Rockfall induced seismic signals: case study in Montserrat, Catalonia

After a rockfall event, a usual post event survey includes qualitative volume estimation, trajectory mapping and determination of departing zones. However, quantitative measurements are not usually made. Additional relevant quantitative information could be useful in determining the spatial occurrence of rockfall events and help us in quantifying their size. Seismic measurements could be suitable for detection purposes since they are non invasive methods and are relatively inexpensive. Moreover, seismic techniques could provide important information on rockfall size and location of impacts. On 14 February 2007 the Avalanche Group of the University of Barcelona obtained the seismic data generated by an artificially triggered rockfall event at the Montserrat massif ( near Barcelona, Spain) carried out in order to purge a slope. Two 3 component seismic stations were deployed in the area about 200 m from the explosion point that triggered the rockfall. Seismic signals and video images were simultaneously obtained. The initial volume of the rockfall was estimated to be 75 m(3) by laser scanner data analysis. After the explosion, dozens of boulders ranging from 10(-4) to 5 m(3) in volume impacted on the ground at different locations. The blocks fell down onto a terrace, 120 m below the release zone. The impact generated a small continuous mass movement composed of a mixture of rocks, sand and dust that ran down the slope and impacted on the road 60 m below. Time, time-frequency evolution and particle motion analysis of the seismic records and seismic energy estimation were performed. The results are as follows: 1 - A rockfall event generates seismic signals with specific characteristics in the time domain; 2 - the seismic signals generated by the mass movement show a time-frequency evolution different from that of other seismogenic sources ( e. g. earthquakes, explosions or a single rock impact). This feature could be used for detection purposes; 3 - particle motion plot analysis shows that the procedure to locate the rock impact using two stations is feasible; 4 - The feasibility and validity of seismic methods for the detection of rockfall events, their localization and size determination are comfirmed.