The coda of artificial shots on volcanos

Abstract

The data of active seismic experiments at Merapi volcano and at Vesuvius volcano are used to analyze the scattering of elastic waves in the shallow structure of these volcanos. The seismograms from artificial explosions located on the volcanos are characterized by spindle-like envelopes, small or missing P-onsets, missing S-onsets, and long codas. These unusual shapes of the envelopes can be explained by strong multiple scattering in the shallow heterogeneous eruptive material and are modeled using the diffusion model. As a result diffusivities of d=0.05 km2/s in the case of Merapi and of d=0.1 km2/s in the case of Vesuvius are obtained independent of frequency between 2 to 20 Hz. Assuming the dominance of shear waves in the coda and a typical S-wave velocity of around 1.5 km/s for the shallow volcano this corresponds to a transport mean free path of only 100 m in the case of Merapi and of only 200 m in the case of Vesuvius. The corresponding length scales for intrinsic attenuation, on the contrary, depend on frequency and are at least one order of magnitude larger than the transport mean free path. These results show that multiple scattering is an important effect for the seismic wave propagation in volcanic environments.