Ringing left over from a meteor impact, or plate tectonics. If more earthquakes happen in phase with the pulsating, it would only increase the amplitude, formi

Ringing left over from a meteor impact, or plate tectonics. If more earthquakes happen in phase with the pulsating, it would only increase the amplitude, forming a feedback loop. I'd look at earthquake phase data to see if they are "helping" the pulsating.