this chart is the "tremors" going on inside the cavern.
(the cavern is directly under the measuring instrument)
i suppose you can call them "very super small earthquakes".
but they are very super small (low energy) tremors
that are being caused by the unstable broken rock stuctures
inside the cavern (huge multi thousand ton rock structures
creaking and cracking slipping slightly) (and, note: if
you had another similar seismograph 3 miles away from the
site there would be no readings at all. that's how low
energy they are)
but all of these tremors are clear indications that the
cavern structure is very unstable and will probably collapse again.
[link to folkworm.ceri.memphis.edu