YMR is set to the sensitivity (125 mv) that if a duck farted @ 1000 ft it would pick it up; Using YMC, which is less sensitive (1600 mv) which is quite close to YMR will help in determining the waveform meaning if it shows up on both it may have significance. It is a best practice to refer to multiple siesmos when possible when interpreting the waveforms.
What you are seeing is Lesser Harmonics not to be confused with Volcanic Harmonics.
LH or Lesser Harmonics are just the background ring of the earth and yes that ring can get somewhat loud in terms of siesmos that are ultra sensitive. These are harmless.
VH or Volcanic Harmonics are distinctly different in shape on siesmos. For less sensitive siesmos it will look similar to a rounded EKG and for sensitive siesmos it’ll look more like a square wave if it truncates or like Mount Everest followed by the Challenger Deep over and over if it doesn’t.
These can be important depending on how long they last and if the grow in amplitude.
Another waveform that is often confusing for many looks like a diamond, starts out small builds and then fades. These are Volcanic Like Movements or VLM tend to occur very deep and are not terribly strong and most of the time only picked up on sensitive siemos but on occasion will show up on less sensitive ones. These are not dangerous.
A similar wave form to VLM are surface events they to tend to be diamond shaped but are much more ragged. In most cases these are felt but tend to be relatively harmless unless of moderately strong magnitude 4.5+.
Lastly the most common type, Tectonics, these typically don’t have a build up or when they do it tends to be short, and they can be proceeded by a building LH. After no or little built there is a sharp movement that will vary in duration on the strength of the quake. following the sharp movement the initial waveform will fade out over a period of seconds to generally a few minutes.