Explosions of Karymsky volcano often produce signals containing a sequence of repeating pulses recorded on acoustic and seismic sensors, known as chugging. The amplitudes of these pulses correlate with the time interval between pulses. For a given measured acoustic pressure, seismic amplitudes take on arbitrary values up to a specific, empirically determined threshold.
Lees, J. M., Gordeev, E. I. & Ripepe, M. (2004) Explosions and periodic tremor at Karymsky volcano, Kamchatka, Russia. Geophysical Journal International,-. doi: 10.1111/j.1365-246X.2004.02239.x