A Template To Obtain Information On Gravitational Mass Movements From The Spectrograms Of The Seismic Signals Generated

We present a method to obtain a parameter (b) that allows to analytically reproduce the shape of the increase in amplitude at high frequencies in time of the SON (Signal Onset) section of the spectrogram of seismic signals generated by gravitational mass movements (snow avalanches, lahars and debris flows) descending a slope and approaching a seismic sensor. This increasing shape is a consequence of the appearance of energy at high frequencies as the gravitational mass approaches the seismic sensor. The developed method to obtain the parameter (b) allows to analytically reproduce the increasing shape of the SON section. Since this shape is related to the speed of the avalanche and the characteristics of the terrain, the parameter allows us to "classify" the mass movement with only one sensor. This methodology includes a link between the propagation properties of seismic waves and the results of the application of an image processing using the Hough transform.

Depending on the type of event, differences are obtained in the order of magnitude of the values of b. The mean value of b for lahars is around 0.003 s^-1, that for debris flows is an order of magnitude greater (0.017 s^-1) and an order of magnitude less than that for avalanches (0.12 s^-1). Furthermore, differences in b are observed within each type of event. This fact allows us to create a template with different values of parameter b to help in the classification within each type of mass movement by only superimpose graphically the corresponding spectrogram with the appropriate template when they are at the same scale.

Once the value of b has been determined, the characteristics of the mass movement should be set according to the judgment of experts. This must be done for each site and for each type of gravitational mass movement. The application to one lahar and one debris flow is presented as an example.