Downloaded the data from the sensor, and troubleshot the issue.
The g-forces didn't hit the setpoint, but the default advance settings were triggering it.
The software has a switchable relay output, with the latch relay being triggered by not only by the actual exceedence of setpoint, but an adjustable sensitivity, software-filtered P-wave pre-alarm/warning state. This is enabled by default, but not appropriate for this use.
So, it was seeing P-wave of the right amplitude of an incoming EQ and triggering, even though the actual event didn't occur.
As an unusual side note, the event that occurred was on 6/26/07, 14:58 PDT, and had the following axial readings:
X-.12 g
Y-.12 g
Z-.02 g
Duration-6 seconds.
Alarm Trigger Threshold-.15 g any X/Y/Z
X-East/West Y-North/South

I checked the current time on the sensor, and it is 1 minute behind, and there is nothing in the SCEC catalog for that particular time. There was an event almost 1 hour later, but not then.
A mystery.....