Alaska Volcano Observatory

A small rockfall event occurred at Cleveland in July 2014, shown by the light-colored trail of dust rising above the path of volcanic debris that tumbled down the east flank of the volcano. The two panels below the photograph show how rockfalls are expressed in seismic data, as recorded by seismic station CLES at the southeast base of the volcano. On top is the seismic waveform - here spanning 1 minute and 30 seconds from left to right - showing the typical spindle-shaped, emerging and slowly diminishing ground shaking signal seen during rockfalls in many settings. Below that is the same information in the frequency domain: warmer colors show higher amounts of energy at different frequencies contained within the waveform. Rockfalls show a different mixture of high and low frequencies than earthquakes. Figure by John Lyons, USGS/AVO, July 29, 2014. For more on interpreting spectrograms, see: http://pnsn.org/spectrograms/faq .