Seismogram on New Zealand earthquake

I have been learning to use the software and database Global Earthquake Explorer for my geophysics course this semester. It seems very useful, although I am still trying to figure out how I can recover ground motion amplitudes in order to have my students calculate magnitudes.

Below is the seismogram from the Christchurch, New Zealand, earthquake, as recorded by the seismograph at South Karori, New Zealand, and retrieved using GEE.