Those of us watching volcanos in Iceland have found these seismic charts to be quite informative.
And the subject of much debate.
First thing that you'll notice is that there is no label on the y axis. Better minds than mine have determined that the y axis is the 30 minute max and min of the number of vertical seismic events at that frequency over a minute.
I know, that doesn't make any sense.
The three different lines are different subsonic frequencies, probably representing different behavior of rising gases in the decompressing magma. When they invert relative positions and increase, there is usually an eruption.