Seismic wave scattering in volcanoes

Abstract

Volcano-tectonic (VT) earthquakes produce high-frequency seismograms characterized by impulsive shear mech- anism; their seismogram coda re ects the random inhomogeneity of the volcano structure. Consequently, this inhomogeneity can be investigated through the analysis of the coda wave envelopes of the VT events. In this chapter, I will review the main observational results obtained from volcanoes around the World, with the 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 sepa- rate the intrinsic from the scattering attenuation using multiple scattering and di¤usion models, and nally, I will report on the interpretations based on these results. The results show that the coda-Q absolute values characteristic of volcanoes are slightly smaller than those measured in non-volcanic zones, and that sometimes their frequency dependence is di¤erent. It is impossible to deduce by coda-Q observations only whether this di¤erence is controlled more by the intrinsic or the scattering attenuation. The application of multiple scatter- ing models allows separate estimates of the intrinsic and the scattering attenuation coe¢ cients. 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, di¤usion 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 an important role at low frequencies. Key words: Seismic scattering, coda waves, volcanoes