about Waveform Data

By using obspy , i can read waveform data ,and plot the seismograms.i have known the meaning of x-axis is time.
ButI have a question what’s the meaning of the Y-axis , and the value of Y-axis can be negative.
Another question is if this time’s wave is a earthquake ,what’s the minimum value of Y-axis.

my English is bad!
thank you for reading!


the y-axis is in most cases ground motion or a derivative thereof. The unit and maximum amplitudes are a bit more complicated to understand - I’d recommend to read a text book to fully understand what seismological waveforms actually are. This for example is a good introduction: