Spectrograms - Landslides, Rock-falls and Avalanches
Mass-wasting events are another common feature observed, particularly on volcanic seismic networks. These signals have a characteristic appearance in both seismograms and spectrograms. On seismograms, the waveform has a spindle shape with a somewhat gradual onset, and slow decay. The duration of these events is highly variable. Landslide events have very broad spectra; in other words, they have a lot of energy at all frequencies. While most small avalanches and rock-falls will be seen only on a station or two, the largest events will be visible across a geographical sub-group of stations and in rare cases on stations far from the volcano. It is often difficult to distinguish short duration rock-falls from glacier slip events.
Above and below: Several types of surface events at Mount Rainier probably related to, ice-rock avalanches or sarac collapse. Note that the larger signals last for tens of seconds with energy concentrated around 2-5 Hz. Note that station WPW, which is located about 18 km east of Mount Rainier shows almost no signal from these surface events. The very short duration, weak events mostly showing only on stations RCM, RCS and STAR are either crevassing events or very small slip events on a glacier.
A series of large rock fall avalanches at Mount St. Helens. There is a small one starting about 19:11:35 and then two large ones starting at 19:13:05 and 19:17:25. Note the data gap in most channels near the beginning of the first event.