A template to obtain information on gravitational mass movements from the spectrograms of the seismic signals generated
Abstract. We present a template that helps in the classification of gravitational mass motions (snow avalanches, lahars and debris flows) by simply overlaying it graphically with the corresponding spectrogram of the generated seismic signal at the same scale. The template is created with different values of a parameter β' that allows us to analytically reproduce the exponential form of the increase in amplitude of high frequencies in time of the SON (Signal Onset) section of the spectrogram when the mass movement descends a slope and approach 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. We present a methodology that includes a link between the propagation properties of seismic waves and the results of the application of an image processing using the Hough transform to demonstrate that this shape is related to the speed of the avalanche and the characteristics of the terrain. Seismic data generated by lahars, debris flows, and avalanches are used for the study. Depending on the type of event, differences are obtained in the order of magnitude of the values of β'. The mean value of β' 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 β' are observed within each type of event.
Once the appropriated value of β' has been determined, the characteristics of the mass movement must be set according to expert judgement. This must be done for each site and for each type of gravitational mass movement. The application of the templates to the data of lahars and an avalanche recorded in two different places of its trajectory is presented as an example.
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