SEISMIC WAVE SCATTERING IN VOLCANOES

Volcano-tectonic earthquakes produce high-frequency seismograms characterized by impulsive shear mechanism their seismogram coda reflects the random inhomogeneity of the volcano structure. Consequently, this inhomogeneity Can be investigated through the analysis of the coda wave envelopes of the volcano-tectonic events. In this chapter, I will review the main observational results obtained from volcanoes around the World, with file aim of quantifying the scattering and attenuation properties of the volcanic areas. First, I will review the coda-Q observations and their frequency dependence, then I will report on attempts that have been made to separate the intrinsic from file scattering attenuation using Multiple scattering and diffusion Models, and finally, I will report on the interpretations based oil these results. The results show that the coda-Q absolute values characteristic of volcanoes are slightly smaller than those measured in nonvolcanic zones, and the sometimes their frequency dependence is different. It is impossible to deduce by coda-Q observations only whether this difference is controlled more by the intrinsic or the scattering attenuation. The application of multiple scattering models allows separate estimates of file intrinsic and the scattering attenuation coefficients. Results Show that volcanoes are highly heterogeneous structures, with a mechanism of seismic wave energy dissipation that tends to be controlled by the scattering phenomena with increasing frequency. For Mt. Vesuvius, Mt. Merapi, and Deception island volcano scattering attenuation prevails at frequencies higher than 2-3 Hz. At Mt. Etna, intrinsic dissipation prevails or is comparable with scattering attenuation for frequencies lower than 8 Hz. At high frequencies, diffusion approximation is appropriate to describe the energy seismogram envelope. The intrinsic dissipation of shear waves (possibly connected with magma reservoirs, which should decrease the intrinsic Q values for shear waves) only has ail important role at low frequencies.