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.