The period, wavelength and amplitude of seismic waves are measured using seismometers and depicted on seismograms (see “What is a seismometer?” and “What is a seismogram?”).
A period is the duration of a single oscillation and is measured in seconds.
A wavelength is the spatial equivalent to a period and is the length of a single oscillation. In the case of seismic waves, the wavelength value is typically in the region of between several dozen metres and several kilometres.
The amplitude is the force or deflection of a single oscillation. Modern seismometers are able to record amplitudes of less than one nanometre (one millionth of a millimetre).