the event detections/false triggers ratio, particularly for a given type of
seismic events. However, the sophisticated adjustments of operational parameters to actual
signal and seismic noise conditions, at each seismic site, which these triggers require, have
often been proven unwieldy and subject to error. Hence, for practical purposes, only two types
are widely used, the level trigger and the STA/LTA trigger (see below) and only those will be
described here.