Observation of coda signals from regional and local earthquakes recorded from a downhole-uphole couple of broad-band sensors at Mt Etna

摘要

Seismic coda of regional and local earthquakes recorded at a couple of broad band seismometerslocated at the bottom of a 125 m deep borehole and up-hole at surface show interesting spectralfeatures. We observe strong similarity between the waveforms recorded from up- and down-holesensors at low frequency (0.1 -3 Hz) and measurable differences in the higher frequency limit. Weinterpret this observation assuming that at high frequency the up-hole coda is produced by body-tosurfacewave scattering in the near surface. We compare the experimental results with numericalsimulations done using the Monte Carlo scheme of Yoshimoto et al. (2000) carried out in theassumption of velocity and scattering coefficient which smoothly vary with depth, with the addition of abody-to-surface wave conversion for the energy particles which reach the surface. The comparison ofthe experimental coda envelopes with those obtained through numerical simulation allow for aquantification of the turbidity parameter at surface.