Complex sentence: It may sense an increase in seismic activity if Meager becomes very restless, but this may only provide a warning for a large eruption; the system might detect activity only once the volcano has started erupting.
Split into simpler sentences: It may sense an increase in seismic activity if Meager becomes very restless, but this may only provide a warning for a large eruption. It might detect activity only once the volcano has started erupting.