TREMORS OF THE FIFTH KIND – OBSERVATION OF NEAR SURFACE MOVEMENTS

According to their origin seismic tremors can be grouped in four categories: tectonic earthquakes, volcanic tremors, collapse events and induced events. The first three are natural events, whereas the last category deals with man-made seismic activity mainly due to mining operations such as blasts and rockbursts, but also reservoir triggered tremors are part of the last category.

The wealth of seismic data from the past years of monitoring revealed an additional group of seismic events which cannot be attributed to any of the before mentioned causes. Such events are being recognised as seismograms showing a more or less distinct first arrival of sometimes low frequent content, followed sometimes by vague secondary onsets - if at all - , of which the coda carries plenty of energy at relatively low frequencies. Few events of this kind have been observed during recent years due to the higher density of seismic stations in the Alps. Some of those events could clearly be identified as rockfalls, whereas other did not leave any obvious traces on surface. That a classic rockfall does really constitute a single events but resembles rather a sequence of events or a landslide will be shown with a video.

Events of this fifth kind, which feature exuberant low frequent seismic wave content, are thought to be a result of near surface movements of the rock mass. It is proposed, that they might incorporate slow slip movements by utilizing existing fault planes, hence emitting distinct shear waves, or they are a result of rockfall-sequences, mountain-splitting or sliding along bedding planes. Geodetic measurements, aerial photography and dense local seismic networks should enable us to find out the true nature of some of theses events, which tend to culminate in the Salzkammergut in Austria. The latter events reached magnitudes of approx. 3 with macroseismic intensities not exceeding 3-4 degrees due to the low frequent ground motion.