IIRC, a quake monitor will transmit a steady tone. In the event of a seismic event, the audio frequency will modulate up and down above and below the normal steady tone. How much the audio frequency modulates depends on the strength of the event.
A trick I recall reading in an old issue of "Monitoring Times", in a review of a external speaker that provided an audio frequency notch capability, is to notch out the steady tone, and if you start hearing the tone, it's because of an seismic event causing the audio frequency to modulate. It could give you seconds of pre-notice, depending on the origin of the quake in relation to the monitor and your location, before the tremors strike your location.
Not having heard this particular earthquake monitor myself, however. I could be wrong, and it may be data bursts, perhaps telemetry of some sort.